SouthernRex

When I had my garbage LT1 Camaro Z28 (1995), it was off by roughly 8-12MPH, as tested multiple times with local speed radars and family drivers attaining a specified speed. I just figured it was the stupid Camaro, because I had about 80 other problems with that. (**** the LT1.)

When I got my awesome 1998 WS6 Trans Am with the LS1, I noticed my car is *exactly the same.* It's about 8-12MPH fast (I have to go 64MPH on dash to go 55MPH on the ground.)

Does this fix with a Frost Tune, and is it super common or something? Is it effecting my transmission's shift points because of being the wrong speed? The car still feels fast and shifts strong (for a stupid A4 with torque reduction) but it is a little annoying having to guess 1-2MPH in my conversion of speed around cops.

1 FMF

iTrader: (1)

my 2002 ss, which is stock, is accurate.
(versus a 2005 altima i had which i verified was ~4mph fast but only above 60 mph)

when you got your {not as awesome as an SS } trans am,
i'm assuming you bought it used? If so what has been done to the car?
More than likely a rear axle gear change was done and the engine computer was not updated to account for that. Or if the engine computer (PCM) has already been reprogrammed then that can itself be the problem, and it's very common for these cars to get modified then sold.

so if you send your engine computer (PCM) out to get reprogrammed then yes this "could" correct the speedometer inaccuracy but only if you know your current rear axle gear ratio and that's what was causing it.

there's a vehicle speed sensor (VSS) on the tail end of the transmission, which is where the PCM gets the speed signal from. It's possible, but not probable, this could be bad and replacing it is easy and not too expensive but would be a shot in the dark in my opinion.

RPM WS6

iTrader: (3)

No, this is not common at all in my experience. I've had four LS1 F-bodies, and none of them have been off by more than ~1mph or so (as verified by the many mobile radar devices in my region.)

As said above, you're either looking at a sensor/cluster/etc. issue, or a gear swap was done but not tuned; if the latter, you would have to figure out the current ratio to get a proper corrective tune done.

SouthernRex

Is there an easy way of figuring out what my gear ratio is?

fst100

iTrader: (5)

here's one method:
http://www.how-to-build-hotrods.com/gear-ratio.html

edit:
If you have a car with a posi or limited slip rear, jack up the car and get both rear tires off the ground. Now put your jackstands under the axle. Make a line on the pinion yoke and onto the rear end with chalk or a grease pencil. Do the same to the tire on the car, so you can see when the tire has rotated exactly one turn.

Now count the number of turns of the driveshaft that it takes (use your chalk marks) to make the rear tire turn one complete rotation. (Transmission needs to be in nuetral or out of gear.)

Posi: Jack both wheels off the ground. Turn one wheel 1 complete turn.

- Count the turns on the drive shaft.

Non-posi: Jack one wheel off the ground. Turn the wheel 2 complete turns.

- Count the turns on the drive shaft.

If it takes about 2 and three-quarters turns, it's around a 2.73 gear ratio. If it's a little more than 3, you have 3.08s. If it takes about 3 and a quarter turns of the driveshaft, you have 3.23 or 3.25 gears. Very close to 3 and three-quarter turns, 3.70 or 3.73 gears. This method won't work very well on a posi that's worn-out and won't spin both tires equally.

murphinator

iTrader: (2)

I have also had 4 f bodies and had a gps in 3 of them at some point , the only one that was not accurate I knew wouldn't be accurate as the rear tires were/are much taller than stock so it reads a couple mph low at 40 about 5 mph off by 60 etc... car has 295/45?/18 on it.

Does your car have stock height tires on it ? If yes than the gears or entire rear was likely swapped at some point like everyone is saying.

not unusual for any car to be off 1 or 2mph based on variations such as new tread vs. worn off tread.

SouthernRex

I'll try that method, thanks guys. Also, I would just be really shocked if they changed it because some middle-aged lady was the only other owner. But she did have work done to it paint wise, and it had a Borla catback on it... so I guess it's possible. (I bought it 35,000 miles.) I *THINK* I have stock tire size. I'll have to check. TiresPlus ordered my tires for me based on my car year/motor so I would imagine as a larger franchise they can't go out of stock size? I know they won't install other aftermarket stuff.

R6cowboy

iTrader: (3)

Check the gear ratio like mentioned above and also let us know what size tires are on the car.

Firebrian

I've owned 2 LT1's and 2 LS1's. Not aware of any inaccuracy in my speedometers. The last time I had to worry about speedo's was on my 1968-1969 Plymouth and Dodges. All of those were around 10% off on the speedo's with stock wheels and stock rear end differential ratio's, which was quite normal for that era.

SouthernRex

The tires are Firehawk 275/40R17 98W. Are those stock size? Also, what is the stock gear-ratio for A4? Final question: For the trick listed above, can I do it with one wheel jacked, or do I need both wheels off the ground? I don't know if the TransAm is "non-posi" or not. lol

RPM WS6

iTrader: (3)

Yes, 275/40/17 was the factory size for a WS6 car.

Being that you have a WS6 model, there was only one factory gear ratio offered for the A4 trans: 3.23 (RPO GU5.) GU5 (3.23) was a mandatory option any time that "WS6" and "MX0" (auto trans) were ordered together. The other factory ratio offered for an LS1 auto trans car was 2.73, but this was not available with a WS6 from the factory.

You have a factory limited slip rear end on your WS6 (as do all other LS1 F-bodies.) This is under RPO G80. As yours is a '98, you have an Auburn LSD unit (or at least you did originally), so you'd need to follow the instructions above for a "posi" rear (meaning jack both wheels off the ground.) Just an FYI...'99+ cars came with a Torsen which acts as an open rear if you try the above method.

gametech

iTrader: (1)

Just to add to this, an Auburn posi that old would likely be completely worn out by now, acting as an open differential.

Boxxx

My awesome LT1 6-speed reads only like 2-3% faster than what it's really doing so i.e. 70 mph on speedo is 68-69 mph on the GPS but it has 275/40-17 now whereas it was 235/55-16, which if the tire size was calibrated from the factory would account for the inaccuracy. Legally speedometers on new cars are allowed a +/- 2.5% leeway on accuracy so it's not enough to drive my OCD up the wall.

abbas461

iTrader: (55)

no its niot common !
u need to tune, the tuner can edit the tire size and rear end then u should be fine

RPM WS6

iTrader: (3)

Not necessarily, just depends on the mileage and life it has lived. The Auburn in my '98 still functions without any problems (verified as recently as October in a small, pre-winter-storage burnout.) Once you get to 80-100k or more it might have issues, especially if it's had a rough life and poor maintenance (lots of people never did that first 7500 mile recommended fluid change either, plus not being careful to stick to ONLY 80w90 conventional lube.)

gametech

iTrader: (1)

I understand your point, but if an 18 year old car has less than 100k miles on it, it should be considered art more than vehicle.

RPM WS6

iTrader: (3)

I don't disagree, but I do occasionally take my "art" out for a drive.

Firebrian

My '99 SS came with an Auburn via SLP and is certainly not shot at 18K miles. I've taken the same precautions as RPM WS6. I did the first fluid exchange at 12K miles in 2012. Considering I don't beat on the car or race it, I suspect the Auburn's clutches will outlast me. The OP bought their car with 35K miles. Their Auburn is probably still fine too.

Boxxx

^Ditto. Even though mine is closing in on 120K the first and second owners before were meticulous and OCD to the book on maintenance and I suspect also like myself they never abused it, the LSD still works great as it's supposed to. That and it's had only conventional gear oil with the Delco additive like RPM WS6 mentioned, which I've read is important on older differentials as there's something about the synthetics that's not nice to the clutch material if I recall correctly.