Getting Better Gas Mileage

[Redfire 05]

With the rise of gas prices over the last few years, and the likelihood that we are stuck with the high gas prices, I thought a discussion on the topic of increasing gas mileage might be useful. Many people are interested in stretching that tank out a little longer and will often go to silly lengths to do so. I often see posts here and on other forums about gimmicks to increase gas mileage that fool people into spending money when they simply don’t work. The truth is there are things you can do to stretch that gas a little further, the most important of which is changing your driving habits, but gimmicks like magnets to place on fuel lines, and adding acetone to your gas are often a waste of money. Just watch the Mythbuster’s episode on the subject. So lets start a discussion on what you can really do to get better gas mileage.

I’ve read several books on the topic over the years and have tried some things that work so I’ll start the discussion out with that. This won’t be super scientific, not because I believe you will be unable to understand it, but because it adds a lot of complexity to things that can be understood in simpler terms, and admittedly I don’t fully understand the math and physics behind all of it. But please feel free to chime in with more technical details if you want. With your help maybe we can make the list more comprehensive as I’m sure I’ll miss important points. In all honesty most of these suggestions are common sense suggestions.

If you think about it gas mileage is controlled by a few factors: Friction (including wind resistance and tire to ground friction), engine speed, engine efficiency and most importantly driving habits. In many cases (not all) the principles used to make a car faster can apply to fuel efficiency as well (for instance reducing weight). So you don’t necessarily have to sacrifice speed and power to get more MPG. But you do need to excerpt some control over your right foot. With the mods I currently have on my 05 GT I get 19MPG city and 27.5MPG highway. You can see the mods in my signature below. I’m making 35 hp over stock before the 90 octane tuen, pulleys and headers, but I still get good gas mileage. I suspect I am at or over 300 rwhp now (281 was my previous best). I’m pretty happy with those power and MPG numbers. My best when the car was stock wha 18 MPG city and 23.5 MPG highway with about 9,000 miles on the odometer. Obviously some of these suggestions will work for everyone and some you probably won’t want to try because it will alter some aspect of the car that you are really happy with (for instance using smaller tire sizes).

Sorry this is a long one, so I broke it up to make it easier to read.

 

[Redfire 05]

Friction

Simply stated, a more aerodynamic car is subject to less wind resistance and thus requires less power (and gas) to cut through the air. How do you make your car more aerodynamic?
1. The cheapest method is to roll up your windows. All of the air coming in and hitting the back window increases drag. It probably doesn’t matter a huge amount at slow speeds, but as speed increases it makes more and more of a difference. With older cars running the air conditioner may actually reduce gas mileage more than rolling down the windows, but A/C units on new cars are much less parasitic so you should probably use the A/C and roll up the windows.
2. More narrow tires help some as they cut through the air more efficiently than wider tires. Plus they decrease the contact patch on the ground reducing friction. I am unsure exactly how much effect this has.
- On that subject some books suggested over-inflating your tires to further reduce contact patch, but that would certainly increase wear in the middle of the tire, so I would not recommend it.
3. Adding big wings to increase down force will also increase drag thus reducing your aerodynamic efficiency. That is how they increase grip. They use the air to force the car down to the ground. They are great for racing, but not so good for gas mileage. Of course some are adjustable so you can reduce the effect when you aren’t racing.
4. I’m not sure if lowering the car helps, maybe someone can chime in on this? It seems like it should help aerodynamically?

 

[Redfire 05]

Engine Speed

Your car will have a sweet spot in the rpm range where it gets the best gas mileage. This sweet spot is determined by how much gas you car uses at a given rpm, the efficiency of the engine, how fast you are traveling, how much wind resistance you are encountering (wind has a huge impact on MPG), and how much power it takes to overcome the wind resistance and probably a hundred other things I forgot to mention. Depending on conditions the sweet spot will change some, but in general lower rpms are better to a point. For instance, under ideal conditions (no wind) on a two way trip my car gets 25.3MPG @ 75mph, 27.5MPG @ 70mph, 28.5 @ 65mph, 30.1MPG @ 60mph in 5th gear. The 70mph and 65mph data come from round trips of 230 miles. The other trips were shorter (~30 miles). I’m not sure where the MPG levels out because I haven’t felt the need to travel any slower over long distances. If you have an 05 up mustang you can easily use the Avg MPG on the display to evaluate the effect of rpms on gas mileage. The gauges are not extremely accurate in all cars but they are useful as estimates.
These trips were not on the same day so conditions were slightly different.
Car Speed = Engine Speed--> MPG
75 mph = 2220 rpms--> 25.3 mpg
70 mph = 2072 rpms--> 27.5 mpg
65 mph = 1924 rpms--> 28.5 mpg
60 mph = 1776 rpms--> 30.1 mpg

The major factors controlling engine speed (besides your right foot) are rear end gear ratio and transmission gearing both of which determine the final drive ratio and tire diameter which can alter rear end gear ratios when you go with a different diameter tire than was stock on the car. So be sure to consider these factors when modifying your car.
1. Going with numerically higher gears (3.73 gears over 3.55) will increase your engine speed and decrease gas mileage at highway speeds. Around town the effect will be different. It might even increase gas mileage if you are able to use overdrive gears at low speeds.
For Example @ 70 mph in 5th gear w/ stock 27.2 inch tires
Gear Ratio = Engine Speed
3.27 = 1986 rpms
3.55 = 2072 rpms
3.73 = 2171 rpms
3.90 = 2276 rpms
4.10 = 2393 rpms

2. A smaller diameter tire will effectively increase, numerically, the gear ratio of your rear end while a larger diameter tire will numerically reduce the gear ratio. So be sure you consider this when changing tires.
Example:
235/55 17 Tire Diameter 27.2 inches = 2072 rpms @ 70 mph in 5th gear
255/45 17 Tire Diameter 26.0 inches = 2167 rpms @ 70 mph in 5th gear

3. I have always been irritated that Ford didn’t choose to go with the 6 speed TR6060 transmission because it has a 0.63 6th gear as opposed to the 0.675 5th gear of the 5 speed TR3650 used in Mustang GT’s.. This would reduce your highway rpms resulting in better gas mileage. Swapping one in your car would effectively reduce the impact of shorter gears. This of course is an expensive fix as a TR6060 costs around $3000. So it’s not worth in from a MPG standpoint.
For Example @ 70 mph in 5th gear w/ stock 27.2 inch tires
Tranny Gear Ratio Engine Speed
TR3650 w/ 3.55 = 2072 rpms
TR6060 w/ 3.55 = 1933 rpms
TR3650 w/ 3.73 = 2171 rpms
TR6060 w/ 3.73 = 2032 rpms

 

[Redfire 05]

Pump up the Power and the Gas Mileage?

The following is a list of suggestions that will increase power and gas mileage … at least in most cases. Our cars are not super efficient from the factory beyond the efficiency of the engine itself there are several aspects of the car you can improve which will help you make more power and increase your gas mileage by smoothing out the flow of the air into and out of the engine. There are other mods you can make to improve gas mileage beyond smoothing the air flow.
1. The addition of a K&N CAI on my 05 Mustang GT alone increased my highway MPG by 2.5. The factory intake is far from free flowing removing the turbulence from the air flow path will keep air velocities high and help performance in all aspects. Obviously CAI’s are designed to allow increased air flow at higher rpms so they will use more fuel as you mash the gas because more air is coming in. So they won’t help wide open throttle gas mileage, but then nothing really will. Not all CAI’s are designed in the same way, so all of them won’t have the same impact on gas mileage or horsepower.
2. The exhaust is another area where airflow can be improved. Older cars often have a very restrictive exhaust system. And although Ford claims to have used 2.5 inch exhaust on the 05 up mustangs anyone can look at the welds just before and after the catalytic converters and see the 2.25 inch pipe welded in between the 2.5 inch sections. Closer inspection reveals crushed pipe on the passenger side where the O2 bung is welded on, and very small 2.0 inch inner diameter pipe coming out of the factory exhaust manifolds. Improvements in these areas can benefit power and gas mileage.
3. Underdrive pulleys increase power by reducing parasitic drag on the engine via slowing down the accessories. This can also be of benefit for gas mileage.
4. Many have seen increases in gas mileage after installing aftermarket tunes. By changing spark and fuel curves you can dramatically change the mannerisms of you car both power wise and fuel efficiency wise. Anyone that has spent time tuning a car with a carburetor knows this. Changing jets and metering rods has a huge impact on fuel flow. When you are running rich you are wasting gas and loosing power. Get the mixture and timing right and you can maximize both.
5. Reduce the weight of your car and … um … yourself. It takes power to move all that weight around. The more weight the more difficult it is for your car to accelerate. You don’t have to go crazy here just remove all the dead weight in the car like the pair of roller blades you never use, the clothes and shoes in the trunk that are growing mold, etc. Consider this too when making mods to your car. Steeda and a few other manufacturers make lighter weight coil springs. Many other aftermarket suspension components are often lighter than the factory stuff too. Several companies make lighter weight wheels for our cars. The factory 17’s for the 05’s weigh about 27.5 lbs. I’ve seen aftermarket 17’s and 18’s with the correct offset coming in at under 20 lbs. Just remember some light weight stuff breaks easier to.

The really unfortunate thing is that most of these parts cost a lot of money so they may not be worth it from a gas mileage standpoint. But if you are going to make the changes anyway consider the gas mileage implications.

 

[Redfire 05]

Driving Habits

Finally, driving habits can make the single largest difference in gas mileage. When I behave myself I achieved a best of 19 mpg city, but as I normally drive I average 17 mpg city. For true lead-foots the disparity can be a lot more. Here are some suggestions that will help maximize fuel efficiency. Individually they may not make a huge difference, but together they do.

1. Maintain momentum: Accelerating from a stop uses a lot of gas so maintain your motion.
a. Remember when you are sitting motionless at a stop light you are getting 0 mpg. So try to avoid stopping as much as possible. When the light ahead of you is red don’t speed up to it and stop. Slip the transmission into neutral and coast up to the light. Depress the brake pedal lightly to slowly reduce speed. The idea is to slow down enough that the light will turn green before you arrive. If it doesn’t that’s still okay, because you still reduced the amount of time you will be stationary. This is easy to do when you are on the road by yourself, but use caution when other cars are behind you as they can get irritated if you impede their progress.
b. On a related matter choose a route that avoids stop lights and signs. Consider the increase in miles you travel when doing this obviously driving twice as much distance to avoid a few stops is not going to be beneficial, but by carefully choosing your route you can reduce your fuel consumption.
c. Don’t slow down so much when turning. You just have to press the accelerator down to speed back up. Obviously don’t be silly here. You don’t have to push the car to the limit around each curve, but don’t be like some and almost come to a complete stop before turning.
2. Accelerate Slowly:
a. This should be obvious, but if you take off like a drag racer from every stop light you are using a lot of gas. The best technique is to accelerate slowly and shift (if you have a manual) at or before 1500 rpms. Obviously this can lead to road rage from the drivers behind you if you really go slow so use some common sense.
3. Use 5th gear in town if you can.
a. The gearing on my 05 GT is perfect to use 5th gear at speeds from 35mph up. If you need more accelerating power down shift. Because of the gearing on my 86’ GT I need to be at speeds above 40 for 5th gear to work okay. Below that the car gets that bouncing gait because the rpms are too low. Automatics are a little different too. I owned a Buick Grand National with an automatic overdrive and it would slip in an out of overdrive to often when driving in-town so I left it in drive to spare the transmission the damage.
4. Car Pooling:
a. Car pooling and sharing the gas cost can seriously reduce the amount of money you use on gas. If you really want to save money on gas this is the way to go.

 

[kooldawg6]

Thanks for posting tips on possible MPG increases

 

[Redfire 05]

Thanks for posting tips on possible MPG increases

No problem.

Here are some things I forgot.
Buy Good Gas:
When it comes to gas just like with other things, you get what you pay for. Be careful buying discount gas. It isn't always bad, but if you get a bad tank of gas it can seriously affect your gas mileage. You shouldn't buy higher octane gas if you don't need it, but you should buy good gas. Some stations like Chevron and Shell use additives in their gasoline that can help keep your fuel system clean which will help you to maintain good gas mileage. These additives may only be in higher octane fuel (I'm not positive). Alternatively you can add a good fuel system cleaner like Chevron's Techron or Marvel Mystery Oil into your gas at regular intervals to keep the system clean.
Cleaning your fuel system
Even though acetone will not improve your gas mileage it is an excellent solvent. If your fuel system is dirty acetone will do a great job of cleaning it out. Thus it might clean your fuel system and result in better gas mileage if your fuel system was very dirty. But it would be unnecessary to continue adding acetone to subsequent tanks.

 

[Redfire 05]

On the acetone myth

Acetone, Ethanol, Propane, Xylene and Toluene are all potential fuel sources and/or additives that can be used to increase octane or clean your fuel system. There is also a myth that won't die about acetone increasing gas mileage. Unfortunately while each of these chemicals is higher in octane than gasoline they all produce less BTU per volume than gasoline. BTU stands for British Thermal Units and is a measure of energy in this case the energy released when the chemical is burned. So in simple terms each of these chemicals releases less energy per gallon than gasoline. What that means is to get the same energy from acetone (or any of the others) you would need to burn more of it when compared to gasoline.
For example:
Propane produces 91,600 BTU/gal
Gasoline produces ~ 125,000 BTU/gal (depending on additives and octane rating)

So to produce 125,000 BTU from propane you must burn 1.364 gallons of propane compared with 1 gal of gasoline.

91,6000 * 1.364 = 125,000

This is why cars running on Propane and Ethanol get slightly worse gas mileage than a vehicle running on gasoline. To get the same power you need to modify your fuel curve to add more fuel. So why run propane or ethanol. Besides being cleaner burning fuels propane at least is cheaper than gasoline and makes it approx. equal in cost. Before the major hike in gas prices it was actually cheaper. The same may be true of ethanol in the future. Because acetone produces around 40,000 BTU you would get far worse gas mileage. Not to mention the fact that acetone is much more expensive than gasoline. So even if you did get better gas mileage it would cost more. As stated in the previous post it is a good solvent so it will clean a dirty fuel system.

The chemicals in terms of BTU/unit ranked from the lowest to highest:
Toluene --> Xylene ----> Acetone --> Ethanol --> Propane --> Gasoline

 

[Art161]

I read somewhere that if you have a number of errands to run, you should go to the place farthest away first and then work your way back to your starting point.

The idea is to get the car warmed up because your gas mileage is better when the car is at normal operating temperature. If you stop at the nearest places first, then the car takes longer to warm up.

 

[TGJ]

One thing I disagree with is that 3.55's will net better gas mileage than 4.10's or other steeper gears. That is not always the case. Sure at 60 MPH, the 3.55's likely do, but what about 30 MPH. At that speed with 4.10's, you can drive in OD and turn enough RPMs to support that. With 3.55's, you would need to be in 4th at the highest gear and would be turning a higher RPM than the 4.10 equipped car.

It all depends where you do your driving. If you are spending lots of time on the freeway/highway, than a gear set like 3.31 or 3.55 would be a better choice than say 4.10s. If you go with that logic though, 2.73's would be the best for highway gas mileage. That stated, if you spend a lot of time driving in the city, 4.10's would be a better choice than the 3.55's.

My 96 TBird is a perfect example for this. The car had 3.27's stock, I put in 3.55's, 3.73's and 4.30's. The car's best highway mileage was with the 3.27's, but followed very closely by the 3.55's. The car's best city mileage is with the 4.30's and cruising in OD where with the other gear sets would be in D and at least 600 RPM higher( 3.73 gear set ). Now the car has a 4R70W auto but it supports the idea I am saying.

 

[wotfr]

Another way to increase your gas mileage is at the pump. The nozzle at the gas station has a vapor return line, so if you can fill up at the coolest part of the day when the fuel is less prone to vaporize and use the slowest setting because agitated fuel is more likely to vaporize.
You get charged for the fuel you pump so by reducing the amount you give back it adds up over time.
Now I know that doesn't effect how much fuel you burn going down the road but it does reduce the amount you pay for that fuel.

 

[Redfire 05]

One thing I disagree with is that 3.55's will net better gas mileage than 4.10's or other steeper gears. That is not always the case. Sure at 60 MPH, the 3.55's likely do, but what about 30 MPH. .

Good point, I guess I was thinking mostly at highway speeds. I edited the original post to reflect this. I'm betting on the 05's though that 4.10's would hurt mileage around town. With 3.55's I'm able to use overdrive at 35mph in my 05 GT. 3.73's might not be a bad choice though.

Another way to increase your gas mileage is at the pump. The nozzle at the gas station has a vapor return line, so if you can fill up at the coolest part of the day when the fuel is less prone to vaporize and use the slowest setting because agitated fuel is more likely to vaporize.
You get charged for the fuel you pump so by reducing the amount you give back it adds up over time.
Now I know that doesn't effect how much fuel you burn going down the road but it does reduce the amount you pay for that fuel.

Those sneaky turds. That's like stealing gas you payed for. I was not aware of that.

 

[SlideWRX]

3. I have always been irritated that Ford didn’t choose to go with the 6 speed TR6060 transmission because it has a 0.63 6th gear as opposed to the 0.675 5th gear of the 5 speed TR3650 used in Mustang GT’s.. This would reduce your highway rpms resulting in better gas mileage. Swapping one in your car would effectively reduce the impact of shorter gears. This of course is an expensive fix as a TR6060 costs around $3000. So it’s not worth in from a MPG standpoint.

I've been pondering something for a while now; The tremec website lists several 3650 tranmissions, and one of them has a lower (taller) 5th gear; 0.62. I think it was on some earlier mustangs but I don't know. How much would it cost to press on a new fifth gear? 'Course, for people that mainly drive around town it may actually hurt mileage if they have to use fourth more often...

 

[stang8urimport]

Until we pay the same as most of the civilized world for gasoline, I'm not gunna complain. I filled up for 3.25 for premium the other day, and I was thankful. Go to Europe or any other country... then you can complain.

I understand this thread isn't complaining about gas prices, but that's my take on the MPG situation.

 

[Hoboattacker]

One thing I disagree with is that 3.55's will net better gas mileage than 4.10's or other steeper gears. That is not always the case. Sure at 60 MPH, the 3.55's likely do, but what about 30 MPH. At that speed with 4.10's, you can drive in OD and turn enough RPMs to support that. With 3.55's, you would need to be in 4th at the highest gear and would be turning a higher RPM than the 4.10 equipped car.

It all depends where you do your driving. If you are spending lots of time on the freeway/highway, than a gear set like 3.31 or 3.55 would be a better choice than say 4.10s. If you go with that logic though, 2.73's would be the best for highway gas mileage. That stated, if you spend a lot of time driving in the city, 4.10's would be a better choice than the 3.55's.

My 96 TBird is a perfect example for this. The car had 3.27's stock, I put in 3.55's, 3.73's and 4.30's. The car's best highway mileage was with the 3.27's, but followed very closely by the 3.55's. The car's best city mileage is with the 4.30's and cruising in OD where with the other gear sets would be in D and at least 600 RPM higher( 3.73 gear set ). Now the car has a 4R70W auto but it supports the idea I am saying.

I disagree 100% about the low speeds steeper gear being more efficient. I easily put my car in 5th at 30mph with the 3.55's. These cars have so much tq that you can't stall it in 5th with the 3.55's. Even with the 3.55's I could start my car in 2nd extremely easily. I can do it even more so with the 4.10's now.

 

[Hoboattacker]

Until we pay the same as most of the civilized world for gasoline, I'm not gunna complain. I filled up for 3.25 for premium the other day, and I was thankful. Go to Europe or any other country... then you can complain.

I understand this thread isn't complaining about gas prices, but that's my take on the MPG situation.

Damn! I hate you for your gas prices haha. I still pay less than most, though, so I can't complain (3.40-3.49 for 93 octane).

 

[1965coupe]

Wow.. woah.. I need some cliff notes!

I purchased, and bought my MUSTANG GT(with a V8) as a performance car.. I didn't buy it expecting Honda-like mileage.

Unfortunately, my car takes premium, and gas is pricey.. so I do my best to get better MPG.. Here's what I do:

Have the odometer portion of the guage set to MPG or ECONOMY.. I basically watch that little gauge and let off the gas when it goes down too far..

I do drive 80.. (with all of the other cars.. only the Priuses belong to the 55/slow-lane club around here (lemmings))

I am careful not to use the AC.. and, as they proved on Mythbusters, use the AC at HWY speeds, versus having the windows down..

Try to avoid wasted trips around town..

Use synthetic oil.. (ok, that probably makes no difference)..

USE GOOD GAS!!! I avoid certain stations because of their cr@ppy gas.

With all of the above combined, I do manage about 20-22 per gallon! Works for me!

 

[Art161]

You shouldn't buy higher octane gas if you don't need it, but you should buy good gas. Some stations like Chevron and Shell use additives in their gasoline that can help keep your fuel system clean which will help you to maintain good gas mileage. These additives may only be in higher octane fuel (I'm not positive). Alternatively you can add a good fuel system cleaner like Chevron's Techron or Marvel Mystery Oil into your gas at regular intervals to keep the system clean.

I sent an e-mail to Chevron. The response from Chevron Fuels Technical Services is, "Years ago Chevron regular unleaded had less Techron than the supreme grade. Now all grades of fuel at any Chevron service station in the United States have the same level of the Detergent additive Techron. The only difference between the 3 grades of fuel is the octane rating."

 

[Redfire 05]

I sent an e-mail to Chevron. The response from Chevron Fuels Technical Services is, "Years ago Chevron regular unleaded had less Techron than the supreme grade. Now all grades of fuel at any Chevron service station in the United States have the same level of the Detergent additive Techron. The only difference between the 3 grades of fuel is the octane rating."

Good to know. Thanks for checking.

In response to some of the other replys. This thread wasn't started to complain about high gas prices, or lament about not getting 4 cylinder gas economy, but to discuss how you can get better gas mileage if you choose to. Many people get tricked into buying gimmiks that don't work or wasting money on acetone or other miracle cures. This hopefully this thread will steer people in the right direction. Honestly if I wanted 4 cylinder gas economy I would sell my 86 GT for a 4 banger to drive every day, but they really aren't very fun to drive. I love my car and I'm happy with the MPG, but it doesn't hurt to stretch the gas mileage during the week so it doesn't hurt the wallet as much when you burn a 1/2 tank of gas at the race track on the weekend.

 

[TGJ]

I disagree 100% about the low speeds steeper gear being more efficient. I easily put my car in 5th at 30mph with the 3.55's. These cars have so much tq that you can't stall it in 5th with the 3.55's. Even with the 3.55's I could start my car in 2nd extremely easily. I can do it even more so with the 4.10's now.

Is this with or without your Twin Screw SC?

You realize that 30 MPH in 5th with 3.55 and the stock 235/55R17 tire is below the stock idle speed and lugs the engine right? With 4.10's, that speed is 1000 RPM and doesn't lug the engine. I have been around for quite a few years and what I saying and have seen is that 4.10's will net better city mileage and worse highway mileage than 3.27's, 3.31's and 3.55's.