Hi have not had this car long and had some issues changed bank one lambda sensor as there was a code for it and smelt rich ..also done an engine service.had an intermitant misfire but after a little diagnostic injector sticking and fuel cleaner has rectified this.but still the trip computer says a erase 9.8 miles to the gallon was reset when last filled driving it like miss Dailey as well .the other reading does go up and down when driving..my question is because the car has 19" xjr100 alloys on would that throw the calibration/ readings out...steve

Stevie. What size tires do you have fitted? If the overall tire height is 26" as was original for a XJR6 it should not alter the speedometer or fuel economy calculator.
Example- when I bought my XJR6 it had 25" tall tires mounted which caused the speedo to indicate faster than was actually travelling and threw off the fuel economy calculation. I made this worse when I swapped the rear diff gears to 3.58 ratio from a 96 VDP. When I had wheels refinished I had a set of 26.6" tall tires fitted, which is the same height as non-supercharged cars were fitted with. Now my speedo is spot on and the fuel milage indicated is closer to accurate, less than .5 mpg off of what I calculate when refilling tank.

Hi John,
Glad to read that you measured fuel economy matches the trip computer. I worked really hard during the development of the XJR6 to programme the software to get the trip computer to match the measured fill.

Yep. Excellent work mate!

Did you keep a sneaky copy of the source code? Must be out of copyright soon.

Wayland , you can go one better and ask Andy to reprogramme your ECU to eliminate the built-in ignition retardation for better performance across the range.
You need to send him your ECU, and there is a cost.

Stevie,
There are many online calculators to allow you to compare the circumference of your new wheels/tyres with the factory fit, and without knowing what width of tyre and aspect ratio on your 19" wheels, I can't be sure of how they compare. However, on the assumption you have not opted for narrower tyres, and given the standard aspect ratio was an already lowish 45%, I would take an educated guess that your circumference is rather bigger than standard. As well as throwing the speedo out of whack, the related fuel consumption is also misstated.
As I think your circumference is larger than standard, when driving like miss daisy, you would actually have been travelling faster than you thought. The trip computer would have measured the fuel consumption correctly, but would have an understated measure of the distance covered, hence showing an abnormally poor mpg figure.
By way of a benchmark fuel consumption figure, on super unleaded fuel, with an Andy bracket fitted, I am getting an indicated 21.5 mpg in miss daisy mode. I have rarely seen anything lower than 18 mpg, but it is usually just me in the car, and I don't drive in the city much. You won't need to look far to find examples of 15 mpg, but not usually much worse. I hope this is helpful.

Well I filled the tank brimmed it today 75 notes drove from my house to portsmouth down the a3 so dual carriage way /motorway then to Hayling Island with the kids and back today milage covered 180 miles and bang on half a tank of fuel used. Still yet to work out the mpg...and that's with Andy bracket now fitted.carnt work out why the trip says average speed 39 mph t hough t would be higher as traveling 70 to 80 most of it

It costs me £35 to fill up from half way which is 33 litres or about 7 gallons. This is 25 miles to the gallon which is what I would expect on a dual carriage way journey at 80mph. I have seen 26 on such journeys but after a few blasts of acceleration (exhilaration) it goes down to 24mpg.

I have the XJ-Sport 4.0 with Andy Bracket.

That's computing miles traveled divided by engine run-time since last reset. 39 is a respectable figure unless you topped up adjacent to the dual-carriageway on-ramp and did absolutely NO stop-n-go, city/neighborhood driving, nor idling.

Capacity is 21.3 US gallons; though I reckon you supercharged boys give up a tad of fuel capacity to that secondary pump. So if I take a .1 off for the pump and use a true half tank, I guess you are flirting with 17 mpg (US Gal) Don't get alarmed! Your gallons are larger by a bit....I always wonder what I'm doing wrong when I see posts about 24-26 mpg...but inevitably, they turn out to be UK gallons and then I feel better. I DID record a smidge over 24 mpg (US) on a trip once; but it was literally top up, on to the freeway, 70-80 mph for 4 hours, exit and top-up again. The kind of drive you describe is close to my daily commute in kind, if not distance - 100 miles round-trip and largely freeway. I topped-up about 5 miles from the house Friday morning and did an oil change Friday night, so a bit of idling for leak-checking and what-not. I'm showing 93.4 miles and an avg trip speed of 41. I reset at every fueling stop - particularly important now that my fuel gauge has started going to E at every top-up. It comes back....over time....but I've lost faith in it. Long term I average 19.8 -20.2 or so on this commute, depending weather and traffic conditions, so I reckon the supercharger is costing you a bit - as I posted in another thread; "Jaguar didn't fit that extra pump because it burns less fuel..."

I know if I'm nearing 430 miles, I'm getting close to fumes. I have a mental-reminder set to top-up by about 350 now that I've lost faith in the fuel gauge.

I had a nice surprise driving my '97 XJR 145mi to meet it's new owner. Cruise set at 70mph for the duration I calculated 27mpg when I topped off at the pump (trip computer showed 24.3mpg) Bummer of it was it turns out the seats are higher in the XJR because at 6ft 5in he could not fit in the car without bumping his head unlike his 1995 XJ6. Anyone else ran into that issue? I just assumed the XJ6 and XJR6 were the same as far as the body size goes.

Did he try adjusting the seats? I'm almost 100% certain they do rise and fall.

The seats do rise and fall. The other issue could be the sunroof, if fitted, as it reduces headroom by an inch or so. If his car didn't have one and yours did....