MPG figures confusing the hell out of me!

[Rubce]

Hi All

As some of you will remember our stag decided to flood the engine bay with fuel last September when one of the needle valves stuck open. I swapped the carbs out for the later type with overflow pipes and overhauled them with new neddle valves, gaskets floats etc. I then attempted to set up the carbs, this being the first time I had ever had to do that job from scratch, The car runs fine but drinks fuel when pottering around and returns somewhere between 16 and 19mpg which is rubbish compared to what I was getting before the carb incident (25 to 40mpg). The car is a MOD with 3.45 diff.

Yesterday we had a family day out in the Stag and went for a run to Haworth which involved a 160 mile journey with about half of it being on motorways or duel carriageways. This is the first "long" distance run the stag has done since that incident even though it has racked up over a 1000 miles in that time. So I was interested to see what MPG it would return on the trip. Having just worked it out I am pleased to see that she returned 37mpg which I consider very good given that some of the run involved winding lanes over the moors near Halifax and Haworth.

So I am puzzled as to why she is giving great mpg on long runs and really terrible on mixed/short runs. Any suggestions on what I have done wrong or need to adjust? I suppose I should go and purchase a colourtune from Halfords.

Regards

Bruce

[Andrew S]

Hi Bruce
I have a feeling that you speedo is out as there is no way that you can get 37mpg. With you diff at a different ratio, this could explain it.

[Rubce]

Andrew

Why do you say I can't get 37mpg? The speedo under reads by 5mph according to the satnav. The GT6 (2 litres straight 6) returns 40mpg on a good long motorway run so that shows old cars can give decent mpg.

Bruce

[Andrew S]

I stand to be corrected but the best mpg I have heard of was 32mpg with a 4 speed conversion

If you are getting that on a run, well done

[iani]

I had the carbs professionally rebuilt on my MK2 GT6 and the consumption went from 17mpg to 32mpg, I've never heard of anyone getting 40mpg out of a GT6, you must be getting some special fuel up there

[Rubce]

I stand to be corrected but the best mpg I have heard of was 32mpg with a 4 speed conversion

If you are getting that on a run, well done

Andrew

Your talking about an auto which are well known to be thirstier than a manual overdrive, especially one a long legged diff.

Bruce

[jleyton]

I DREAM of getting 37mpg

20mpg normal - which is a lot of short journeys and hill climbing. 25 mpg ish on a run.

If you can find the secret to your 37mpg I'd patent it and sell it if I were you !

Cheers

Julian

[Andrew S]

Hi Bruce, I hear what you say, but the 4 speed auto is the same as a manual, I believe.

[Kevin Garrod]

What distance and circumstances are you getting 37 mpg? Just to compare I could get a reading of over 90 mpg on the trip computer when I had my jag, no throttle and downhill or low single figures when I booted it.
The average is what counts, I can get over 300 miles out of a tankful in the stag, but 260 ish is more usual.

[Rubce]

I had the carbs professionally rebuilt on my MK2 GT6 and the consumption went from 17mpg to 32mpg, I've never heard of anyone getting 40mpg out of a GT6, you must be getting some special fuel up there

Ian

The GT6 normally chucks out between 25 to 30mpg. However, on a motorway journey from Cheshire to the Lake District she will return 40mpg if driven steadily at 60 to 70mph. Go above that speed and the mpg drops off dramatically. As you will know the killer is round town when the twin chokes are in operation and you almost hear the fuel gushing through them!

Bruce

[Rubce]

Hmm, when we were discussing MPG last summer a number of people on here were getting 30+mpg in their manual stags.

Bruce

[Rubce]

What distance and circumstances are you getting 37 mpg? Just to compare I could get a reading of over 90 mpg on the trip computer when I had my jag, no throttle and downhill or low single figures when I booted it.
The average is what counts, I can get over 300 miles out of a tankful in the stag, but 260 ish is more usual.

Kevin

As I said, 37mpg over a 160 mile trip.

Cheers

Bruce

[martin]

Twin chokes ? you've got Webers on a GT6 ?

[Rubce]

Twin chokes ? you've got Webers on a GT6 ?

Sorry Martin, I meant the single chokes in the twin Stromberg carbs.

Bruce

[Rubce]

To go back to my original question, why do you think there is such a huge differential between the short trip and long trip figures? Surely it can't be right that the long journey economy is more than double that of the short to medium trips?

Bruce