rv93

I have a 2002 Z28, and would like to get a set of 2002 SS wheels. They look better, have a smaller sidewall, and wider tires. From what i calculated, the outer diameter of the tires is 0.1% smaller with the SS wheels compared ot the Z28 wheels (im planning on using stock sized SS wheels). Would that small of a different require recalibration of the speedometer? and will the wheels fit without any modifications? thanks

Injuneer

Which tires did your Z28 come with from the factory - 235/55-16 (26.18") or the optional Z-rated 245/50-16's (25.65")? The 275/40-17 is a 25.66" tire.. (Theoretical dimensions, based on specification)

Assuming you started with the 245/50-16's, you will not notice the difference in tire diameter. If you get theoretical and calculate the outside diameter of a 245/50-16 (25.6457") tire compared to a 275/40-17 (25.6614") tire, the difference (0.061%) is less than the build tolerance on the tires. For all practical purposes they are identical. At a true 60.0 MPH the slightly larger 275/40-17 would cause the speedo to indicate 59.96 MPH.

Bottom line - Don't worry about it.

If you started with the 235/55-16's you will have a larger difference, with the speedo indicating 61.21 MPH at a true 60 MPH. That would probalby be worth correcting the tire diameter in the PCM, since it is adding extra miles driven to the odometer.

Note also that each specific tire has a manufactured nominal diameter that may be slightly different than the theoretical/calculated diameter.

Yes, the wheels will fit with no modifications. The wheel wells and the axle hubs are identical between the Z28 and the SS. If you were putting newer wheels (97-02) on a 96 or older model, you would have problems with the newer wheels not fitting over the over-size rear axle hubs, requiring some sanding of the wheel hub holes. But with your 2002 that is not a problem.

rv93

thaanks a lot, that was really thorough. yes, i have the 245 tires (didnt know that was an option), by the way, is there a way to find out if a manufacturer has different than standard diameter? the tires i think i want are yokohama s.drives, if that helps

Injuneer

Go to a site like Tire Rack and look them up. They have the tires you want listed. Hit "specs" and they list the overall diameter (25.7" - note - they round to one decimal place) and the revloutions per mile (808). In most cases, the nominal diameter is 25.7" for that size tire, but the revs per mile are where the difference shows up.

I really don't think it's going to make any significant difference, but you can convert the revs per mile to a rolling diameter. You would then have to compare it to the factory tires. It's not worth the effort unless the rev per mile are at an extreme difference from the average for tires with the 275/40-17 size.

6SpdLT1Z

I wouldn't worry about it, we are talking about a fraction of a percent. I run a size larger tire on my truck with a more significant difference and I havent done a tune ir recalibrated my Speedo. My speedometer is like 1% slow. If anything the only thing I notice is a little drag on the highway but I'm running a inch and a half larger diameter tire. A lot bigger difference than you are looking at. 275/40/17 is basically a factory upgraded tire size for 245/50/16.

rv93

275/40/17 is the factory upgraded tire size on the car. i normally wouldnt worry about it, but im a bit **** about this car. i have my motorcycle to have fun modifying because i can, so im trying to do everything right on the car. i guess if gm didnt reprogram the speedometer for the SS, i really shouldnt worry about it though.