My speedo over estimates the speed by about 4 mph at 70. In other words, to motor along at exactly 70mph will be when the speedo reads 74mph. And it gives a fuel consumption regularly some 1.5 mpg better than actual ‘full to full’ readings taken over a number of fills (there is another interesting thread on fuel consumption: MPG study). Can you give your figures for 1 or 2 below, or both?

1. So, what speedo reading would you have for exactly 30mph actual, and exactly 70mph actual, as measured on a
satnav?
2. How do your consumption figures on the computer compare with a calculation using full to full fuel quantity readings and distance on the trip? *. Please state whether UK or US gallons

Direct fuel consumption measurement (in case anyone is unsure)
* fill to the brim. Zero the trip meter. On the next fill, once again fill to the brim, and note quantity of fuel in gallons and the trip reading in miles. Mpg is then miles/gallons. A more accurate result will be obtained with a number of fills, and total fuel from start to last fill, and of course the total miles over all fills.

 

Have you looked into the various tire sizes for a root cause explanation? I could see Jaguar calibrating the speedo (based on the ABS sensors) on some early version of the car (even possibly the 17" revolvers), but somehow not re-calibrating with the later 20" packages. The various Jaguar official sizes are listed under the armrest...

 

Fuel consumption is within 0.2 gallons consistently.

Would have to evaluate the speedo to answer question 1, though it seems to be reading a couple mph over real speed.

John

 

When I first get a car I check MPG, odo and speedo against a good gps. Here's what I found on the '03 xk8:

Computer MPG is optimistic by a little over 1 MPG.
Odometer is very accurate.
Speedometer is very accurate.

 

Surely the overall tyre diameters would be the same, whatever the rim? Larger wheels would have correspondingly lower profile tyres. Or have I misunderstood, somewhere?

 

Generally, yes, but what does the math say?

"Height" of a tire is the diameter of the wheel plus twice the width of the tire multiplied by the ratio. Units have to be converted, too (diameter in ", width in mm). There are tire calculators out there, too.