jayblev95

I just picked up a 1995 z28 and it seems in good mechanical order. But I noticed a couple of things. First, the guy I got it from took it in trade and said the previous owner installed a shift kit so it shifts very firm. It also seems to take a while to shift. Even part throttle it seems to only shift around the 2500-2700 rpm range. I'm not sure if he did any tuning or not but would like at least the shift points back toward stock since it seems excessive. Dont know if that can be done. Also I noticed the speedometer is off by about 5 mph. For instance if I do around 54-55 the guy following me said I was doing 60. And when I'm at 60 or a reading of 55 the rpms are around 2000-2100. Didnt know if this is normal. Im just not aure what was done to it and dont know where to start. Any help would be greatly appreciated. Thanks.

Not Dave

I'm going to guess the stock gears were replaced with something a little shorter so the speedo is off. Shift points on an auto (being it's controlled by the computer) were probably jacked with by a tuner of some sort. Both the shift points and the speedo error can be adjusted through someone that tunes computers. I'm sure someone here can tell you what the gear ratio is just by the RPM at certain speeds. If you have stock size tires that helps too but if they're undersized it'll show a higher than actual speed. You can download many apps on a smart phone that use GPS to tell you your actual speed, I have one on mine and it confirmed my car hadn't been jacked with. My car is an M6 so my gearing is completely different than an A4 car so can't tell you what it should be, though you can probably do a search and find the info somewhere (if Shoebox doesn't have it already). Cars with auto trans came with two different rear end gear ratios, 2.73 and 3.23. The RPO code on the door will tell you which one it came with.

jayblev95

Thanks. Does anyone know or have a tuner close to east Tennessee? I found a dyno shop but I have to have a hp tuner for them to use. Or can anyone do tjis without the dyno.

13qtr

95 is ob1.....can't use hp tuners. you'll need tuner cats or Lt1 edit. Don't need a dyno .

Injuneer

If someone installed a shift kit in the trans, playing with the tune isn't necessarily going to fix the problem with the trans. You have to find out what has been done with the trans internals first.

Once you figure out what has been done to the trans, and the rear axle ratio, you could have the PCM tuned by a mail-order tuner. Example:

Home | PCM for Less

With the later models, the Service Parts Identification Label is often in the glove box.

If you have standard size tires (either 235/55-16 = 26.1" or 245/50-16 = 25.7") you will see roughly this RPM for various rear axle ratios, at a true 60 MPH assuming you are in O/D with a locked converter:

2.73 = 1480
3.23 = 1760
3.43 = 1860
3.73 = 2030
4.11 = 2230

Problem is, if it still had the PCM programmed for the original gears (2.73 = RPO GU2 or 3.23 = GU5), the speedometer error would be greater than 5 MPH at a true 60 MPH, and the indicated speed would be higher than the true speed with higher numerical gears. Note also that the error due to gear or tire differences will be a percentage, not a fixed value.

jayblev95

It has the 17inch camaro ss wheels so I'm sure that will make a difference. And the shift firmness I dont mind cause its better on trans internals but just it waiting to shift under light throttle until around 2500-2700 rpm is a little much. So if I can just figure that out I will be happy.

Injuneer

No. Wheel diameter will not affect anything. Only the tire outside diameter affects the speedometer, and if the tires were selected correctly, they will match the original tire diameter. For example, if your car came with the stock 16x8.0" wheels with 245/50-16 tires, the outside diameter of the tire was 25.7". The correct setup on a 17x9.5" wheel would be a 275/40-17, which has a 25.7" outside diameter. I run 17x11.0" wheels with a 315/35-17 tire, and it also has a 25.7" diameter.

When you increase the wheel diameter, you decrease the "aspect ratio" of the tire to keep the tire outside diameter the same.

jayblev95

Yeah I just checked and its got 275/40-17s on it. The code in the glove compartment says GU2. So it looks like it came from the factory with a 2.73 gear. So it seems someone definitely changed the rear end gears.Im still trying to track down the previous owner to see what was changed or done to it

jayblev95

So my next question is if I keep the gears, its seems like they are 3.73, what would that do to the driverain s reliability. Would stuff start wearing out faster or will it do fine. It may sound like a dumb question but I just don't want to hurt anything.

Injuneer

What happens is you run with the trans in one gear higher, and the engine RPM stays the same. The only time that doesn't work is when you are cruising in O/D. Then the engine is turning 37% higher RPM, so there might be a small increase in wear, and a loss of MPG. That's the tradeoff for the performance increase of a rear axle that makes it feel like you added 37% more rear wheel torque.

jayblev95

Since I dont plan on taking this car on long trips I'd say I have my answer then. Im definitely keeping the gears and will get a tune to correct my speedo and shift points. Thanks for all your help guys. Now I'm in the garage to start replacing window motors. Fun fun.

Injuneer

Did you look at Shoebox's guide to replacing the window motors?

jayblev95

I did and it came in very handy. It was a very easy process. It seemed to me the hardest process was getting the door panel back on properly. The motor replacement was very easy and only took me about 15 minutes or so. I have a working driver side window now, woo hoooo!.