Jack Straw

I've got the stock rear end gears in my '06 GT and it feels like way too tall a ratio. I do like the long-legged highway cruising aspect but as many of you know sometimes it seems to take forever to get on the boil in 2nd, 3rd, and 4th.

I'm running a 27.4" rear tire diameter and being math challenged I'd like to know about how many RPM's I'll be turning in 5th at 75 mph with either the 3.73 or the 4.10 gearset?

This car is almost a daily driver but will not often be used on long interstate hauls. Engine is stock but for a Steeda CAI and tune. Cams are likely over the summer and maybe long tube headers as well.

I appreciate any input from you guys who have changed the ratio.

Jack Straw

OK, I got off my butt and did a search. Yep, there's plenty of info pertaining to my question.

I'd still like to hear from y'all about your experiences with the lower rear end ratios. Like 'em, hate 'em??

berzerk_1980

I have 3.73s in my 2014. They are fun around town but I run out of 1st at like 10 mph which means I'm high revving or shifting in the intersection or everyday right and left turns.

The revs are a little high at highway speed (approximately 2500) but not really bothersome with the right exhaust. If I had 4.10s I think I'd hate driving on the highway.

07 Boss

Don't fear the gear. Kinda wish I went with 4.30's instead of 4.10's but that's me. These motors like the higher RPM's anyways. I think I cruise at about 2700 rpm's at 75. On my way out to Cali a few years back I hummed along at 2900-3000 for hours. It's not going to hurt anything.

akdoggie

I agree. 3:73's on the highway. I had 331's from the factory and overall weren't bad. I found a 2012 GT rear end with only 2:73's and bought it. I immediately went to new 373 gears and installed them before putting it under the car. My rpms' went from 1950 to about 2300 rpms on the highway. I found the torque feel was considerably more than I would of thought. It was the best combination I could come up with. I thought about 410's or even 390's, but went with 373. I'm very happy with the outcome and ease of this install.

karman

3.55 gears are pretty good for the car if you have a tune to eliminate low end factory lag.
Do you have a tune on the car?

Right now I have a stock car and a tuned car each with 3.55s and manual tranny.
You can feel the difference at lower RPMs, the tuned car has more pep especially in the low end.


Having run both 3.73s and the 3.55s in 2006 GT cars...
I would say there is not enough difference going from 3.55 to 3.73 to justify it.
My car with 3.73 was an auto which came with 3.31s, that made the 3.73 a big enough change to make a difference.

Jack Straw

Hi karman,

Thanks for the insight on final drive ratios. At the moment my engine is stock but for a Steeda CAI and an off road x pipe. I just ordered a Steeda 87 and 91 octane tune for my present set up. Cams (FRPP Hot Rods) are a couple of weeks away and long tube headers to follow shortly after that.

I suspect you are right in saying the difference between 3.55 and 3.73 may not be great enough to justify the cost. As my '06 is mainly a fun car for the frequent spirited run on some twisty roads, mostly third and fourth gear, I'm leaning toward the 4.10's.

karman

Wait to change gears until after you have installed the other stuff.
Also, most people recommend re-tuning after installing things like Cams and Long tubes.
I don't think Steeda lifetime tunes include modified Cams. This probably requires a Dyno tune to get it right.
You may want to go with the Long tubes first and find a good local Dyno tuning shop.

Jack Straw

On the Steeda tune request page there is a question specific to cams and in the drop down the Ford Hot Rod set is one of the choices so I expect they can get in the ball park after I re-cam but indeed, a good dyno shop is indispensable as the mods get more substantial.

M90CaliforniaSpecial

Go with 3:73 not 4:10! Had 4:10's on my Bullett and it was mistake... 1st gear is pointless, gas mileage is garbage, freeway is high rpms but an absolute monster off the line. I just bought 3:73's and having them installed this weekend. Ifeel it'll be a good compromise from 3:31's that even my supercharger couldn't move fast enough for my liking.

Jack Straw

Thanks guys for all the input. I'm goin' with 4.10's. They'll be installed Monday!!!

m05fastbackGT

I would only recommend upgrading to 4:10 gears on an auto trans that's N/A..

For a supercharged application, I wouldn't go any lower than 3:73 - 3:55..

I totally agree with M90CaliforniaSpecial's recommendation and know from personal experience that he's spot on about the disadvantages of using 4:10 gears especially when running FI..

Jack Straw

I'll let y'all know if i like the 4.10's next week. If I don't like 'em I'll just change back. Not a big deal to me at all.

M90CaliforniaSpecial

You'll be grinning ear to ear the first time it throws you back in your seat thats for sure!

07 Boss

My personal experience tells me that I should have gone 4.30 with my Supercharged automatic. Just saying there is no blanket answer for FI or NA cars. It's personal preference and driver skill set/car set up. Traction is not usually a gearing issue but a tire and suspension issue. If I could cross the line at 6500 in 4th, that would put me right around 128 mph. Need more gear and more power but it will get there one day.

Jack Straw

07 Boss brings up a good point........personal preference.

After many years of being a BMW nut one of the main things I've noticed in the Mustang scene is the emphasis on 1/4 mile performance. Sure, most of the 'Stang set appreciate handling mods and good brakes but the main thrust of most of the forums seems to be toward drag racing. I'm not making a judgement here, just an observation.

My personal interest has always been in twisty roads, auto-crossing, and, when I was younger, track days. SCCA road racing was unfortunately beyond my budget.

My goal in changing the final drive ratio is merely to allow the engine to stay in it's power band for the kind of driving I most enjoy which tends to be high second gear through third, with an occasional peak at fourth gear which seems to be the nature of the back roads here in central Arizona (at least the ones that are paved!!!).

m05fastbackGT

You're absolutely spot on.. As it's a personal preference and for track use, I agree that you need a lower gear ratio to get off the line faster, as there's no question about that..
I also fully agree about traction being a suspension issue and not a gearing issue..

However when it comes to those who use their cars strictly for street performance as daily drivers, I totally agree with M90 California Special's post in which a lower gear ratio such as a 4:10/4:30, your always going to be running at higher RPM's on the highway at speeds above 60MPH and your fuel mileage is going suck miserably as a direct result..

I'm actually considering taking out my 4:10's and putting the stock 3:55 gears back in for those reasons

07 Boss

You are right about fuel mileage. I think I average 11.3 according to the trip computer. RPM's are not that bad though. Only a couple hundred (total guess) over what the 373's would run on the freeway. I run the rpm's up on my car anyways and rarely see below 2500 no matter where I'm at.

M90CaliforniaSpecial

Jack - It is total preference, my opinion is strictly based on everyday driving which would include fuel mileage and obviously occasionally freeway time. Are you going from 3:31's or 3:55's? Either way man your going to be very happy, the 4:10's on a slightly modified stang will be a huge improvment but when you start dumping a bunch of money on performance and no your longer N/A I would definitely say no go on the 4:10's (no offense 05fastback).

M90CaliforniaSpecial

Jeez 07Boss, didn't realize you were 4:10s as well. I can't imagine the torque on a blown 4:10 car!