Nice, I'm an engineer too (building services/mechanical engineer), which explains my equally overly **** response to your issue.

Have you found out what type of tires you are running? TBH it sounds like you must have a more of an issue than just overall tire diameter.

 

The tires are the "standard" Pirelli PZero that are spec'd to go with the 20" BBS Montreal wheels. If there is a calculation problem, it can only be one of two things. One possibility is that the miles that show up on the trip odometer are incorrectly low (that would be the case if oversized tires were put on or rear end gear ratio was changed without adjusting the odometer gear or programming). Figuring about 10% more driven miles than is shown on the odometer would result in my manual calc matching the computer's.

On the other hand, maybe the odometer is correct and the computer's calculation is too high. I assume the computer's calculation is similarly based on knowing how far the car has traveled (by the same means as what shows up on the odometer or maybe by some other means?) and measuring the fuel flow.

Or maybe I have too much time on my hands

Doug

 

Here is another weird thing I noticed. There is a long downhill stretch of freeway I hit sometimes (405 North from Los Angeles into the San Fernando Valley) that is at least a few miles long. When hitting this you can basically remove your foot from the gas pedal and use engine braking and still maintain 60+ mph speed. When I do so in my Corvette, the instantaneous mpg read-out of course goes up to the max 99 mpg. However, when I am coasting down in the Jaguar, the instant readout keeps bouncing back and forth between as low as 14 mpg to up to maybe 50 mpg. If I gently push the pedal enough to just open the throttle blade, the instant readout keeps steady at between 30-40 mpg.

It is odd that the mpg readout would drop to 14 mpg while coasting with NO foot on the pedal. Seems like the mpg computer is a bit wacky.

Doug

 

My computer does the same thing. It's pretty accurate. Yesterday at the end of a trip it showed 15.3 gallons of gas used. The reading on the pump showed 15.2 gallons delivered.

My 88 Vette's computer use to be spot on until I changed injectors. Now it's ALWAYS 2 mpg too high.