I have recently started driving in PDX/HPDE events. My '04 C5 has 17" fronts and 18" rears. My driving instructor tells me I should be running the same size tires on both. Anyone have some input for a brginner?

 

Your instructor is not very knowledgable about Corvettes. That is the "stock" setting for the car, and GM did it intentionally. The EBCM (Electronic Brake Control Module) has been programmed for this difference in wheel sizes, and uses this in calculating when to turn on Active Handling, for example.

The car has larger tires at the rear because the engine generates an ENORMOUS amount of torque ... so it needs REALLY big tires at the back to handle that. The front tires steer the car, and do not need to be as large as they only have to deal with cornering forces, not the added force of acceleration.

Ignore that instructor ... at least that piece of advice ... hopefully he taught you about looking ahead ... driving with your brain ... being smooth ... driving slower to go faster ... and all the other "tricks" that make you a much better driver.

Just be sure to use the skills you learned at the HPDE on the street .. it will amke you a better street driver, and will make "good" habits ingrained in your driving style, on the road and on the track.

 

Your instructor is wrong! GM designed C5's that way and many other sports cars are the same way.

 

Those guys got it right, Chevy designed C5s that way for a reason, read 'All Corvettes are Red' and you'll know why plus lots of other good info about the C5 design....
STW, DocOhm

 

The standard C5 tire setup is as mentioned above. However, most people end up running equal diameter tires front and rear. If you start to look into track tires you will soon find the General did not choose a very good combination for track usage. The biggest selection of race tires is in the 17 inch sizes and the SCCA T1 race Corvettes are allowed to run 315/35 x17 on all 4 corners with 17 x 10.5 inch rims in the front and 17 x 11 rims in the rear. They can also run a 275/40 x 17 tire in the front on a 17 x 9.5 rim. All of these tires are a 25.3 inch diameter. John Heinricy the GM Corvette engineer who does quite a bit of racing found the 96 Grand Sport rims and tires worked best when he was doing some performance testing with the C5 back in the late 90s and that is how those sizes got in the rule book.

I am currently running 315/35 x17 tires on all 4 corners and if available I buy used ones from T1 racers who only run a couple of heat cycles on a set of tires.

Bill

 

Also the ABS braking system and the traction control system are calibrated using the heights on the frt and rear tires--The actual tire size isn't as important as the difference between the frt and rear which is apprx. 1 inch---If you change it much either way neither system will work correctly and it isn't safe--usually you'll get an ABS light on or a traction control error message---there was a chart floating around on this site at one time calculating various tire combinations and showing a go or no go sizing

PS---Racing applications are different as they don't use the traction control system nor the ABS system--most use hi-dollar aftermarket brakes with a porportionning valve to asjust the frt to rear braking

 

The stock difference is 1 inch in diameter between front and rear tires. The computer will work with 1/2 inch difference, less than that and you will have problems.

 

Here's a little secret Ill let you in on, Bill's god sent to us for a reason.
Use his knowledge. He's done more than read an article or two about corvettes.