Speedometer incorrect - 98 tahoe

[Popeye65]

I am a new owner of a 98 tahoe LT. It is not moddified in any way (from what i can see), the Speedometer is incorrect as in shows 100Km/hr when actual speed is 60Km/hr (verified via GPS).

How can i correct this issue?

[ricebike]

check tire size vs manufacturer's recommendation

are they the OEM tire/rim?

[777stickman]

GPS is a good way to measure speed. The best way is to go by mileage (or in your case Km) markers on the highway. At an actual speed of 60Km/hr it takes 60 seconds per Km. Adjust your speed to achieve the 60 seconds per Km and record what the speedo is reading.

I agree with checking the tire sizes as to OEM. Could the gear ratio's have been changed in the past? The 40Km/hr difference indicates a much lower gear ratio (higher numerically, such as, 3:42 down to 4:11).

In answer to your question as to how to correct this. The only way I know of is to take it to a good repair shop that can re-calibrate the VCM to what gear ratio and tire size you have. By tire size I mean the height of the tire. Once they get it close it may take a couple of VCM adjustments to get it right.

[Popeye65]

Thx for all the posts.

Verified that the stock tires were 245/75R-16", It has now 265/75-16". As per my calcluations the difference is 1.5" between the tires. This as expected will have a effect but it should not be a 40km/hr difference.

I also did verify the gearing, stock it came with 3.73 in rear diff, that too is confirmed but rotating the tire and counting the driveshaft revolutions (approx is correct).

As per '777Stickman' i am now left with the "VCM" path.

Any other ideas folks. Really do not want to sink $$ if i don't have to.

[j cAT]

Originally Posted by Popeye65

Thx for all the posts.

Verified that the stock tires were 245/75R-16", It has now 265/75-16". As per my calcluations the difference is 1.5" between the tires. This as expected will have a effect but it should not be a 40km/hr difference.

I also did verify the gearing, stock it came with 3.73 in rear diff, that too is confirmed but rotating the tire and counting the driveshaft revolutions (approx is correct).

As per '777Stickman' i am now left with the "VCM" path.

Any other ideas folks. Really do not want to sink $$ if i don't have to.

your stock tires are smaller than the ones now on vehicle....this means that the speedometer should read LOWER not HIGHER...with larger tires on the vehicle you will indicate on speedo a lower speed than actually moving..

this means that you may very well get speeding tickets..
I actually know of this...

[J-Ri]

100 KPH = 62.1 MPH www.convertit.com

I'd start with the possibility that it is from an area that measures speed in miles per hour and the IPC is just set wrong (or PCM, wherever the setting is). Should be an easy fix with a Tech-2 scan tool if that is the case.