Just came back from our 1st trip with the "Big Cat" (05 XJ8L) and was pleased to get 26 MPG @ 80 MPH over 540 miles per tank! Such a nice car with the added benefit of decent mileage, long range and good performance! I get around 22 MPG during mixed Hwy/City driving. A small price to pay for the quality of the car! Just turned over 100K on the trip!

 

I get 22-25 with normal day-to-day driving with around 400 miles to a tank of gas (before the warning light comes on). I get around 28-30 on the highway.

 

Went on a 215 mile round trip today, Northen NJ to north central CT and back. Basically highway for 90% of the trip but I was driving like a maniac the whole way; running with the 80-90mph traffic whenever possible including the run up 684 behind a Suburban at 90-95mph.

25.1MPG on the OBC average, door to door. OBC average 69mph.

When I got home I topped the tank off, it took 8.6 gallons so the computer seems to be just about dead accurate.

 

We averaged 31.1 MPG from Dallas to Dauphin Island, AL last summer, A/C on full blast and doing 80-90 MPH almost the whole way. Just incredible.

 

Well that is interesting..some over 34 MPG is impressive for a large car. Updating your car Oldmike will cost the $76 plus but you might have covered more miles.
My Holden/Vauxhall Combo vans cost nearly that to fill it here.

 

Doesn't the altitude and petrol grade (RON) number have a lot to do with the MPG as well?

The higher the altitude the better the fuel consumption especially if you are using a RON of 93-97?

Each state that I have ridden through (USA grand Tour this year) has a different RON level than the one before and of course there's the ethanol issue to boot.

However, in Texas using premium gas (93 RON) mostly Highway at 431ft above sea level I get 23.5mpg at 70mph average.

 

We picked my my wife's '03 XK8 4.2L in Utah and drove back so Spokane together ... I was in my '07 Corvette Z06. We carefully monitored our fuel and 700 miles later when we arrived home the XK8 used 0.6 gallons less than the 505 hp Vette. The onboard computer on both cars was reading 29.x mpg. Not too shabby. Our '04 XJ8 4.2L gets similar highway mileage.

 

Fuel sold in North America is rated by AKI, not RON.

Peak performance on these cars is achieved with 91 AKI so using anything higher is just a waste of money.

Overall performance is reduced at higher altitudes due to the thinner atmosphere, as a result a lower octane gas is required. Typically 89 AKI is sold as 'premium' in these locations and 85 as regular. Using higher octane than that is still a waste of time.

 

Should you not get better economy at higher altitude. Since your oxygen sensor is reporting less oxygen, your ecu is going to lean out the mixture. This will reduce your power, but improve your economy.

My other thought was density of the air. Drag seems to be directly related to air density, and at the altitudes I had been been running (4000 - 10,000ft) the air is 16-20% less dense than at sea level.

Less drag, better economy. Leaner fuel, better economy.

Research Octane Rating (RON) is the European version of AKI i presume, me being English and all!!

Plus it was on this....

 

Less air and less fuel means less power available which means a heavier foot on the pedal to get the same job done. Difference in drag is negligible as moderns cars are pretty slippery in the first place.

 

The difference between my old XJR6 and new X350 is chalk and cheese the X350 is so economical
I was worried at first it being a 4.2 V8
I constantly get 800 k,s to a tank best I could get with the XJ6 was 600k,s to a tank

 

A few weekends ago I went on a quick road trip from Dallas to Austin TX. 200 mile trip each way. On the way back to Dallas I got 28 MPG at 70 MPH. Not bad for an 04 XJR. By the way I filled up with Chevron 93 Octane.

 

A couple of weeks ago I topped my 2007 VDP off with 93 octane in Indianapolis, reset the trip odometer and drove to Columbus, OH and back. I stopped just east of Indy to top off again. The computer said I averaged just over 32 mpg. Dividing actual miles by how many gallons I topped off with calculated out to 30.2 mpg. All interstate driving. Mostly cruising around 75 mph.

And to think the new XJs have gone to a supercharged V6 for fuel efficiency and state they only get mid 20s on the highway. I will stick with my V8 thank you very much.

 

You cant go wrong with the upgrade! Good luck..

 

"And to think the new XJs have gone to a supercharged V6 for fuel efficiency and state they only get mid 20s on the highway. I will stick with my V8 thank you very much."

Have to agree Greg,plus the V8 sound is worth a bit more too.
Interesting point brought up is the amount of fuel used to the actual amount the car takes. Often mine is almost 100% correct and other times it is underestimated (but still in the ballpark)
Still impressive fuel economy for a full size V8 car.

 

I live in a small town, and my local journeys, including slow trips down narrow, winding country lanes, was 16.6mpg

This week, while away, I got a flat tyre, and had to drive home at 50mph on the narrow spare, almost all motorway at an even pace. I got 36mpg for that.

I think that when I turned the aircon off, the "instant" mpg improved by one or two.

Mine's a 4.2 Supercharged.

 

I get an overall average of 20.5, however the guy before me had managed 25 over the previous 8000 miles, on a run I have seen 32 and am sure it will do more on a long run on open roads with little traffic etc. Most of my mileage is short town stuff with blasts on the dual carriageways, regularly red lining etc. and I use sport mode, it's fun and compares well to other lesser cars used in the same way, so i'm not complaining. figures are mpg (british).

 

Latest tank of mixed town, country roads, and some highway, came in at 19.58 miles per (US) gallon. (93 octane with 10% EtOH)

Over the last 750 miles, I have averaged 19.21 mpg, just banging around my local area. Anywhere from 30 to 80 mph, with 80 not very often or long and only on Interstate Highways. That is automatic reckless driving in CT., and the Staatspolitzei drive unmarked "stealth" vehicles.

 

Drove to Branson, MO this past Saturday and got 32 mpg on the flats at 70 mph.