Hello all,

I did some searching on the threads but didn't find the answer I was looking for, so I decided to post this question. (Apologies if I just missed the information in the current threads.)

I have a 1996 with the automatic transmission, LT1 engine. The previous owner had put a set of Z06 replicas but used an 18" rim instead of the 17" size. He did, however, stay with the same tire size specified on the placard with the exception of the rims. Now I have 255/45R18's on the front and 285/40R18's on the back. I find that the speedometer is about 2 MPH off; I show 60 on the speedo and 62 on my GPS.

Questions:

Is there any way to re-calibrate the speedometer?

The "Service LTPWS" light will not go away on the panel. Is this an issue with the rim/tire size or bad/missing sensors in the wheels? (I had the dealer reset the indicators on the warning panel but they reappear consistently.)

How do I check the offset of these rims and know whether or not I need spacers? I've noticed some rub marks on the top of the front wheel wells with the hood up.

Again, apologies if I just missed this info elsewhere and if you have anything else to add that may be off-topic but useful for a first-time Corvette owner, please feel free.

Thanks,
Dave P.

 

I replaced the differential from 2.59 to 3.07 gears and have the Z06 wheels also, got me a Hypertech Tuner, allows you to adjust the speedo, change the temp that the cooling fans come on, adjust shift points and rev limiter. Worked very well on my 95.

 

The '96 PCM is different than the '95's. Corvette went to OBD-II in '96.

Dave, you should post this question in the C4 Tuner section.

 

Speedometer/odometer can be calibrated in the PCM BUT for the error that you mention it may not be money well spent. You need to do speedometer checks like this at your convenience. Using the GPS drive at 45, 55, 60, 70, & 75 MPH and record what the speedometer reflects to arrive at a true "percentage" of error. Do that a couple times and mix some other MPH if you like.

If your rub is on the upper wheel house that bolts to the hood where is it? Center where the tread would scuff it, towards the outer edge of the panel where the outer side-wall would scuff it or maybe the inner of the panel where the inner side-wall would scuff it. Spacers can only correct the last! What does the rear do?

If you remove the tire/wheel on the back-side of the wheel the dimensional characteristics of the wheel should be cast into it. Diameter, width and off-set are normally all contained there. If it's not then the game changes. Pull a front and rear from one side and note the information you find and also maybe catch a snapshot.

The LTPWS lights even though they were said to be reset I'd say still have a fault and you need to get the error code and do some diagnostics. Do you have an FSM? The codes should be quite easily recovered by yourself.

I'd avoid any "over the counter" tuner enhancement product whether applicable or not. Do you need the services of a tuner for anything other than the calibration of odometer/speedometer? I'd think likely not.

What rear axle ratio is the car built with?

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Not knowing the wheel widths of the wheels involved another option might be for the next set of tires is change the "profile" of the tires to get closer to the original diameters.

A possible front would be 255/40 and for the rear 285/35 - this change would also be the only likely fix for a situation where the tread of the tire is scuffing the center of the upper wheel-house panel because of the additional height other than lifting the ride height of the spring. That is not inexpensive and the car with the profile tires you have is already riding "high".

These tire size changes are actually the sizes that are suggested by most when doing the change the previous owner did. Those tire size changes get the speedometer/odometer very near "spot-on"!