Will 3.73's kill my mileage?

buddyf87 · Jul 1, 2005

I've got a 94 Cobra thats virtually stock (CAI, headers and o/r h pipe on the way), and looking for a cost effective way to decrease times. Higher gearing seems to be the way to go, and I'm wondering what it's going to cost when it's all said and done (parts, installation, etc). Also, this is a daily driver, and the gas mileage is bad enough as it is. Any other ideas on mods would be appreciated too, as well as sites with good prices. Thanks.

Venom351R · Jul 1, 2005

Your mileage will be effected some but not enough to break the piggy bank. Having a good tune up will cancel out any effects of having a higher set of gears.

CManT1914 · Jul 1, 2005

Depends on where the majority of your driving is. In town, it should not be reduced, as you can simply use a higher gear (i.e. 4th instead of 3rd). On the highway, you're gonna be turning around 2300 rpms in 5th at 70, so it will take away 1 or 2 mpg if you do a lot of highway driving. Before, when I had 2.73's, I could get 25mpg on the highway. Now I get around 23, so it's not that big of a difference, and it's certainly not enough to warrant not getting them.

wytstang · Jul 1, 2005

Gears can be had cheap at www.gefracing.com. The install is what's $$ that usually ranges from 400-450 depending on your area. Under drive pulleys are cheap and give a good gain for the buck. No the chances of you having charging problems and or heating problems are slim to none.

Swarzkopf · Jul 1, 2005

Mustangs are not cars meant to get great gas mileage out of.

The only way to really increase performance and not effect gas mileage is nitrous.

Pokageek · Jul 1, 2005

I've seen this question before on other forums. The answer is for HWY maybe 1-2mpg difference max.

Many say they get BETTER gas mileage with the 3.73's because the car is in just the right RPM band for efficiency. I have hear up to 24mpg several times from several mustang owners. Guess it depends on the mods too but sounds high to me.

I can tell you right now with my stock eng, I can get 300miles on a tank thats just about bone dry too. Safer is 285 or so. 3.55's here. Anybody know the gas capacity off hand? That'l tell you what mpg I am getting.

edit = 15.4 gallon tank = 19.5MPG

My '90 averaged 15mpg. Looks like the '94 is more efficient.

mikeflys1 · Jul 1, 2005

15.4 gal iirc

cleanLX · Jul 1, 2005

with 3.73's and stock motor:
steady cruise at 70mph I got over 26mpg.
hauling ass between 80-100mph I got over 24mpg.
Babying it along at 55-60 I got 29.

Around town I got 19-20 plain driving, but could work in 10mpg if I felt like it.

Pokageek · Jul 1, 2005

"but could work in 10mpg if I felt like it."

LOL.

cleanLX · Jul 1, 2005

Pokageek said:
I've seen this question before on other forums. The answer is for HWY maybe 1-2mpg difference max.

Many say they get BETTER gas mileage with the 3.73's because the car is in just the right RPM band for efficiency. I have hear up to 24mpg several times from several mustang owners. Guess it depends on the mods too but sounds high to me.

I can tell you right now with my stock eng, I can get 300miles on a tank thats just about bone dry too. Safer is 285 or so. 3.55's here. Anybody know the gas capacity off hand? That'l tell you what mpg I am getting.

edit = 15.4 gallon tank = 19.5MPG

My '90 averaged 15mpg. Looks like the '94 is more efficient.

next time you fill up, you'll notice there is a counter for how much volume you purchase. this is in gallons(liters up north).
assuming you were totally empty is a poor means of calculation.

Pokageek · Jul 1, 2005

Thanks! I will do this!

dstanggt50 · Jul 1, 2005

I think it will take away maybe a couple of mpg, but it will be well worth it over your stock gears.

mackey · Jul 1, 2005

I actually got better fuel efficiency with the 3.73 gears over the stock 3.08's. One reason is in town it is easier to get the car moving.

grrrrrrr · Jul 1, 2005

I got better efficiency with the 3.73's too... and it's a **** ton more fun to drive... I got mine for 550... I've heard of people getting them for sub 400's, but I would expect to pay approx. 500 for everything when it's said and done... I was a little worried before about gas mileage especially because this is my daily driver and gas prices have sky-rocketed but I wasn't dissappointed, and I don't think you will be either... Do it, you'll be glad you did...

5spd GT · Jul 1, 2005

24mpg with 373's here (h/c/i)

I'm at 2400rpm at 70mph...

GRGT1994 · Jul 1, 2005

For all those saying they got better mileage, please confirm that you are measuring mileage with your speedo corrected. If you speedo is not corrected, neither is your odometer, and your mileage would be worse than you think.

Not that I'm calling you guys out, there's several very knowledgeable guys posting on point. But when I see claims of mid to upper 20s I start to wonder.

BTW, the poster never mentioned whether he has a 5 speed or AODE. If you have an auto, then 3.73 will definitely NOT degrade fuel economy significantly (and you might want to think about more gear).

And welcome to Stangnet.

buddyf87 · Jul 1, 2005

It is a 5 speed. And what will I have to do to adjust the speedometer / odometer with a 3.73 swap from the 3.08's? I wasn't aware that this was needed.

mackey · Jul 1, 2005

GRGT1994 said:
For all those saying they got better mileage, please confirm that you are measuring mileage with your speedo corrected. If you speedo is not corrected, neither is your odometer, and your mileage would be worse than you think.

Not that I'm calling you guys out, there's several very knowledgeable guys posting on point. But when I see claims of mid to upper 20s I start to wonder.

BTW, the poster never mentioned whether he has a 5 speed or AODE. If you have an auto, then 3.73 will definitely NOT degrade fuel economy significantly (and you might want to think about more gear).

And welcome to Stangnet.

Yes, I did have my speedo corrected and I get better efficiency with the gears in.

CManT1914 · Jul 1, 2005

buddyf87 said:
It is a 5 speed. And what will I have to do to adjust the speedometer / odometer with a 3.73 swap from the 3.08's? I wasn't aware that this was needed.

You'll need to get a new tranny gear, they only cost like $15 or so. And you can swap it out in 5 minutes.

http://secure.50resto.com/product1.cfm?SID=1&Product_ID=226&Category_ID=145

You'll want to get the 3.55 one, unfortunately they don't make a real gear for the 3.73's, so your speedo will still be a little bit off, but only by a couple mph at 70 (negated if you use bigger than stock tires).

fiveohh94gt · Jul 1, 2005

buddyf87 said:
It is a 5 speed. And what will I have to do to adjust the speedometer / odometer with a 3.73 swap from the 3.08's? I wasn't aware that this was needed.

to correct the speedo you have to change the gear on the speed sensor which is plugged into the side of the tranny, i cant remember how many tooth gear is needed with 3.73's off the top of my head though, 23 maybe?
EDIT- yea its 23 tooth, here's a chart

Will 3.73's kill my mileage?

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