ACCLR8

Does anyone have any easy method of recalibrating my Speedometer / Odometer after moving down to a 17" rear tire? I have moved to a 17" for strip days but no longer have a tow vehicle and I am worried about miles piling up. 2002 C-5 Coupe 6-Spd

iceburgh

LS1 edit or Hypertech is the only way I know of doing that. :yesnod:

6Speeder

It's not the diameter of the wheel that will throw off your speedo and odometer, it's the diameter of the tire. It's probably not off as far as you think. For example, if you are running a tire a full inch smaller in diameter (like a 315/35-17) you're still only off by 4%. You drive 1000 miles and it registers 1040. No big deal. :cheers:

USAZR1

I have seen mini-tubbed C5's running 345/35/18 tires and was wondering what needs to be done to allow this.

tscales

You can run 335/30-18 with minitubbing.

I do.

Tom

SoCal C5

I could be wrong but I rember reading that the speedo reads one of the front wheel speed sensors. The speedo is not driven by the tranmisson like in the past. If I am right you should not have to do anything.

If you have two sets of wheels as your post seems to imply you can test this. With the first set of wheels cruise at a constant speed (say 60 mph) and read the tach. Then do the same indicated speed (in the same gear) with the second set of tires and read the tach again. If the two readiings are different I am rignt and you have nothing to worry about. If they are the same then I am wrong, the speedo is driven off the rear wheel speed, and you may need to get a hypertech if you are worried about the differance.


[Modified by SoCal C5, 3:02 PM 7/8/2002]

ToplessTexan

iceburgh68 is correct, PCM programming is your only real option (if your tire diameter has changed enough you want to mess with it.) A used HPP3 is probably as cheap a solution as you'll find. The VSS signal is made available to a number of things through the PCM, including the instrument cluster. A signal conditioning hack on the VSS signal (e.g. similar to the MAFT solution for the MAF signal) would be asking for trouble with things like the IC, ESC, and EBTCM which may well have a fairly "hard coded" understanding of the VSS signal.

Mike Mercury

Close, but not completely accurate. Speed pulses come from a notched reluctor wheel in the rear differential. That is why a rear axle ratio change does not alter the speedometer calibration; but a change in the overall rear tire heigth will throw the speedo off.

Godspeed

I switched to a 335x30x18 out back (with minitubs) and used LS1 Edit to calibrate. The stock tire size is 26.7" OD and the new tire was 26.1". My speedometer was 3-5MPH off until I did the calibration. :cheers:

ToplessTexan

The percent error in your speed is going to be like

percent error = 100 * (PCM height - actual height) / actual height

with the indicated speed greater than the actual speed if the PCM height is larger than the actual tire height, less than if not.

Modulo issues like crush, flex, and ballooning of course. :)

How fast do you drive Jeremy? :p: