can gears improve gas mileage and acceleration?

[bad68coupe]

just wondering if the changing of gears can improve gas milage and acceleration. also how difficult is it to change gears in a third member compared to the other kind of rearend, not sure what it is called but it has a differential inspection plate that bolts to the rear?

 

[Stanger007]

I swapped my 2.73s for 4.10s in my 95 and my gas mileage went down a few MPG - I drive mostly highway though and didn't care about the mileage drop - the 4.10s were worth it!

If you drive a lot of stop and go, a lower ratio (higher numerically) can actually improve your gas mileage.

BUMP on the difficulty of swapping out the gears - I let a shop do mine since it requires some specialized tools.

 

[tos]

Sorry you cant have both. You trade one for the other. To get better milage you go to a gear ratio that has less acceleration and vice versa. The best thing is to probably find a ratio in the middle. Maybe a 3.00 ratio or 3.50. One of these would give you decent accel plus be good on the highway. The 3.50 would be better for acceleration and the 3.00 would be better for mileage.

 

[Max Power]

What Stanger said can actually happen. You can improve gas mileage if you drive primarily in the city in stop and go traffic. Most cases though, performance gears will hurt mpg.

Of course, most vintage stang owners don't care that much about mpg.

 

[bad68coupe]

It just doesn't make sense to me. If it takes lets just say, 100 Calories of energy to turn the drive shaft once, and with that 100 calories, with my old gears i would roll 5 ft. now with my new gears i would roll 7 feet because i am doing more with the same amount of energy and i would get there faster because i can get more rotations out of my tires per every turn of the driveshaft.

 

[66forfun]

yes, i believe if the gears put the engine closer to the optimal power range. but that all gets thrown out the window when you are mashing the gas at every stoplight because it is so much more fun!

 

[Stanger007]

Let me explain what I said about gaining mileage with a lower ratio gear (higher number) a bit more and maybe it will make more sense.

The gears don't create power but they do a couple of things for you...

First they provide more torque by causing the driveshaft to spin more times per tire revolution.

Secondly this allows your engine to rev with less effort and this gets you into your powerband more quickly.

If you do a lot of stop and go driving you are constantly having to get your car moving - the better application of torque (added torque multipication because of the lower gearing) will help you get up to speed more quickly. This is why trucks have lower gearing than cars - it helps get heavy weight moving.

If you do a lot of highway driving (like me), you will lose mileage because you are having to spin your engine higher to maintain the same speed on the highway.

Make sense?

Wes

EDIT: My spelling really sucks.

 

[bnickel]

basically if you drive only in town doing a lot of stop and go stuff and rarely get on the freeway or the highway a lower gear WILL get better mileage than the stock 2.79 somthing on the order of like a 3.25 or 3.50 would be about right maybe even as high as a 3.70 gear.

if you do use the freeway or the highway then the 2.79 gear you have will be better for mileage.

in town the lower gear allows the engine to not work so hard to get the car moving but on the highway the higher rear gear allows the car to stay moving with less work from the engine.

a good compromise for both in town and highway would be a 3.00 or 3.25 rear gear. i have a 3.25 gear in my car with a 351w and fmx auto trans. the last motor i built for the car before the current setup got an average 21mpg on the highway with a 26" tall tire, never measured in town though.

 

[fasttback]

basically if you drive only in town doing a lot of stop and go stuff and rarely get on the freeway or the highway a lower gear WILL get better mileage than the stock 2.79 somthing on the order of like a 3.25 or 3.50 would be about right maybe even as high as a 3.70 gear.

if you do use the freeway or the highway then the 2.79 gear you have will be better for mileage.

in town the lower gear allows the engine to not work so hard to get the car moving but on the highway the higher rear gear allows the car to stay moving with less work from the engine.

a good compromise for both in town and highway would be a 3.00 or 3.25 rear gear. i have a 3.25 gear in my car with a 351w and fmx auto trans. the last motor i built for the car before the current setup got an average 21mpg on the highway with a 26" tall tire, never measured in town though.

Yes, bnickel stated it well. If your worried about gas mileage (efficiency), stick with the numbers above. As always, there are other variables to toss in such as motor/weight characteristics.

My experience was from a 3.00 to a 3.50 gear on a 289. Since I did mostly city driving, this improved gas mileage and performance (I actually had to detune my carb in order to stop it from bogging off the line).

 

[bnickel]

Yes, bnickel stated it well. If your worried about gas mileage (efficiency), stick with the numbers above. As always, there are other variables to toss in such as motor/weight characteristics.

My experience was from a 3.00 to a 3.50 gear on a 289. Since I did mostly city driving, this improved gas mileage and performance (I actually had to detune my carb in order to stop it from bogging off the line).

thanks

 

[chromedog]

i did a tire change on my daily driver, (a 94 explorer) swaping 70 series tires for 75's... not much of a change, but a change, nontheless.this effectively raised my gear ratio slightly. i would have never noticed a difference, but my family and myself took off on a 1500 mile trip the day after i changed tires. when we returned home, i dug out my gas receipts, and averaged them together. my car, which normally does 24 miles per, had gotten just over 25, and i feel it was due to the tire change. the way i see it, the engine is turning a bit faster, but it is not turning as hard, due to the higher gear.

 

[bnickel]

i did a tire change on my daily driver, (a 94 explorer) swaping 70 series tires for 75's... not much of a change, but a change, nontheless.this effectively raised my gear ratio slightly. i would have never noticed a difference, but my family and myself took off on a 1500 mile trip the day after i changed tires. when we returned home, i dug out my gas receipts, and averaged them together. my car, which normally does 24 miles per, had gotten just over 25, and i feel it was due to the tire change. the way i see it, the engine is turning a bit faster, but it is not turning as hard, due to the higher gear.

the mileage difference could be totally from speed error and not have anything to do with the tires (other than the different speedo reading of course)

 

[ForceFed70]

It just doesn't make sense to me. If it takes lets just say, 100 Calories of energy to turn the drive shaft once, and with that 100 calories....

The problem is that you are simplifying things too much.

It's never a case of "It takes 100 joules of energy" Different engine RPM, temperature, slope, and even the efficiency of the engine comes into play. It's always a changing number.. you have to look at an average number not an instance in time.

 

[Great68]

Any mileage improvement from gears is negated the first time you go on the highway.

From 2.80's to 3.55's my gas mileage decreased substantially.

 

[Max Power]

Gas mileage is a byproduct of engine speed AND load. 3.50 gears reduce the load around town, and if all you do is drive around town, quite often the reduction in load is of greater benefit than the increase in engine speed a detractor.

 

[bad68coupe]

alright i think i got it now thanks for all the help, i think i am going to end up with a 3.00 gear ratio, cause i will be driving between dallas and tyler about once a month so i will mostly be doing in town driving.

Now how hard is it to do gears on a third member?

 

[Max Power]

All diffs are third members. There are diffs with integral carriers, and diffs with removable carriers. It takes special tools and a knowledge of how to set gear patterns. It's not for a novice.

What gears do you have now? I am willing to bet that they are real close to 3:00 to 1.

 

[ultrastang]

Get the best of both worlds

If you are running a C-4, C-6 automatic, or Top-Loader 3- or 4-speed manual transmission (all 1:1 in high gear), then you are greatly limiting the capability your car has in both performance and economy.

The best choice is to run a lower gear ~3.25-3.70:1 in conjunction with an overdrive transmission, such as an AOD or T-5 manual 5-speed (typically .68:1 in overdrive).

The overdrive will allow you to take advantage of off-the-line acceleration with a lower rear end gear, but at cruising speeds, the engine RPMs will drop drastically once the transmisssion goes into overdrive.

As an example, if you had a 26" tall tire, 3.00:1 rear end ratio, and non-overdriven transmission, your engine RPMs at 60 MPH would be ~2,326 RPMs. Given this very same senario, except coupled to an overdrive transmission, your cruising RPMs at 60 MPH would be 1,582 RPMs --744 RPM drop.

A T-5, in overdrive, coupled to a 3.50:1 rear end ratio would cause the engine to "see" the rear end ratio as being 2.38:1 (3.50 x .68 = 2.38:1). Even though the rear ratio may be 3.50:1, in overdrive, the engine thinks the rear ratio is 2.38:1 --That's even higher than a stock 3.00:1 ratio with no overdrive, so you would actually get better mileage with the 3.50s over the 3.00:1s, and still have much more accelleration in 1st - 4th gears with a T-5, or 1st - 3rd with an AOD.

http://www.ultrastang.com/Rearinfo.asp?Page_ID=3

 

[bnickel]

If you are running a C-4, C-6 automatic, or Top-Loader 3- or 4-speed manual transmission (all 1:1 in high gear), then you are greatly limiting the capability your car has in both performance and economy.

The best choice is to run a lower gear ~3.25-3.70:1 in conjunction with an overdrive transmission, such as an AOD or T-5 manual 5-speed (typically .68:1 in overdrive).

The overdrive will allow you to take advantage of off-the-line acceleration with a lower rear end gear, but at cruising speeds, the engine RPMs will drop drastically once the transmisssion goes into overdrive.

As an example, if you had a 26" tall tire, 3.00:1 rear end ratio, and non-overdriven transmission, your engine RPMs at 60 MPH would be ~2,326 RPMs. Given this very same senario, except coupled to an overdrive transmission, your cruising RPMs at 60 MPH would be 1,582 RPMs --744 RPM drop.

A T-5, in overdrive, coupled to a 3.50:1 rear end ratio would cause the engine to "see" the rear end ratio as being 2.38:1 (3.50 x .68 = 2.38:1). Even though the rear ratio may be 3.50:1, in overdrive, the engine thinks the rear ratio is 2.38:1 --That's even higher than a stock 3.00:1 ratio with no overdrive, so you would actually get better mileage with the 3.50s over the 3.00:1s, and still have much more accelleration in 1st - 4th gears with a T-5, or 1st - 3rd with an AOD.

http://www.ultrastang.com/Rearinfo.asp?Page_ID=3

exactly true, a lower rear gear and OD trans would be the best of both worlds. especially if you do a lot of combined driving. however if all you ever do is city driving then just a bit lower gear would be fine and if you all ever did was highway driving a higher gear would be best.

personally, i'm going to be using a 3.25 gear and and AOD because i really like to drive my car on the open road and it will be doing some cruising around town as well but there is nothing better than to just get out on the highway and take a couple four drive to the middle of nowhere and back.

 

[Max Power]

An I the only guy that loves the sound of my engine at 3500rpms? If you get decent heads, a lumy cam and decent exhaust, it's music to my ears.