Hey all,

Was just driving 100km/h on my meter and the gps said i was going 80km/h.

Is this to do with the fact i put 18 inch rims instead of the factory 17 inch? Or is that fiction?

How can I solve this issue? 3 years ago it was 1:1 the same as the gps.

Oh and the gps is correct, i tried it with a few gps devices and they all said 80.

Who can help me out?

 

Not nearly enough information.

Did the disparity occur at the time you replaced the rims?

And what is the difference between the diameters in the tires?

A change in the "rolling diameter" of the tire will affect speedometer accuracy. There are various calculators on the internet. You can determine the rolling diameter of your original tires and the current tires by looking them up at tirerack.com

 

Hmmmmm..... Speedo 20% off. ................Okay I will take two. Can;t turn down a sale on Speedos.
No, seriously, a tire/wheel size change does change the speedo reading, but not 20%. A 5% change is extreme. There is something else wrong here.

 

malf. vehicle speed sensor?

 

compare your new tire size with the old size with this:
Tire size calculator

 

Lug_nut is correct, there is no way that going from 17 to 18 inch wheels will produce that kind of error. Maybe 5 kph, if that but if you match the new wheel and tire correctly, it will not change the speedometer at all. As you increase the wheel size, you need to decrease the sidewall height of the tire. In the case of the Jag however, I think even this is unnecessary.

Mark

 

FWIW GPS always reads slower than the speedo. There's been a lot of discussion on this - to no useful conclusion.
In the XK an indicated 117 km/h is GPS 110, in the beat up Corolla it's an indicated 120 - but nowhere near your disparity. COuld it be a cumulative thing with tyre size added in ?
Also FWIW the ACC radar sets off some French speed cameras - not very desirable but no tickets yet.

 

By law, speedos always read spot on or can read high-but never lower. As it would be impossible for OEMs to calibrate each speedo to be 'perfect' they design in a fudge factor so they end up reading slightly high.

If yours is on the upper limit of that fudge factor and the tire circumference (rim diameter is irrelevant) is substantially lower than original tire size, at 20% error is possible.

 

Your error is in the wrong direction. An increase in tire/wheel diameter will cause the speedometer/odometer to read low. I have the exact same configuration, a one inch increase in diameter. I did a very accurate calibration over 20 miles using highway mileage markers and the GPS. Both results agreed exactly. The speedometer/odometer is 5% slow/short. With a larger wheel/tire diameter the circumference will be greater and the car goes further with each rotation, but the onboard instrument does not account for this.

EZDriver

 

What kind of error do you get at other speeds? Is it always a 20kph difference or does it remain a constant 20% error?

If it is a constant speed error (for the most part cuz when your stopped it will be hard to read -20kph), you might have something similar to my issue which is a "sticky" speedometer. When mine gets heated on the face by direct sunshine (which can happen a lot in Arizona) something must be expanding and causing things to "stick". I've seen this manifest itself as both high and low readings.

Unfortunately, I'm not offering much of a solution. I just try to drive with my hand on the wheel in a position that shades the speedometer.