LTX_Alex

Good evening, I wanted to know if I swap out my 2:73 gears for 3:73, 3:42 or 4:10 gears what would be the pros and cons to each one. My car is a 96 Trans Am with A4 (I have the ability to use 1st gear) my top speed is 115 and I wanted to know how I can increase the top speed if anyway possible too.

Thank You!

ThoR294

iTrader: (6)

Remove governor. For best performance though, you want 3.73s. Higher top speed.... More power, more gears in your trans, taller tire, lower numerical rear. But that's silly....

hrcslam

3.73 get's my vote too. It tops out in 4th at 174mph (theoretical), which you'll never hit without the Salt Flats; or likely even with them. Good acceleration, but will also keep your revs down around 2500 at 75mph (255/50R16).

Also agreed, remove that speed limiter.

shbox

Your theoretical top speed will be greater with the gears you currently have (not factoring in wind resistance).

ahritchie

iTrader: (1)

Sell it and get a 6spd car.....160mph stock

96capricemgr

iTrader: (1)

With the overdrive trannies that have been in cars since the 80s gearing is generally not as big a factor in top speed as people assume.

Actually with the 2.73 in third gear your topspeed would be pretty close to the same as 3.73s in 4th gear. I suspect given wind resistance the 2.73s might actually let the engine rev just a little higher for a given speed and therefore allow a very slightly higher top speed than the 3.73s in 4th. On pure math the 3.73s in 4th has the advantage but at 160+mph wind becomes a big factor.

As said a top speed limiter setting in the pcm is what stops you at 115mph, car is still in third gear.

leadfoot4

iTrader: (1)

Agreed, and just an FYI.....If the LT1 powered F-bodies were ordered with the 245/50/ZR16 tires, the speed limiter wasn't present in the computer programming, rear gear notwithstanding.

I seem to recall a 'Car&Driver' magazine article from the day, road testing an LT-1 powered Z-28, and achieving a top end of 155 MPH.

OVA1

iTrader: (12)

When I first got my 96Z, A4, 3.42 rear... it was stock minus the obligatory bolt ons and it went straight to 140... eased up to 145 fairly well and if ya had enough room and a favorable wind and did NOT have the T-tops open up... it'd snuggle up to just over 150* (indicated).

*Warning: Bring a gas can, cause when she's up in that zone... the Dog WILL EAT! 140+ for ten miles... about 3/8ths of a tank, starting from 1/2 tank.

rjmarazzi

iTrader: (1)

This ^^^ If you got z rated tires there was not a limiter.

ahritchie

iTrader: (1)

http://www.motortrend.com/roadtests/...o/viewall.html

^155 Not even broken in, once it's loosened it up with 40K, it will pull all the way to 5700rpm in fifth....then again, that is with the proper trans. Driving an auto that fast is just plain dangerous IMHO, you don't have nearly as much control of the car at that speed as you do with a manual.

MasterTomos

iTrader: (15)

Cars that were limited to 115mph factory came with non-Z rated tires. Cars that were ordered with Z-rated tires had a 155mph speedometer which was actually inadequate as many people have broken past 160mph in stock fbodys.

3.73's would actually reduce your top speed vs 2.73's although while increasing acceleration, so I'm not sure what some of these folks are talking about saying that 3.73's would give you a higher top speed.

Like others have said, to increase your top speed get a mail order tune and get that governor removed from the computer and you'll easily get around 155mph top speed without any other mods assuming the car is running well. Also, make sure your tires are in proper working order before doing this

ThoR294

iTrader: (6)

im just saying that 3.73s are the best because you get way better acceleration. why top speed? unless you live on the salt flats...... lol

Shownomercy

iTrader: (1)

Because running out of gear at top end of 1320 is lame.

hrcslam

Because top speed is where horsepower and resistance match. Horsepower is (tq x rpm)/ 5252. And tq isn't linear on an internal combustion engine.

Increasing leverage (gearing numerically higher) increases actual torque at the wheels. So at the same speed, the higher gears will have more torque to the wheels everything else being equal.

Basically, there's a difference between theoretical top speed (where the max rpms, gearing, and tire size are calculated) and actual top speed (where to horsepower, weight, aero, trans slip, tire deformation, and gearing are factored). And Actual top speed is always lower than theoretical. Actual and theoretical at lower speeds are very close, especially on a manual transmission.

For Example:
My Camaro as it sits* has a theoretical top speed of 304mph in 6th (0.5:1 OD). But doesn't have near the power to actually go faster than about 150-155mph in 6th. But in 5th (0.74:1 OD) it's theoretical top speed is 205mph and it should be able to get to 175-180mph, maybe even a little more.

*6700 rpms, 3.42:1 rear ratio, 26.1" diameter tires
That would suck, but (theoretical)....

4L60e, 3.73's, and 245/50R16 (25.6" diameter)

@5850rpms
3rd-119.6
4th- 171.84

@PCM limits (7000 rpms)
3rd- 143.11
4th- 205.62

Shownomercy

iTrader: (1)

I was talking 1320 stuff, and not many will race in OD gear.

IDK if I would want to do WOT in OD on a salt lake either though

BALLSS

look at your tire rating. I suspect, as mentioned, you don't have "Z" rated tires so your PCM was calibrated at factory with a limiter to keep top speed within the "speed rating" of the installed tire.

If you do get PCM tuning to adjust that...make sure you have a tire rated to drive at the higher speeds

"lower" gear ratio than 2:73's will allow the car to accelerate faster but your RPM's will be higher at = speeds with lower gears

In your case changing gears will not make you get a higher top end speed. You need to re-program your PCM to allow it to go faster than 115....if changing gears to a lower ratio (3:42, 3:73, 4:10) your RPMs at the higher speeds will increase vs 2:73's

96capricemgr

iTrader: (1)

As has been sort of covered, those of us telling him 3.73s might actually increase top speed have some comprehension that with numerically lower gears the engine can be far enough below peak HP to not be able to fight the wind adequately. Gears like 3.42-3.73 actually get the engine higher into the powerband so it can push through the wind.

The 2.73s only work for a top speed run if he leaves it in third, putting it in 4th would lug the engine and slow the car down.

hrcslam

Yeah, that's why I posted the numbers for 3rd gear too.

That's on a 25.6" tire too. I can see with 4.10's, a TH400, and 27" slicks hitting 135 in the 1/4 at 7000rpms.

OD probably doesn't have the strength to handle that speed in that time, that's a lot of power. Unless the car was 1800lbs.....

If I'm hitting 117mph in the 1/4 with a 5850 stock redline, I'm one happy camper! That should be mid 11's.

BIGCAT7274

i would go 3.73 and remove the limiter.

my old 94 with a4 and 2.73 was out of steam at 146 in od. just not enough power to pull the gear split between 3rd and 4th.

HellTeeOne

I've heard of anywhere from 156 to 163 out of stock ungoverned M6 cars with factory gearing.
The fastest I ever saw out of mine with headers, CAI, full exhaust and stock gearing was 164, GPS verified. Car was making ~290rwhp at the time and the run was made on flat, level ground. That was the only time I made such an attempt with the GPS in the car.

I've driven an A4 3.23 car to an indicated 155, only mods were a CAI and a muffler.
Lengthy WOT runs in overdrive are generally very hard on stock A4 trannies, from what I understand.

As far as modded cars, who knows? I do know one of the magazines did a top speed test back in the 90s with a Lingenfelter 383 Firebird. IIRC those cars were rated at 440 flywheel and made around 380-ish rwhp. They got a verified 192 mph out of it with 3.42 gears.
I haven't done any upstairs runs since I did all the motor work on mine. I'm too old for that stuff now.