XK8 / XKR ( X100 ) 1996 - 2006

Odometer is 20% off

Old 06-03-2012, 06:21 PM
Junior Member
Thread Starter
Join Date: May 2012
Location: Amsterdam
Posts: 7
Received 0 Likes on 0 Posts
Default Odometer is 20% off

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?
Old 06-03-2012, 06:36 PM
plums's Avatar
Veteran Member
Join Date: Feb 2011
Location: on-the-edge
Posts: 9,731
Received 2,117 Likes on 1,585 Posts

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
Old 06-03-2012, 07:27 PM
Lug_Nut's Avatar
Senior Member
Join Date: May 2012
Location: Canada
Posts: 143
Received 10 Likes on 9 Posts

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.
Old 06-03-2012, 09:01 PM
Cerberus66's Avatar
Senior Member
Join Date: Mar 2012
Location: Dallas Texas
Posts: 230
Received 14 Likes on 12 Posts

malf. vehicle speed sensor?
Old 06-04-2012, 12:12 AM
OregonJag's Avatar
Senior Member
Join Date: Jun 2009
Location: Lake Oswego, OR
Posts: 346
Received 79 Likes on 53 Posts
Default tire size calculator / speedo correction

compare your new tire size with the old size with this:
Tire size calculator
Old 06-04-2012, 04:51 AM
mjlaris's Avatar
Veteran Member
Join Date: Oct 2010
Location: Denton, TX
Posts: 1,079
Received 179 Likes on 146 Posts

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.

Old 06-04-2012, 05:14 AM
steveinfrance's Avatar
Veteran Member
Join Date: Dec 2011
Location: Limousin, France
Posts: 6,282
Received 680 Likes on 587 Posts

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.
Old 06-04-2012, 12:52 PM
Join Date: Oct 2009
Location: Perth Ontario Canada
Posts: 11,058
Received 2,202 Likes on 1,804 Posts

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.
Old 06-04-2012, 04:12 PM
Veteran Member
Join Date: Dec 2010
Location: Holly Lake Ranch, Texas
Posts: 2,063
Received 252 Likes on 158 Posts

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.

Old 06-05-2012, 12:37 AM
beg3yrs's Avatar
Senior Member
Join Date: Nov 2010
Location: Tucson, Arizona USA
Posts: 326
Received 35 Likes on 30 Posts

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.
Related Topics
Thread Starter
Last Post
E type ( XK-E )
04-16-2016 04:00 AM
XJ6 & XJ12 Series I, II & III
01-03-2016 08:55 PM
Clare Jennings
New Member Area - Intro a MUST
10-04-2015 05:43 PM
XJ XJ6 / XJ8 / XJR ( X350 & X358 )
10-02-2015 03:25 PM

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)

Thread Tools
Search this Thread
Quick Reply: Odometer is 20% off

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

© 2019 MH Sub I, LLC dba Internet Brands

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.
  • Ask a Question
    Get answers from community experts
Question Title:
Your question will be posted in: