I swear everytime I drive the Vanden Plas, I seem to zoom past all of the other cars unless I drive like 5-10 under. Has anyone else noticed this? Thanks

 

A change in tire circumference might be a factor.

Are you running oversize tires?

 

My XJR speedometer is dead nuts on. I confirmed this with a GPS. There are several threads on this on the forum and the consensus is that the accuracy of the speedometer is pretty good.

 

I am running 225/60-r16 tires. I think that is the stock size. Maybe the speedo is dead on and everyone else just cowers and backs down to let me by

 

I'm going to check tire size as soon as my wife gets home, but I noticed the same thing a week or so ago on a run to Dallas. I hit the interstate and not being in any real hurry set the cruise between 70-75 and it seemed I was passing everybody. I set it the same for the ride back and pulled up a GPS speedo on my phone. I've hand clocked my MX5 to calibrate custom gauges and found the GPS to be very accurate @ 60mph. According to it I was bouncing between 74-76 with the cruise set at 70, which means I was pushing 80 for 3 hours on the way to Dallas. DPS must have been feeling generous.

 

my XJR speedometer usually reads about 1mph under my current actual speed

 

My 02 XJ8 VDP seem to be extremely accurate when I check it against some of this city's "your speed" sign readings. I have 225/60/R16 tires.

 

Just a thought, but each specific vehicle might have a calibration that is unique to the tires that were original equipment. In other words, if a car was built with a wheel/tire upgrade option, it may receive a calibration that is different from the base model.

The calibration should match the tire fitment sticker on the specific vehicle.

 

lol that's the other thing. I have only gotten 1 ticket in the Jag and it was going 40 in a 35. The speedo was dead on 40 there. I can drive up I-35 right at 80 mph and I am blowing cars off the road, but never seem to get attention from state troopers.

 

What specific tire brand/model and size are you running?

 

What specific tire brand/model and size are you running?

 

goodyear eagle F1 all season 255/40/R18

 

Thanks for that.

The tirerack chart shows 26" overall diameter and 803 revolutions per mile on that tire.

 

My speedo reads 1 MPH slow, according to my Navigon GPS.

 

Ok, ... victim #3:

What specific tire brand/model and size are you running?

 

Sumitomo HRZs 255 40 18

 

Nice tread pattern

26.1" overall diameter, 798 revolutions per mile.

For those who are wondering what the point is, it is an attempt at compiling representative "revolutions per mile" figures with a known speedometer error.

The speedometer knows nothing except revolutions over time, so enough "revolutions per mile" observations will directly translate into a pretty good guess as to the most desirable "revolutions per mile" target when purchasing tires. This is closely related to the diameter but not the same thing because it varies according to sidewall flex. The manufacturers publishing this figure will run the tires at load against rollers to determine this figure.

Only 2 reports so far, but, they are consistent at 798-803 revolutions per mile resulting in a 1 mph under reading by the speedometer.

assuming that the 1 mph error is at 60 mph,

meaning that 15 or so "revolutions per mile" will only result in a difference of 1 mph or so, and that the target is 814 "revolutions per mile" when considering tires if speedometer accuracy on a X308 XJR is being considered.

++

 

iive confirmed it with my jag im running on 275 35 20 and gps showes 90 im doing 90 on tthe speedo other cars ive got im doing 90 on the gps its showing 94-96 on the metre

 

+1, checked with my TomTom.I read somewhere that the XJ was one of the LEAST ticketed vehicles on the road. I don't remember where I saw that. But I agree, I've blown past police a few times and never even turned their heads.GoodYear Eagle GTs 255/40/ZR18I've noticed that most of my cars showed I was going faster than I actually was, and I've owned quite a few Brand New cars with New Stock Tires.

 

That's because speedometers are not allowed any error to the downside, but are allowed "X" percent to the upside.