Two days ago I did a 75 mile round trip to meet a friend for lunch. I filled up with gas and reset the computer. It was in the low 30's F outside and I'm using a 10% ethanol mix here in NJ. About 10 miles of that was on local roads with half a dozen stop lights and then I'm on route 80 for most of the trip.

Naturally the gas gauge reads close to 1/2. Such precision instrumentation on this car ... BTW, when I filled the tank it read very close to empty certainly under 1/8. It took 11.1 gallons to fill it. Maybe Jaguar gallons are different?

I didn't dawdle in the center lane all that much but I also wasn't having a ton of fun either. Just a cruise at 65-75mph but certainly not the most efficient drive as with cruise control turned on.

So when I get back it says 19.1 mpg as an average. It's really hard to achieve anything in the 20's with this car except on open roads with no traffic.

Then I drive 3 miles locally round trip to pick up my wife and the average drops to 18.1 mpg. The instant readouts during this short drive are 9-11 mpg.

For an STR does this sound normal?

I am thinking that since your gas guage is off its throwing off the trip computer. You arent far off from what I get. I average between 16-18 around town with little getting on it. With your ghage being off, I wouldnt rely on the mpg computer. Try figuring it out manually and see what you get.

I've found that the colder it gets the lower my MPG, up to 2 MPG lower than what I normally expect. Ex: my Winter commute is averaging 18.5 MPG and my Fall commute was right around 19.8.

MPG is accurate. I get 9-11 around town.

Yes your petrol mpg will be worse when cold, but the situation is not as bad as the diesels; those petrol engines warm up faster.
On the other hand, in the real world of brim-brim vs. miles covered, don't expect a light fuel consumption from a car weighing almost two tonsUK and powered by a 400horse motor. Yes, 400horse under your foot feels almost sexual, but you're gonna have to pay for it!
Leedsman, with 2.7D S-type.

Granted mine isn't an STR but I've found my avg to be pretty accurate--I have a combination interstate/in-town commute in generally wretched traffic so lots of stop-n-go even on the interstate.


What makes me laugh is watching this one go all over the place --WTH is inst fuel anyway?

Supposedly, it's an estimate of your current MPG at that second, rather than an overall average. Coast down hill on a highway and it'll say 99, but floor it and you can watch your MPG go as low as 2!

In my car, I've noticed on a full tank that the first 1/4 seems to drop much quicker than the rest. And even when I'm on empty (to the point where it says my range is 0) I've never filled up with more than 15 gal.

There is a few gallon reserve that's not shown on the gauge or calculated in the miles to empty.

This is probably why we arent far off cause you're in a much colder climate then I am. So mileage sounds about right.

Yes me too. This has been covered a bunch of times on here. But the last couple of times even with the gauge issues the MPG calculations have been dead on compared to my hand calculations.

We seem to get the same percentage of ethanol here summer vs winter and now that it's 10% all year long (has been since before I bough the car) perhaps this is normal mileage?

That's what I'm asking about. If my mpg numbers are accurate (I believe they are) is this what everyone else is getting?

And please only STRs.

Thanks guys.

Bob S.

FWIW, on a recent business trip to CA, a couple of pretty sharp engineers I was visiting told me they lost 20% when they took out MTBE or whatever their latest bright-idea regulation is.

Your numbers look really close to my STR. Numerous times I have wanted to see how high of an avg I could get on a tank of gas, but haven't made it yet. That right foot pedal is just way to tempting to stomp on and the avg plummets quickly then!.

When I had the loaner '10 Supercharged 5.0 XF for 1.5 months it was avg from 17 to 22 mpg and I was running it hard. Holy cow that car was fast.

That must be tough. Going from 470hp back to a measly 390?

inst = instantaneous i.e. over a very short time

not meant to be useful
but can be amusing / interesting

MY 05 STR also did badly on the MPG.
i average around 18 MPG with mix driving.
and when my tank is empty (on the gauge) then fill up only to around 13 gallons instead of 18 gallons which is the size of the tank. (so i do get rip off)

but STR is fun to drive..

...Blame it on the Ethanol for inaccurate readings on the gauge.

Quixotic mysteries?

My 19.1 was logged mostly during what SHOULD have been efficient driving. Oh well it would seem that it's a very hard car to drive frugaly. My Turbo cars are easier in that respect. The trouble is I never want to be frugal when I drive those.

What's the point of that? It's like having a gorgeous girlfriend and deciding you won't have sex with her because ????

Oh that's right, gas is heading north of $4 a gallon here in the USA. If only our num-nuts of a POTUS had pulled a Noriega capture on Gaddafy perhaps this mess would be no where near as awful as it is? Just parking a carrier out there would help. But I digress ...

I wonder if there was a way to disengage the supercharger would it then run more frugally without a map change?

The little 2.3 liter Mercedes "Kompressor" Motors do this. They had a clutch on the S/C pulley similar to what most cars use on the A/C pulley. They would lockup the pulley based on a bunch of factors, such as the TPS, engine RPM etc.

A lot of the guys that wanted more power out of them, would hardwire the clutch to permanent lock.

I don't know or think that this is a viable idea on the STR... But just food for thought.

Take care,

George

I get 24 MAX on the highway, windows up, no A/C, and TRYING to feather the gas. Normal highway driving is around 22MPG. I dont even want to talk about "city" driving.....terrible.

I have read on here people getting 26ish, I dont know how that is possible whatsoever.

Ill take that kind of gas mileage on a 8cyl supercharged vehicle ANY DAY.