I have a 2000 S-type with a 4.0 gas engine and have found that the car has always reported the MPG inaccurately. Originally it reported lower-than-actual MPG when compared to actual MPG calculated at each fillup. Over 48 fillups, the car had understated the mileage by 0.36 MPG. Early on, when I had it in for a regular service appointment, the dealer updated the on-board computer’s algorithms. I guess Jaguar had realized that the MPGs looked worse than actual and that needed to be fixed. However, they overdid it and, since then, it has overstated the mileage by an average of 0.32 MPG based on 281 fillups. In the U.S., that falls under false advertising. I recommend that you compare Torque’s reported trip MPG to actual (trip miles at fillup divided by gallons), and ignore what the car reports. And the Instant MPG can never be very accurate because it is based on momentary/random measurements. It would be nice if Torque offered a smoothing-average function in setup to make the Instant MPG less volatile, and thus more usable.

Quote from johnjordan on January 19, 2013
My car, Jaguar 2.7 diesel S Type has on board instant MPG read out. When I use the Torque read out it is miles different from the car one, sometimes by as much as 100%, i.e. Torque says 25mpg, car says 50mpg.
Which is right do you think and why such a big difference?
OK, first off I presume you have Torque Pro, not Torque Lite. you need this.
Second, there is no “instant fix”. You will need to experiment over at least 100miles, better over several 100s.
Third, you have to presume that the on board ECU readout is correct, this is what you will try to emulate.
Go into vehicle profile and enter your vehicle details.
The last entry is “MPG Trim Adjustment”, its probably set at the default 1. If you are getting a Torque Pro mpg less than the on-board computer (as you have experienced), you will have to INCREASE the Trim. Try by increments of 10% allowing around 10-20miles to see if things are moving in the right direction. Make further adjustments until you have over-compensated, then trim back 2% at a time until you find the Torque Pro figure is matching the ECU readout on the dashboard panel.