ieatmaro

is that even possible? i thought gears only worked for manual cars. and if not what are the advantages and disadvantages of this? i know it makes ur car pull harder in the lower rpms but how much top speed will i lose? sorry if this is a really stupid question but in all the car knowledge i have i never looked into gears cuz i was getting an auto. please dont tease about the auto. i live in ny and its a daily driver and im in rush hour traffic all the time so i didnt wanna end up hating my car.

Ducky

Of course it's possible. Rear end gears have nothing to do with the transmission. Just like a manual, you will accelerate faster in a given gear, but you won't go as fast. I don't think you have to worry about top end speed though since even with 3.73's you could go well over 150. I have 3.73's in my manual, and I tach about 2k RPM right at 70 mph, so it's not bad. I'm sure your auto will be close to that. Auto owners tend to go higher to 4.10's however to compensate for the extra drivetrain loss, but that is your call.

OAC_Sparky

Assuming you have the standard US-Spec 5.0 Auto with a 3.15:1 axle when you go to a 3.73 you're changing the ratio by 15.5%

So whatever RPM you're running at a given speed multiply it by 1.15 and that will give you an idea of what you'd run with a new gear.

What you will find is that you will get up to speed quicker -- you will lose top end but unless you're driving like an asshat way over 140mph you'll never see it.

Where it will bite you is in your gas mileage.

Also bear in mind that if you change the gears you will have to budget for the cost of getting your speedo recalibrated to match.

ieatmaro

yeah i figured as much. i really need to find a reputable place that can install all of this for me and find out exactly how much it will all cost. i still dont even have the car yet since its a 2012 GT so i have plenty of time to figure it all out. is the mileage going to be effected significantly? or is it like 1 or 2 mpg?

OAC_Sparky

I would wager closer to 2-4 depending how you drive. I'm guessing that's the main reason that the gearing is so high in the Autos because their EPA tests are different, and the ratio affects the mileage which affects the CAFE numbers.

That's why the manual drivers got a choice of lower gears but got stuck with skip-shift.

Ronin38

You might want to check here for some more info. on gearing before you just stick something in there "because"...

Start with Post #43:
https://themustangsource.com/f800/wh...matter-486829/

The gear ratio's are different in the auto. compared to the manual, too, so it sounds like 3.73's would be way too tall for the auto...

Has anyone changed the rear gears with an auto. yet?

RedCandy5.0

Depends on what size tire you are running at the track.
Here is what Justin from VMP says about gearing

Our car goes through the traps at 7050RPM with a 28" M&H (275/50/15) and 3.55s, 138 trap speed. If we were running a shorter tire (26 or 27) then stock 3.15s would be fine. I don't see a lot of point to gearing the hell out of the autos, power adder or NA.

jsnyng

I have 3.73's in my auto. I run 70MPH @ 2350, 75 @ 2500, and 80 @ 2650. It's the best bang for the buck mod on the automatic and keeps you in the power longer and helps get you through the gears faster. If my car was a drag-only, then I'd do 4.10's but I'm glad I've got 3.73's for the highway driving I do. I lost 1MPG on the highway and gained 2MPG in the city so it was primarily a wash on the mileage.

ieatmaro

Sounds promising. How much was installation if u dont mind me asking?

Everett

There's your answer.

jsnyng

I got mine installed for $150, but that was the "friend" price. There are several very reputable shops in/near Atlanta that quoted me between $200-$250 for the install.

ieatmaro

ima ask my dealership how much itll cost. i can guarantee you they will say atleast 500. they wanted to charge me 300 for an axleback exhaust....

jsnyng

What part of the country are you located in? I'm sure we could find you somebody reputable to do it.

ieatmaro

queens, new york.

jsnyng

I hear these guys - http://www.realspeedautomotive.com/ - do really good work. I don't know their prices, but they have a good reputation for gear work. According to Google Maps, it's about a 40 mile drive for you.

Everett

Install if you supply parts should be around 300 tops.

OAC_Sparky

How much did they charge to recalibrate the PCM for your speedometer? Or did you tune it yourself?