Hey guys, I'm sure this is probably a stupid question, but if I had a stock 05-09 GT and wanted to put in 4.10 gears, how much would that lower the top speed...if at all? I'm not really worried since I'm not going to be racing it or driving 140 mph on the highways, just curious. And how much would that increase the 0-60 time by? Also, would I need any other mods if I wanted to do that? I read somewhere that if you change the gears like that, the speedometer doesn't give the correct reading? Thanks in advance!

 

I don't know how much top end you would lose but you would lose some. You will need a tuner to correct the speedo after a change of gears

 

You'd have to do some math with the gear ratios and tire size and what not, but potentially you could lose some top speed. However, you'd get to that top speed quicker. aggressive gears like that are going to affect your cruising rpms. If you do a lot of freeway daily driving that's something to keep in mind because your fuel economy is going to be affected.

Edit: I did the math, assuming your car is a 5 speed manual, with 4.10 gears 5th revs out at over 180 mph. So, you won't loose any top speed. Your car will loose it's battle with the air resistance long before it reached that point.

 

I do pretty even amounts of city and highway driving. I was looking at the FRPP 4.10 gears on American Muscle and most of the reviews said the gas mileage didn't change much at all, and some even said it improved by a couple MPG?

And just to give me a rough idea, how much would it cost for the gears to be installed and the speedometer to be tuned by a shop?

 

Well, based on my math, stock, at 65 MPH the engine turns approx. 1900 RPM in 5th gear. With 4.10s, the engine turns approx 2200 RPM in 5th at 65. Not a huge difference, I admit, and we didn't buy mustang GT's for the gas mileage anyway

 

It won't improve your mpg's at all that's for sure. It will knock it down, but not by a ton IF you keep your foot out of it. I saw only about average of 1 mpg less when I did 4:10's when I measured the gas MPG. But considering I normally get about 11 mpg now, I didn't care much. lol

The car will wind up MUCH quicker then before, esp first three gears.

 

I thought I was bad with my 13-15mpg

 

11 mpg? That's insane, haha. So I'm guessing if the car is just totally stock other than the 4.10 gears, it would average around 18-20 mpg?

 

That's about what I get, mixed driving, manual trans, 4.10's, heavy 20" wheels & bigger Brembo brakes, CAI, offroad Xpipe and a 93 tune (300rwhp/311rwtq).

The rpm with the exhaust, does get a little annoying at higher cruising speeds on highway for longer than say an hour.

 

It will NOT affect your top speed. Our cars come with a speed limiter of 145 mph. So unless you disable the speed limiter with a tune that turns it off your top speed is 145 no matter what gear ratio. Now if you turn off the speed limiter in your tune, your top speed will be whatever speed you are going when you redline in 5th gear. HOWEVER, our cars don't make enough power to redline in 5th gear so it's a moot point. You'll never hit top speed anyways, even on a 1/4 mile dragstrip your nowhere near hitting top speed.

 

Gotcha. Thanks guys!

 

Exactly!

Think of it this way -- the only way that changing the rear end ratio would affect your top speed is if you are hitting redline in 5th right now. Are you doing that?

(hint: probably not . . . 6000 RPM in 5th with the 3.31 rear would be over 200 MPH)

Assuming you have the standard 18" wheels/tires which are about 27.25" overall diameter, 6000 RPM in fifth with 4.10 rear ratio is 175 MPH

On the highway, the 4.10 rear WILL reduce gas mileage for sure. RPM is directly proportional to rear end ratio (at the same speed) and fuel consumption is roughly proportional to RPM . . . so when you go from 3.31 to 4.10 rear you are increasing RPM by 4.10/3.31 = 24.9% (in the same gear at the same speed) so you are also going to increase fuel consumption by about 25%. (a little less in the real world due to other things like wind resistance) Around town the affect is smaller because you are going up and down through the gears so you are not just cruising along at 25% higher RPM.

 

I did the 4.10's some time ago. First gear is pretty useless around town. I think the '05 was a 3.73 stock, so gas mileage down a tad, nothing serious.

 

05-09 GT 5MT
05 & 06 stock rear gears were 3.55's
07 and up standard was 3.31's with an option for 3.55's

 

No....... Thats the annoy all your neighbors gear

 

I love the 4.10's in my car. I still average around 19 mpg on the highway and the response from the car is great.

 

but back to the original question.... I don't think top end was reduced. It might even have increased a bit. Hard to tell as the speedo isn't all that accurate over 100 anyways, and with a re-calibration, who knows.... However, I would suspect that the best top end would be with a gear that allows you to just barely reach red-line in 5th gear. Maybe a 4.3, or even lower. As an example, many corvettes reach top speed in 5th gear rather than 6th.