2:73 = Bad Gas Mileage???

I personally think that if you have a lower gear ratio and beat your car on take offs, you're gonna burn more fuel than i will with my 4.10s because it takes more effort to get the car moving while mine can get there easier with the 4.10s.

We've had this debate before and people disagreed with me on that, but logic tells me otherwise. You should still make killer highway mileage though.

LiquidGT said:

I have a 273 rear end and I did a 5 speed swap, I was wondering if my engine was being bogged down and giving me 16mpg mostly highway driving like a normal person. Yes I know its a mustang and bla bla, but I feel it should be better. I heard of bigger combos getting 19-21mpg. At 65mph is 1500rpm in 5th gear, it can even spin lower in the city. The motor is fresh and has a lot of new sensors and no codes. My dads 5.0 explorer gets better mileage with 373s and it spins at 2300'ish rpm at 65mph.

I have a hanging idle too, which dosent help (need to reset the idle and mess with the 10 pin connectors)

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Sounds like you are probably lugging it too much. With 2.73's, I'd think you could almost be running be running in 4th gear at 55 mph. Cruising at anything below 1800 rpm in 4th or 5th gear, and I would rationally assume your gas mileage is going to suffer a bit.

85_SS_302_Coupe said:

I personally think that if you have a lower gear ratio and beat your car on take offs, you're gonna burn more fuel than i will with my 4.10s because it takes more effort to get the car moving while mine can get there easier with the 4.10s.

We've had this debate before and people disagreed with me on that, but logic tells me otherwise. You should still make killer highway mileage though.

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Your logic is...not very logical.

If there is one thing we can all agree on, it's that the higher the engine rpm, the more fuel is burned, period.

With a higher *numerical* gear ratio over stock, such as your 4.10s, you are going to burn more fuel.

Understanding this is simple.

With a higher numerical gear ratio, the rpms are higher in everything you do. When you leave from a dead stop, your engine is at a higher rpm before the car even moves. Cruising, your rpms will be higher in every gear.

More fuel is being burned over fewer miles driven. While results will vary (the amount of mileage lost), you will get fewer mpg.

"it takes more effort to get the car moving while mine can get there easier with the 4.10s. "

What does "getting there easier" have to do with maintaining or improving gas mileage? Your car "gets there easier" because the motor is at a higher rpm when it leaves from a stop, or is at an already increased rpm when you step on the throttle, so you essentially have more horsepower under your foot at any given speed because of the increases in rpm in all gears. But that increased rpm requires more fuel.

Again, fuel consumption is related to engine rpm. Just because it feels easier (quicker) to get from point a to point b, the distance from point a to point b is the same, but you are burning more fuel to get there...

You're taking this way way beyond what i was talking about. I'm not retarded ya know, i know 4.10s don't equal better mpg. What i said was that on TAKE OFF my engine is under less load than a car with a 2.73 gear. It's basic physics man. Sure if i were shifting at 4k RPM all the time i'd get crap mpg, but cruising around town at 35mph, i can shift and run at much lower RPMs without lugging my engine than someone with 2.73s can, so around town there's no way i'm getting less mpg. I can shift at 2k and cruise at 1500RPM or less...and you said yourself that fuel consumption is related to engine RPM. Obviously on the highway or at 40+ speeds this won't be the case because i'll have to run higher RPM to get there, but around town in city streets, and especially on hills, you can't tell me i don't get better mpg.


Lower gear ratios require more torque to make the car move. Torque cost you mpg.

Higher gear ratios multiply torque and at lower speeds require less torque to get a certain speed. What i'm saying is that at a lower speed where you don't have to run high RPMs, i'm going to see better mpg. This is all strictly lower RPM that i'm referring to.

jtfairlane said:

Your logic is...not very logical.

If there is one thing we can all agree on, it's that the higher the engine rpm, the more fuel is burned, period.

With a higher *numerical* gear ratio over stock, such as your 4.10s, you are going to burn more fuel.

Understanding this is simple.

Again, fuel consumption is related to engine rpm. Just because it feels easier (quicker) to get from point a to point b, the distance from point a to point b is the same, but you are burning more fuel to get there...

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I almost spit out my morning coffee when I read this.

First of all, just because he has 4.10's does not mean he is running any higher rpm until you get to 5th gear and freeway speeds. Just because he has 4.10's, does not mean he shifts at 3000rpm all day instead of say 2000. It doesn't matter what gear ratio you have, you're still operating the engine in the same rpm range under normal driving conditions, you're just using a different gear. Cruising at 40mph, maybe 2.73's drive around in 4th, and maybe 4.10's drive around in 5th and are running the same rpm. The freeway is a different story because you've run all out of forward gears. Yes running too crazy of a gear like 4.10's can make mileage suffer, but not necessarily which brings me to the next point.

Fuel consumption sure is related to engine rpm, but not the way you think. Running a lower rpm does not mean you are getting better mileage. Engines are most efficient somewhere in the midrange and it totally depends on the camshaft profile. Weight of the vehicle and wind resistance play major parts in what rpm the engine needs to cruise as well. If you have a light vehicle, 1500 isn't so bad, but if your stang is 5000lbs, you better be running that engine at 2500 because it's under too much load at 1500. Under your "logic" driving around at 250rpm or less would be ideal. If that is too drastic, let's call it an even 750rpm, or idle. Trying installing a final drive ratio that will get you 750rpm at 65mph and tell me how good the gas mileage is.

I had an E-303 cam in my stang with the stock 2.73 gears with everything else stock. I was getting 22mpg freeway that way and 13 in town. I installed a 3.45 rearend from another car and my mileage increased on the freeway to 24mpg while my in town remained at 13. And that's just from a gear swap, because the rearends were the same. The E-303 couldn't handle the low 1650rpm that the 2.73's gave, it needed the 2100 my 3.45's give. Since then, I have the stock cam back in and a remanufactured stock carb and still get 24mpg.

In short, I agree with 85_SS_302_Coupe and I think you need to understand the logistics of internal combustion engines a little better before putting someone to the grindstone.

FINALLY someone gets it



I wish there was a way to test 0-40mph fuel consumption based on 4.10s vs. 2.73s. I may shift more often but my engine is under less stress at those speeds, therefor gets better mpg. 2.73s are HIGHWAY gears and suffer at lower speeds...exactly opposite of higher gear ratios which shine at low speed and suffer at higher speeds.

i just love debates like this

As NKau said, you probably will want to keep it in 4th under about 65 mph. With 2.73's I wouldnt want to be in 5th till about 70 mph. The rear-gearing is just too tall otherwise.

One needs to remember that the injector fires at one particular time during 4 strokes.

Two crank revolutions equal a complete 4-stroke cycle.

So the higher the rpm, which is revolutions per minute, the more times the injector is going to be electronically triggered.

The injector squirts fuel out

I believe there has to be a mix between lugging and over revving past the point of efficiency.

LiquidGT said:

I've been driving pretty conservative, especally with the crappy weather around here. If I punch it, its only at half throttle. It takes a fair amount of slip to get going in first gear and I have to wait to get to 2500rpm to go in 2nd or it bucks a little. I understand the 4.10 debate and the rpm sweet spot theory, I'm thinking the cam is out of its range for super low rpm driving, so I'm going to try driving in 3rd or 4th on the hwy (4th is only 1800 rpm) and shifting higher in town.

Is the hwy rpm for 3.55's 1900-2000'ish, because I'm going to start driving there.

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There is actually a formula out there that given your rear end ratio, tire diameter, and final drive ratio (.63 for 5th, 1.00 for 4th) will tell you what rpms you are actually turning at any given speed.

Where is the car driven most? In town? 55-60 on the highway? 70+ on the freeway? Even mix? I'd strongly consider getting anywhere between a 3.55-4.10 gear. I don't know why these cars were ever equipped with 2.73's.

My 2.73s around town net me HORRIBLE milage. (the rear needs to be overhauled)

The AOD + 2.73s puts me cruising in 3rd @ about 1000RPMs. Way out of the powerband, causing bucking and other nasty symptoms. A rear end swap or rebuild in definitely in the near future.

LiquidGT said:

My friend has a turbo coupe 8.8 with 4 lug disc and 3.55's collecting dust in his garage.

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Wave a few hundred bucks in front of him and get it People may tell you not to go with that low of a gear, but I loved them. They don't really feel any different than 3.73's, but you do notice that 1st gear is a little longer.