How much mileage do you think I'd lose? I'm already in the low 16s with Bamachips' 93 race, i don't know if i want to go any lower.

local dealer quoted me $600 installed for FRPP 3.73s, is that about right?

 

Sounds about right , I think I paid about $200 for the gears and $400 for the install at Steeda ...

 

I went from 3.55's to 4.10's. My hwy mpg droped by from 23 to 22. Around town I don't know because I can't keep my foot out of it.

 

Why 3.73's??

 

According to my calculator, using the stock tire on the Mustang GT, the engine RPMs at 70 mph are 2865 with 3.31 gears, 3229 with 3.73 gears, and 3549 with 4.10 gears.


So if you do a lot of highway driving, you have to balance the extra performance against the higher RPMs and lower fuel economy. As they say, pick your poison.

 

it came with 3.31s, and i felt that i would be able to tell the difference with 3.73s and not lose as much mpg as i would with 4.10s

the thing is, almost all my driving is rush hour driving, so i never really get the benefit of highway driving as far as my mpg is concerned.

 

With an automatic, stock tires, and 4.10 gears my engine rpms at 70 are 2500 and at 80 I'm at 3000. The only thing I would say is that with 4.10 gears, o/r h and stingers highway driving can be bummer when in the drone range.

 

I had 3.73s installed in my wife's 2007 GT Automatic with a Bamachips 93 perf tune and still get around 21-22 mpg on the highway. I decided on 3.73s because she drives more than 30 miles to work every day. The difference in performance and drivability is noticable but not as what I was expecting.

Kirk

 

thanks for the feedback kirk. hmmmm....well i have a manual but now i'm thinking the money would be better spent on LCAs or maybe sway bars to upgrade the handling.

 

Do 4.10s. The difference in mileage is very small compared to 3.73s you can still average 16 or so overall. I get low 20s on the interstate at 70 mph. If you do alot of stop and go driving you won't notice the difference in mileage; and 4.10s will allow you to take off in 2nd gear if you choose so there'd be one less shift to make during stop & go.

 

I would only stay away from 4.10s if you drive 50+miles daily on the interstate. I have a 5-spd w/ 4.10s and at 70, I am about 2600rpm.

I average 19.5 mpg in town
On the highway at 70-80mph, I generally average 20.5-21 mpg.

 

I have 4.10's in my 07 GT automatic, and I get 23 mpg on the highway (just like the EPA rating on the window sticker says it should for a stock GT with 3.31's). I had the 4.10's installed when the car was only 2 weeks old, so I can't attest for mileage with the 3.31's. However, I went from 3.55's to 4.30's in my 03 Mach 1 after about a year of ownership and my highway mileage stayed exactly the same. My 07 has the factory 18" wheels/tires and turns 2,800 rpm's at 80mph and just 2,100 rpm's at 60 mph.

Around town and at the track you'll notice a big difference with 4.10's (even 3.73's would be noticable). Much better acceleration, but also more fuel consumption (unlike the overdrive and easy highway drives). My city mileage was about 15-16 mpg with the 4.10's in my 07 GT (window sticker said 17 for a stock GT with 3.31's), so not much of a drop in mileage.

 

Since I posted the results of the RPM calculator, it occurred to me that it's an older piece of software I picked up years ago. I doubt it takes into consideration overdrive transmissions.

 

I had the 3.73's installed in mine. (automatic) and there is no loss in mileage around town. The car gets up to speed easier and quicker. in mixed driving I get 22 mpg in ethanol season and 24 mpg without ethanol. (Arizona) I paid just under $200 for the gears and $350 for the dealer to install them. I am glad I did it.

 

I plan on getting some 3.73's in the spring for my auto. I just hope whine isn't an issue after the install.

 

Go with FMS gears, yes it makes a big difference.