I have had a few cars now with a trip computer that gave instantaneous and average miles per gallon, just like the one on my 2005 XKR. I got in the habit of noting the average mpg after filling the tank and then occasionally manually checking. For the other cars I have owned, the manual check was generally spot on with what the trip computer was calculating (maybe very slightly different by one or two tenths). However, the Jaguar trip computer has been consistently higher (e.g., the trip computer might calculate 15.5 mpg when I am calculating 14.4 mpg). The percentage difference is much greater than I have seen with my other cars and it has always been in the same direction (computer calculates more than I calculate).

I know that changing rear end gears or having nonstandard tire diameters will give a false reading on distance traveled and, hence, wrong mpg unless the computer knows about these changes. This car has 20' wheels but it came from the factory that way and I assume the computer was 'aware' of this.

Has anyone else bothered to spot check the computer's mpg estimate with a simple manual calculation?

Thanks,

Doug

 

I haven't done it manually doug, but next time i fill up (today) i'll start a couple tank cycles and let you know if the 19's on mine are accurate with the computer.

Other xk8/r owners please do so as well.

 

okiedokie

 

I'll check as well with my 2000 Convertable. When I've checked with my other cars - Cadillac STS's, El Dorado, and Corvette the car computer has always been optomistic, giving a much greater mileage figure than my hand calculation.

I hear the computers are pretty accurate - counting injector pulses and all that - but my real world observations say otherwise.

 

When I noticed this I was wondering if the distance that showed up on the odometer was somehow derived differently than the way the trip computer is coming up with distance. That would be the only way that I would be getting a different result from distance divided by gallons.

Doug

 

My computer is .3 higher than a manual calculation. I've been checking my mileage long hand since I got the car at the end of November.

My 2004 Ford F250's computer shows results based on 500 miles driven. Use both A and B computers and reset A when you fill up but don't reset B until you are at 500 or more miles and look to see if there is a difference between the two.

I have not done this as of yet, I keep forgetting to reset the B computer (or even look at it).

 

I was expecting a couple of tenths difference but my difference has been much greater. For example the computer will show 15.7 and I will calculate 14.5, etc. My calculation is pretty consistently lower than the computer's by the same degree. I never think of resetting the B computer.

Doug

 

I have checked the computer vs. manual on several cars and found the computer consistantly reports 0.1 or 0.2 over my manual calculations. You have to run 4-5 tanks to get a reasonable approximation of 'full'. I get consistantly 20.6 - 20.8 mpg in my style of driving, which does not include much traffic but lots ofsteep and twisty roads. I'm happy, very happy.

 

My typical home/work commute is only about 4 miles each way on Los Angeles area streets. The mileage I am getting with the Jaguar in these condition is about the same as my 95 Corvette (about 13 mpg). When I add in the bit of weekend driving that might include some freeway and less street congestion, the mileage manages to get up to about 15-16 mpg. On the Vette, steady freeway driving gets me as high as 25 mpg but I have not had the chance to see what the Jaguar will get.

On a side note, my wife's 2000 Acura is amazing in terms of mileage. She does the same type of street driving as me to and from work with a bit of freeway driving thrown in and we consistently get 18 to 20 mpg. If we take a long freeway drive (like to San Diego) we get 30 mpg. This is darn good for a 4 door roomy sedan. That 3.2 liter V6 really sips gas even under stop and go street driving.

Doug

 

My commute is 60 miles per day. Half of this is at 55-60 mph on a 2 lane road (I work grave shift, no traffic), the other half is city streets, 30 to 40 mph. I'm getting 19.9-20.3 mpg (long hand). The computer is .1 to .3 mph high. Not off that much.

Going to take a 280 mile freeway drive soon so I'll be able to see what my mpg is at 70 mph. Can't do 80's like you can in California, Oregon State Cops will nab you and they use planes to track you around here and in Washington.

 

I found my car's 'sweet spot' at 67 mph, according to the trip computer instant mpg.

 

Reset the trip computer in the Jag and recheck. The trip computer can record up to 9999 miles and it will be calculating the fuel mileage based on all of those miles and road conditions. To get an accurate reading from the trip computer you need to reset it every few tanks of fuel or at least reset either the trip a or b and use that one for "current" mileage calculations and let the other serve as a long term mileage indicator. I find most jags computers to be fairly accurate off only a few tenths or so. One other thing to keep in mind is the computer is using the fuel level sender in the tank to make its calculations and any bad readings along the senders range of travel will affect the accuracy and sender problems are not unheard of in these cars. If the senders readings get bad enough it will set a code in the ECM that does not turn the check engine light on until it is really bad and by then you would most likely have other symptoms like the gauge showing 1/4 tank but it only needs 5 gallons to fill it up.

 

If by reset you mean hit the zero button, I do that for the A computer at every refill. I generally fill up when the tank gets to about a quarter tank and consistently fill it to full. I have refilled about 4 times since owning the car and have not yet had the computer average on A come close to my hand calculation. This is the only car I have owned with this discrepancy but I suppose it is not important.

Doug

 

I too fill up every time and reset the trip A computer to 000. On my 01 XKR the trip computer actually comes in a bit lower than my manually calculated mileage. Usually about .5mpg. So if the trip computer says 14.5 my manual calculation is usually about 15. But that may be my wishful thinking and rounding up :-)

My commute is about 12 miles with a mix of 55 mph highway and 25 to 40 mph stop and go traffic. I'm usually in the 15 to 18 mpg range depending on how often I get after it. My old Mustang Cobra was pretty consistent at 18 mpg so I figure with the heavier Jag and the supercharger 15 to 18 isn't bad for what it is.

 

It's not so much that I am all that concerned about the fairly large discrepancy in mpg so far between the computer calculation and my own calculation. What I am concerned about is that if there really is some glitch with the computer, could this have an impact on anything else?

Doug

 

Doug, checked my math versus the computer, and found my computer was conservatively high by 0.5 mpg. Apparently, she's as optimistic as I am.

 

97 4.0 vert.

Hwy at 65-85mph 400 miles (top up) 22.4 mpg actual; 23.9 trip computer

Local driving (top usually down) 18.0-19.4 actual; 19.7 trip computer

 

Well I filled it up again and did another check. The computer was telling me I was getting about 16.9 mpg which seemed about right for the mix of driving I was doing. When I calculated myself based on odometer miles driven versus gallons to fill, I came out with 15.5 mpg.

I have a theory but it could be wrong. I remember that for a previous car I owned back in the 1970s, I replaced the original rear tires (F70) with larger diameter ones (H70). This resulted in the speedometer reading lower than actual speed and also the odometer showing less than actual. I recall that to correct this, it was necessary to swap out the gear that drove the speedometer cable. These days, such a change is done by reprogramming the computer.

My understanding is that the standard wheels on the XKR were 18" and my car has the 20" BBS wheels. I wonder if there was some type of reprogramming that was supposed to be done but wasn't.

Doug

 

I'm quite new to this forum ( in fact i just joined today) but back in the day when i used to cycle to school, i had a little speedo attached to the wheel. Now bear with me here, but to set it up you had to put in the diameter of the wheel and i imagine that the cars computer is much the same.

If a larger diameter wheel (overall diameter, including the rubber too) is any larger than when it left the factory and the computer doesn't know about it, then its definatly going to skew its calulated distance travelled and then also your MPG.

Thing is, if it is giving you a higher value than you calculate, then chances are that the overall diameter is less than the stock. Now this would make sense if the stock 18" rim had a very large high-profile tire (quite likely), whereas your 20", despite the larger metal rim, probably has a very low-profile tire.

If you know the model of tire you are currently running, ie 285/30 zr20 (or something similar), then you can calculate the total diameter and compare to the stock tire sizes (shown below).



Stock sizes from jag owners manual

Lamina
8J x 17 Pirelli P Zero 245/50 ZR17 99Y Assimetrico

7-Flute
Front 8J x 18
Rear 9J x 18
Front - Pirelli P Zero 245/45 ZR 18 96Y Direzionale
Rear - Pirelli P Zero 255/45 ZR 18 99Y Assimetrico

Double five
Front 8J x 18
Rear 9J x 18
Front - Pirelli P Zero 245/45 ZR 18 96Y Direzionale
Rear - Pirelli P Zero 255/45 ZR 18 99Y Assimetrico

Milan
Front 8.5Jx 18
Rear 9J x 18
Front - Pirelli P Zero 245/45 ZR 18 96Y Direzionale
Rear - Pirelli P Zero 255/45 ZR 18 99Y Assimetrico

Paris
Front 9J x 20
Rear 10J x 20
Front - Pirelli P Zero 255/35 ZR 20 97Y Direzionale
Rear - Pirelli P Zero 285/30 ZR 20 99Y Assimetrico

Detroit
Front 9J x 20
Rear 10J x 20
Front - Pirelli P Zero 255/35 ZR 20 97Y Direzionale
Rear - Pirelli P Zero 285/30 ZR 20 99Y Assimetrico


To work this out you would need to know the original style rims on your car when it left the factory, as it is unlikely that the computer has been been updated since then. Working out the difference between the stock 18" and 20", the 20" has a diameter of 7.7mm less than the 18". Thats only about a difference of about 1% in tire circumferance, so unless you have very very low profile tires, i can't imagine that the tire diameter is causing the rather large (~10% difference) that you have reported.

Hope some of this helps out.

 

I realize that perhaps I am being **** about this but, what can I say, I am an engineer. For the last few cars I have owned that had trip computers, the computer mpg was generally spot-on with my manual calculation or, at most, a one or two tenths off. With the Jaguar, I have not yet gotten as close as I feel it should be comparing the computer with manual. Something seems amiss but I guess I shouldn't fret as long as the car runs well. As an aside, I will be bringing the car in for the J004 bulletin with entails some reprogramming (I think) and I wonder if that will change things.

Doug