Completed a couple of local trips this week. 400 plus miles included a couple of steep 6,000 ft. passes. Mostly at 85 plus MPH wherever possible. Average MPG 26.5. 2005 VDP. Impresses me. Also the speedometer is spot on when compared to smart phone GPS. What are other people getting?

Alas most of my time is around town and relatively short jaunts. I get between 12 - 16 mpg. Not great but given my trips not unexpected. (Almost double what my Ford pickup gets for the same trips.) If I want to up the average calculated by the car I only have to make a quick 20 mile run up the hill and back. Then the average goes to about 24.

During my first month of X350 XJR ownership, doing roughly the same kind of journeys, I'm averaging 21mpg, which is 4-5mpg more than the average my X308 XJR was giving. Those are proper English gallons too, none of this litre/100km nonsense.

I do mostly city driving, right now my average mpg is showing 12.3. Highway driving I might see 16 to 17 mpg. I think I can get a little better if I fix some bugs. Otherwise I knew what I was getting myself into when I swapped pulleys and got the tune.

I am getting 24.5 mpg on a daily trip of 85 miles highway and 25 miles city from my 2005 VDP. Better mpg than my Ford Ranger 3L V6!

I average 23-24 mpg in 50/50 mixed driving. On long trips I've achieved 27-32 mpg with no special driving. Stays in high 20s when the car is fully laden with 4 passengers and luggage. Easily get 30+ when it's just me plus 1 passenger. Typically 70-75 mph.

I average about 21MPG but do drive over some rather steep mountains and some decent traffic too

About 17 mpg around town and 26 mpg on long highway trips with some hard acceleration and speeds up to 90 mph when its safe.

Thats with all my mods most of which, as with most mods, do not come into play unless near WOT.

I am in the UK, so slightly different...
Best I have had is 38mpg (32mpg us) at a very gentle run for 50 miles

Last Sunday I did a 310 mile journey, 34mpg UK calculated.

I am convinced that the power mods have made the car more economical as a by product.

Was very pleased with when mileage was much better than expected, on highway IF the cruse is LEFT set @ 60, it goes past 30 mpg, on both XJ8 's.

Mine has now improved a bit. I usually get gas at a local Chevron. The last time I filled up I took a chance on the local box store (a Costco). So far my average is actually up to about 13.6. I also added a can of Sea Foam, just like I do in my motorcycle every few months. (The Sea Foam really does help my motorcycle so I figured it can't hurt to help clean my fuel injectors a little since the Costco gasoline is "Top Tier" but does not have Techron.) Perhaps next time I will try a Shell or Union 76? Or at least a different Chevron. Gas is getting over $.4.00/gallon here now.

Never used Sea Foam in anything as it was intended for two stroke boat motors, & my bikes, & cars are not.
My newest bike is a 91, still equipped with carb's, so i have used A T F with good results to keep all those small openings free from clogging up. Works great on opening partly clogged idle jets on used ones that have their idle turned up to keep them running. Can't use it in cars whose states have emissions checks though, as the converter will gum up on you, & a bypass won't work.
Costco high test works well in all my motors, they sell the Techron fuel injector cleaner in the store, it was on sale last month, this month it's Mobil one motor oil.

Are you thinking the Sea Foam would harm the converters? That has not been my experience. Although I love Techron as an additive and use it very often, (I can buy Sea Foam in gallon containers and also use it to stabilize the fuel in my other vehicles ) it is often more expensive. I only go through a tank of gas a month on average in the XJ8 so I have more concerns with condensation in the tank as I get toward 1/2 a tank.

No ATF will harm, or plug them though! But A T F works well for old bikes with carbs.Condensation is a problem

When I first came to OZ nine years ago when on the (new) freeway and cruising at 100 kph (62 mph) I reset the computer to zero
and after driving for half an hour the computer was reading 44.5 mpg.Cruise was on all the way.

Artificial but interesting.

Now i get 24.5mpg mainly town driving-I must try and improve that.

I forgot to mention that my mpg figures are the same with 87 Octane and 93 Octane gas. I am using meth/water injection.

I'm averaging 23 mpg UK gallons in the first year of xjr ownership.
Highest I got on a long motorway run was 26, worst 20.
That's based on my calculations, the trip computer is always about 10% below what really happens.
Mostly shortish journeys, I live in the countryside about 6 miles form the nearest town.
No modifications