For some reason my Jaguar XJ8 gets poor mileage around town. I think the computer is showing 14.9 MPG. A couple things I noticed, the computer shows my average speed and only like 16 mph also on my car I have 18 inch rims and I think it came with 16. I change the air filter I changed all the plugs any thoughts as to why this may be?

 

Sounds normal to me. By a strange coincidence, the EPA estimated fuel economy, around town, for your car is......15 mpg!

The reason for your poor fuel economy is that you have 4000 lb vehicle with automatic transmission and a 4 litre V8.

What were you expecting to get? Why did you think you'd get far more than the EPA estimates?

 

+1 Sounds right, especially with the hills you have around the 'Burg.'

I am getting 15.8 around my town (which is flat) averaging 19 MPH, in the Winter. Drops another Mile Per gallon during the Summer when the air is on. Running about 24 MPG on the open road.

 

I've had my 1998 XJ8 4.0L long wheelbase Sovereign since it was 9 months old with 9000 miles on the clock. It now has 115000 and the fuel consumption over all those years is 23.2 mpg (UK gallons). That includes about 6 months when the O2 sensor had failed and it dropped down to about 19 or so. On a clear motorway run it will be 26 - 27 mpg.

 

The wheel diameter in itself isn't a factor, but the overall diameter of the wheel/tire combination is.

Since all the computations are based on the transmission and differential gear ratio and a given rolling circumference, any major changes throws all that out the window. Your actual road speed, mileage, etc. can be radically different from what the car "thinks" is going on.

For instance:
The computer determines your wheels have turned X number of times on this trip, while using 1 gallon of fuel, so your fuel mileage is Y.

While in actuality (a radical example just for illustration), your wheel/tire circumference is 20% oversize. So you've actually gone a bunch further on your gallon of gas than the poor car thinks.

Yet another reason I don't like ghetto wheels and rubber band tires.

 

The wheels that are on the car are Jaguar Rims XJR Pentas

 

I've generally found the EPA estimates, for cars that I've owned, to be pretty close. The only exception is my Passat TDI which is rated 33/43, but will get 50 quite easily on a long trip.

 

Now this may or may not be an issue, although the XJR uses the same diff ratio, it uses a totally different gearbox. A Mercedes gearbox compered to the ZF gearbox in the XJ8.


Were 18 inch rims an option on a normally aspirated XJ8?

 

On my truck, I run 235/75/15 tires. The tires it came from the factory with were 225/70/15. Using a tire size calculator online (google it, they're everywhere), it says my 235's are 4% larger than the factory tires which the Odo and Speedo should be calibrated for. So when I figure MPG, I always add 4% more miles traveled.

Punch up your tire size you're currently running -vs- the Jags standard issue and get your % difference, then figure it in. You might gain a MPG or two.

 

14.9 isn't far off of what you should be getting. I do a lot of highway travel, but when I fill up, and stay in town, I usually am at about 15-15.5 MPG, when I am on the highway with little or no traffic doing 55-70 I can push about 26MPG however, my combined average is about 18.7MPG which to be honest, isn't great when you think about it, however, at the same time it is really good when you consider A: The age of the vehicle, B: The size of the vehicle and, C: The size of the engine.

I was looking at some old Merc's, and Bimmers with inline 6 cylinder engines, that would get about the same gas mileage. Furthermore, my father's 2010 Chevy Cobalt (little 4 banger) gets about the same fuel economy as me on the same fuel. But for its size, it isn't too horrible. I usually get about 320 miles (US) to the tank.

 

I usually drive close to a 50/50 on a tank of gas. That's 50% city driving around town and 50% freeway driving. Around town I get about 18-19 on the open road with cruise set at 70 I get about 23.9-24.9. Avg per tank in around 22.6 MPG.