Socko

So I was at the track 10 days ago and i hit my rev limiter in 4th. the weird thing was my speedo said i was going 144.7. I don't have a tuner yet but my car is tuned. So i looked in the calibration and it says my tire diameter is 25.21". I assume this is supposed to be the rear wheel diameter so I checked a factory tune and it was the same. Why is it set this way as the stock c5z rear tire diameter is 26.1? even the front is over 25.21?

Thanks!

tblu92

Even a regular C5 has the tire diameter somewhat smaller in the tune than actual Tune says 26" and the actual stock tire size measures 26.50
Also be aware that a tire can have the same P metric size but different manufacturers can have different tire heights----
GM by making the tire size smaller in the tune than actual size will show a larger MPH on the speedo--maybe they do this to keep us from getting tickets ??

Socko

making the tire size smaller actually makes the speedo read under how fast you are going, as you are going farther then you think you are. C5Z stock ring at 3.42:1 and 1:1 4th trans ratio with a 295x35/18 measured at 81.75" circomference so 26.0" diameter yields 149 by my math at 6600 rpm. Speedo says 144.7. At 65 i think its only 2 mph though as they are both flat lines on a different pitch, but i am really doing 67. So the damn car is trying to get me arrested!

tblu92

OOPS !! Sorry you are correct I got it backwards--sorry---Well then I have no idea why they do it that way !!

SVT_Z06

Guys,

That number is the LOADED TIRE RADIUS.

You can use the following formula (295/35-18 to 305/35-18):

26.4 / 26.1 = 1.0115

25.21 * 1.0115 = 25.4999
-----------------------------------------
New actual height / old actual height = difference

Old actual height * difference = new loaded radius

Enjoy

Socko

I was assuming their must have been some logic to it. Thank you!

SVT_Z06

No problem, man.

V