Poor gas mileage

[doctormap]

I know this sounds like a silly question but am I being unrealistic in expecting better gasoline mileage? I usually get between 13-14 miles per gallon on 91 octane gasoline. I have read on Jaguar forums of people getting much better mileage, like 25-26 mpg. Is that possible? And if so, what could be done to achieve that? I just had the car in the shop for repairs and upgrades and the mechanic says he tuned up and repaired the carbs, so what else could be the problem? It runs very nicely, the only issue it has right now is that sometimes the brakes tend to hang up when starting up from a traffic light.

[abowie]

Sounds like my car.

[kgnich]

Depends on usage of course. Mine does 18mpg for short journeys or up to 24mpg on long runs. US gallons are less than UK gallons too!

[christopher storey]

Your 14 mpg is equivalent to 17.5 mpg on imperial gallons . Although some of us with short wheelbase cars, unmodified engines , and using standard UM needles on 3 SUs, rather than the richer needles that many people use , get as much as 23 mpg , your figure does not sound as though it is so far outside the envelope that there is something wrong with the car, particularly if it happens to be an automatic

[malcolm]

I'm on a standard set up with needles etc. In imperial measures, I average 22mpg overall, with a mix which includes a fair amount of long runs. If I stick to speed limits on motorways (I did it once as a test!) then on a motorway run I can get 25mpg. Around town I'm probably down to 18mpg.
I drive a manual 4.2 series 2, 2+2
I don't thrash it all the time, and I imagine the right foot would dictate the mpg to a considerable extent. Also, an automatic would probably use up at least 10% more fuel, maybe more. The Borg Warner 3 speeds weren't very efficient.

[PeterCrespin]

15 mpg US S2 FHC

Depends on gearing, right foot, type of roads travelled. Your car probably has twin Stranglebergs like mine plus an emissions distributor, and low gearing.

It's an E-type. If you could get, say, 18 per US gallon you'd save what, a hundred dollars a year maximum? Forget it. Pour fuel in and the smiles come out all over.

Pete

[Nickleback]

15 mpg US S2 FHC

Depends on gearing, right foot, type of roads travelled. Your car probably has twin Stranglebergs like mine plus an emissions distributor, and low gearing.

Me too, averaging 16 mpg as above with T***o Momentum 99 octane fuel

[malcolm]

15 mpg US S2 FHC

It's an E-type. If you could get, say, 18 per US gallon you'd save what, a hundred dollars a year maximum? Forget it. Pour fuel in and the smiles come out all over.

Pete

It's not the cost that would worry me, I'd simply be thinking about whether or not something was wrong, and would it go better if that was fixed etc

[Mark Gordon]

" It runs very nicely, the only issue it has right now is that sometimes the brakes tend to hang up when starting up from a traffic light."

Mark, I agree that you should get better gas mileage. Does your mechanic really understand how to tune a car with (two?) carburetors? I get around 21mpg with 3 SU's and a 3.54 ratio. Strombergs shouldn't be that much different. But a bigger and more immediate concern is the brakes failing to release. It's a fairly common complaint, one that I dealt with for a long time until I finally replaced my master cylinder when it started leaking. One quick test to determine if the problem is with the hydraulic or the servo vacuum side (more likely) is to disconnect the vacuum line connecting the vacuum reservoir to the intake manifold, plugging the connection to the manifold so that you don't have a vacuum leak. Then go for a careful test drive. You'll still have unassisted hydraulic braking with just a little more pedal pressure needed to do the job. If the hanging up disappears, you know that the problem lies in the servo assist side. The most common problem is that the small piston located on the front of the MC which connects to the reaction valve. It seems that that piston hangs up when the brakes release causing the vacuum to still be applied to the brakes, thereby still applying light force to the hydraulic side of the system and pressure on the brake caliper pistons. The fix is to remove the reaction valve, pull out the piston (noting which way around it goes), apply some red rubber brake grease to the piston, reinstall and then hook everything back up.

[MarekH]

You can discount the brakes very quickly by seeing whether they are hot or not.

kind regards
Marek

[PeterCrespin]

I think we can accept his word if he feels the brakes are hesitating to release. There's nothing else that gives the symptom of failing to roll in neutral with the brakes off? The hot brakes test is helpful but a little care is required, since of course if the test run and final stop from speed use the brakes, they will be hot anyway. Coasting to a halt after a mile or so of fast unbraked use helps higlight any issues. An IR thermometer (or spitting on the discs to see if they sizzle) helps save fingertip skin...

Of course, dragging brakes also hurt mpg if it happens more than briefly from rest.

Pete

[doctormap]

First, thanks so much for all of you who have offered possible solutions to the problem. I am convinced that the poor gas mileage stems from two issues. One, that the two carbs are not set up correctly. I believe that the mechanic is competent, but he may not be as competent to adjust the carbs as much as it needs. Two, the brakes failing to release properly after a stop. I have tested it frequently during my in-town driving and frequently find the car will not roll freely when my foot is off the brake pedal. I don't feel competent to mess with that myself, not having any "red rubber brake grease" on hand, so I will take it in to the shop and get the repairs done by my mechanic.

By the way, my driving, every day, is about half highway and half in town.

[Mark Gordon]

Mark, you aren't originally from Kettering, OH, are you?

[doctormap]

Nope. Not from Ohio.