I expect 6% (or so) is deliberate. Most speedos over-read by 5-10%. I think they're not allowed to under-read. I don't understand why you're surprised at yours over-reading.

 

That's why.

 

In re-re-reading the .pdf file,
http://jaguar.bttlxe.com/xtype

http://jaguar.bttlxe.com/xtype/Addit...%20Cluster.pdf

here's an interesting comment on page 7.

"ETM displayed speed figure will be
approx 3% higher than speed
indicated by speedometer.
Allowed tolerance – minus
nothing/+ 10% + 2.5 mile/h."

Mine is 6%, so I'm double what Jag states. Their comment '+10% +2.5mph' is ambiguous. Does it means 3-10% is acceptable or 10% above 3% is the limit? Or is there another 2.5 mph to be added?

 

I was playing around with the GPS on my Blackberry and in comparing it to my speedometer reading there was only a maximum of 1-2 mph difference for speeds from 50 mph to 70 mph. The speedometer reading was either slightly higher or dead accurate. I was counting on a 5 mph difference and the usual 5 mph grace which is why I usually go a little under 10 mph more than the posted speed limit. I guess I'd better throttle back a bit, especially since VA is experiencing a budget shortfall.

 

Bear in mind it depends on which rims+tires (sizes) you have, how much the tires are inflated, and the setting put into your car for the rim+tire sizes when it was last set (in manufacturing if not done since then). Plus, how worn your tires are.

 

I had forgotten about this thread. Over the summer, I had a new set of tires installed. The over-indication of the speed has reduced from 6% to about 3%.

 

Most of the cars I have owned have over-indicated. My current S-Type's speedo leads by around 5-6% when compared to my GPS. I suspect the tyres are to blame.