Well I finally got a ticket in the jaguar late last night. I was leaving the beach and right where the speed limit changes from 40 to 50 the policeman stopped me. He said I was doing 49 in a 40. I said that's strange because my speedometer was barely over 40 maybe around 42 or 43 at most. I took the ticket didn't argue with the man but noticed today when getting off an over pass the signs that check your speed clocked me at 45 and the speedo was at 36 maybe 37 just barely over 35mph. Is it possible having larger wheels (20" diameter) could cause these inaccurate readings? I have the low profile tires and I understand these aren't exactly what the jaguar calls for it seems to ride well since I fixed the front suspension and does not rub one bit anymore after the shock and shock mount replacement. One thing I should state as well is that I noticed my rear tires need to be swapped as they aren't rotating in the right direction. Could that cause this issue? Thanks in advance
Marc

 

Marc, am going to guess that you live somewhere warm, as I live half a mile from the beach here in the UK and 'late last night' it was 2 degrees C

OK you'll have to share what your tyre profile size is and others with 19" or the odd with 20" will tell you what you should have fitted.

IIRC the speedo takes it's reading from the fronts so shouldn't be affected by your rears..however if you've got rotational tyres fitted and they're the wrong way that'll be one heck of a drive!

Here in the UK if you speedo reads below the actual speed of the car it is illegal

 

Basically, you need to compare the diameter of the rear tires as specified by Jaguar (should be under the lid of the center armrest) to the outside diameter of the actual tires on the car. The percentage difference is how far off your speedometer will be. And yes, "taller" tires will make your speedometer read low.

The diameter is the wheel size plus twice the height of the sidewall.

The sidewall height is the width of the tire times the aspect ratio taken as a percentage.

Alternatively: https://tiresize.com/comparison/

 

And note that fmertz said rear tires, not front.

 

Changing wheel and even sometimes only tire size can definitely impact the speedometer. Back in the 1970s I got a ticket in my Roadrunner which was running H70 tires on the rear versus the standard F70 (I am not sure how the old designations compare with current). Anyway, the speedometer was reading just slightly above the freeway speed limit so I took the car to a local AAA club that had a speedometer check setup (I don't think these are common anymore). Sure enough the speedometer speed was several mph lower than the actual speed and I was able to beat the ticket!

Doug

 

Speedo reading slow means the rear tires are too big.


Stock tire height is 26.5, but the Speedo actually wants 26.0" to be correct on my car.


Most 20" tire sizes are 27.5" tall, which is 6% different. The speed differences you are describing are like 15% off which would make for a 29"+ diameter tire... which is pretty big tire for an XK8.


I would think your speedo is out of calibration.

 

Marc, don't know where this happened, but fyi in New Jersey USA the law states that a "Reduced Speed Ahead" sign must be present before speed limit drops like that. I don't know if it is state rules or Dept of Transportation (federal) rules but tickets have been dismissed for improper signage.

John

 

Here in what could only be described as the speed camera capital of the entire world (state of victoria) you would have more chance of a lottery win than getting out of a contribution to government coffers due to speed error.
My xkr is factory fitted with 20" detroit and my speedo actually reads about 8% higher than actual and speed according to obd2 port is different again.