Well checking the speedo is easy ..use a GPS but the odometer (mileage display) more difficult.
I wanted to do this to check the accuracy of the on board MPG meter .
On your phone you can download an App called Drivescore . What it does is check your driving for harsh acceleration ( oh yes in a jag !!) braking ete but it also records the mileage travelled.
So on the screen below I have done 205miles using the phones GPS . So record the odometer milage and the Drivescore and when you have done enough miles compare the difference in the two.
Warning ...if you go in another car for any reason it will add in that milage so turn it off when not in the jag. Press the little head top right and select off.
Will let you know how we go ....I think US cars have more accurate speedo.odometer than UK

 

This has not proved simple. The odo on the car does not have tenths of a mile and neither does the mobile phone app.
This means the accuracy is poor ,even on a 100mile trip 100 could be 100.9 and on the app 99 could be 99.9 so for any accuracy you need a long long trip. Trying to add trips together also means accumulated inaccuracy.
So for now I give up until I find a better app and do it on a long drive across france.
In general terms with a new LH front tyre it seems to be accurate.

 

Friend. Use the elapsed mileage selection on the display (press button on end of turn signal stalk. You'll get tenths displayed, plus wth you could just stop next to a mile marker on empty road. Press reset, then cruise a mile or two!
John

 

I don't believe that many of us bought our vehicles because of MPG. Just sayin'😆

 

Will try that ..have found an app that does 1/10 mile...

 

Deleted, I figured it out

 

Maybe he is hoping the onboard MPG meter is under-reading by a factor of 10.

 

I normally top up the tank ..record the mileage and do that again keeping an eye on the MPG ..now if the MPG meter is accurate I wont need to bother....

 

I can tell you that when I look at average mpg in torque it bears little relation to the one the car shows even with the 'enhanced accuracy version' in torque.

 

On the Sun Tester ( engine analyser) it said "Test Don't Guess" so thats what we do