Gears and gas

Chris-86GT5.0 · May 22, 2008

I have my baby... 86GT Vert in completely stock condition.

I also have another 86GT Vert that is modded, but the engine needs rebuilding.

I have a Turbo Coupe rear disc rear end with 3.73s that I thought about throwing onto my stock baby but now I am not so sure.

It currently has 2.73s in it which means I get about 23 MPG on the freeway.

I know if I put those gears in I'll be getting considerably less. I can cruise at 65mph just above an idle. With gas prices only going up I can't see doing the swap.

Quickness... or MPG... that is the question.

93 LX · May 22, 2008

just keep it stock and if you want a bit more pep go with 327s instead..

LiquidGT · May 22, 2008

I know what you mean, I have 2:73's, 5-speed and I'm getting 19-21 mpg with mixed driving. I too have a T/C rear with 3:55's waiting in the wings, and 3:27's have hovered over my mind every now and then, but I'm gonna put the T/C rear in as is.

Pros: quicker acceleration and braking, plus its easier to maintenance disk brakes.
Cons: slightly worse gas mileage.

Just get a 4 cyl beater to drive around in, because when gas prices go up more 23mpg hwy aint going to be chit.

1987stangman · May 23, 2008

I just have a stock 8.8 but 2 months ago I installed 3:55's over the stock 2:73's and as long as I keep it in 5th on the highway when driving to work everyday gas is so far not affected much. I driver 28 miles ONE WAY 5 days a week so gas is just something I have to deal with now : (

lang williams · May 23, 2008

When I had my aod DD i got better city mileage with 327's over the 273's. high way wasn't much different an the car was much more enjoyable.

90mustangGT · May 23, 2008

What suprised me was when I swapped 2.73 to 3.73 was that gas milege in the city improved slightly, and on the highway it remained about the same, but the difference in getup and go was much better.

93 LX · May 24, 2008

90mustangGT said:
What suprised me was when I swapped 2.73 to 3.73 was that gas milege in the city improved slightly, and on the highway it remained about the same, but the difference in getup and go was much better.

I will say when I put 3.73s in my Capri it was like 50 more HPs at the line. The car really scoots but the difference in city driving is now I can cruise in 5th gear so yes, city milage is much better.

Not sure how highway milage is the same as quoted above. I used to cruise at 65 at 2000 rpm, now at 65 I am at 2400. I now drive closer to 55 where rpms are closer to 2000 in 5th. this is where a 6 speed would be great! LOL

BTW stock gears were 3.08..... so your RPMS at speed may be different.

Chris-86GT5.0 · Jun 17, 2008

93 LX said:
I will say when I put 3.73s in my Capri it was like 50 more HPs at the line. The car really scoots but the difference in city driving is now I can cruise in 5th gear so yes, city milage is much better.

Not sure how highway milage is the same as quoted above. I used to cruise at 65 at 2000 rpm, now at 65 I am at 2400. I now drive closer to 55 where rpms are closer to 2000 in 5th. this is where a 6 speed would be great! LOL

BTW stock gears were 3.08..... so your RPMS at speed may be different.

Yeah... in this thing I can cruise at 70mph and be less than 2000 rpm. It REALLY helps for highway mileage. I get about 25 on the freeway driving 65.

Fast63 · Jun 17, 2008

I'm going to have to disagree with 2.73's being the best gear for mileage. In my 85, I switched from 2.73's to 3.45's and gained 2mpg on the freeway from 22 to 24 on average. I once got 27mpg with my 3.45's. If you did happen to lose any mpg's with 3.73's it would not be much, like 1 or 2mpg's at the very most. 2.73's are actually bogging the engine at freeway speeds. Contrary to popular belief, lower rpm doesn't necessarily mean higher gas mileage. It's all about turning the right rpm of engine efficiency.

Darkwriter77 · Jun 17, 2008

FWIW, I've noticed that a lot of heavier cars like Crown Vics and such will actually GAIN city mileage from going up in gears. If your car is geared too low from the factory, it will have to work that much harder to pull its own weight from a stop - think of trying to pedal a 10-speed bicycle in 5th gear from a dead stop, versus taking off in 1st gear.

Once you're moving down the road, the 2.73's MAY have an advantage over 3.73's as far as mileage ... or, then again, maybe not. If you live in an area with hilly or mountainous terrain, higher gears might make it easier for the motor to pull you along at the same speed. It's not just the cruising RPM that matters, but also how hard the motor actually has to work (and, thus, how much fuel it uses) that determine its efficiency.

85gt79fj40 · Jun 18, 2008

I've been driving my 85 pretty much daily 90 miles a day for the last 2 months. with a stock motor w/headers and 2-1/2 inch exhaust no cats, an edelbrock performer 289 intake 600cfm holley vac sec carb and 3.55's i'm getting a very consistent 22mpg. The same I got with my bone stock 86 with 2.73's about 15 years ago. The idea of a more efficient commuter is on my mind all the time, but so far the cost of a decent one would take a few years in gas savings to pay for itself. And it would be boring. Even if I had a corrolla or something I probably wouldn't drive it.

TRWXXA · Jun 18, 2008

85gt79fj40 said:
The idea of a more efficient commuter is on my mind all the time, but so far the cost of a decent one would take a few years in gas savings to pay for itself.

Even if you were giving up a 15mpg truck for a $12K econo-box, it would take about 7 years for you to recover the cost of the new car, in gas savings alone.

In general, you can expect to see some better city mileage if you go up in gearing. But you can expect to lose just under 1 mpg in highway mileage for each step up in gearing you go. A lot of the "improved" highway mileage you hear about can usually be attributed to poorly calibrated speedometers/odometers.

As it has already been mentioned, 3.27 gears are a good compromise. The engine won't feel like it's lugging as much at low speeds, and you won't lose more than 1 or 2 mpgs.

BStevens · Jun 19, 2008

i agree with TRWXXA...

Yes, you are going to lose gas mileage... But, I'd prefer the power to be there. Thats why I went with 4.10's. Everyone has a case by case situation. You need to justify your own.

Again, Power or Economy. It is a Mustang after all

blackstangt · Jun 19, 2008

Gearing is going to affect gas mileage differently for everyone. First off, you can probably save some gas by driving easier and slower, leaving 10 minutes earlier and following an suv or minivan in the right lane for a little less drag on the highway. Peak gas mileage will always occur at minimum throttle and (almost always) in your highest gear. Switching from 2.73's to 3.73's might give you better gas mileage, unless you drive over 80 mph everywhere... 3.73's will help your city mileage a little and allow you to get into fifth gear on roads you previously could not without speeding drastically.

I have 3.73's and I probably get my best gas milage around 50-55 mph. Draft a large truck, 60-65 mph. If you're not cruising much faster, you should be able to get better gas mileage from the 3.73's.

I'd do the swap unless you have plans for the rear in the other stang...

Gears and gas

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lang williams

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