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MG MGF Technical - False Economy?

Morning Folks

I am puzzled (and a little concerned). Since my car went in for brake repairs a few weeks ago (Stoneleigh weekend) the fuel economy seems to have dropped by about 50 miles per tank. I used to get 400-420 (topping out at 450 once) all the time, now I am lucky to get 370. This seems very odd to me. I have not become a more aggressive driver and still am doing the same mix of short and longer journeys. The only thing that I can think that also changed around that time is the tyres. Is it possible that a change in tyres could lead to a loss of fuel economy? Or could the garage have done something that has affected it? There doesn't seem to be any resistance from the brakes so I don't think they are binding.

I am genuinely puzzled by this one. Any suggestions?


Phil Brindley (Work)

Check your tyre pressures first!
Will Munns

Dragging brake?
Tim Jenner

Phil - I have to say I find your "normal" figures extraordinary. If I remember correctly the F has a 50ltr tank. Presuming you don't normally run it totally out before refuelling lets presume a normal refuel is around 45 ltrs. Thats 9.9 gallons. If you are doing 420 miles each time thats an average of 42.42 mpg. Very very good for an F. Even on 400 miles that is still 40.40 mpg.

I have had my F now for 8 years and the most I have EVER got from a tank full is 378 miles and that took 46.77 ltrs to refill. That works out at 36.74 mpg, which for me is reasonable but pales in comparison to your numbers. In all my fillups over those 8 years the times I have had over 40 mpg is less that 10 and those have been on long, very gentle summer runs. My personal average over the 8 years is 36.22 mpg.

So in answer to your query I would say that nothing is wrong with your F. You have just joined the rest of us.

regards, Alan

I'm not surprised by the figures.

I used to get 400+ miles out of a tank regularly but was doing 52 miles per day for work mainly dual carraige ways and normally sat at 80mph and did not hang around when moving up the gears as my moto is get upto speed as soon as possible. Okay I normally filled up with 50 litres each time and once managed to fill up with 53 litres so was pushing the tank right to its limit. I dont think I have ever got less than 350 miles out the tank except when doing track days.

As for the drop in consumption check tyre pressure as already mentioned check pipes in engine bay maybe a air leak or something messing around with the mixture, simple things like this can have huge affects on engine performance these days as sensors detect everything and adjust the mixture to suit.

Tom Randell @ work

Hmmm, I'm running at 28 psi all round, I'll try upping that to 30 and see if that makes a difference.

The 450 mile run was extraordinary. And the re-fill was 50.4 litres so very close to running out. I usually put in 48-50 ltrs. maybe I should start filling up a little earlier :-/
Phil Brindley (Work)

Wow thoes are stunning mpg. I am currently doing some mpg testing. I do the same 50 mile trip every day to work, some motorway, some A roads. So far with RON 95 fuel I get 28.6 mpg whereas I get 30.4 mpg with the higher octane rating.

So at best running the car from full to empty I could only get 330-340 miles from a tank.

Steve Ratledge

My main daily drive is on the A40 into Wetminster and then out to W London. Normally nose to tail traffic.

I filled up last night, got 49.7 ltrs into the tank and reset the trip meter. I had done 263 miles since my last refill - eeeek!
Billy Bob

My fuel consumption figures are up there with Phil and Tom. I would be disappointed if I got less than 360 miles out of town; mixed fast A roads and a nice bit of 3rd/4th gear twisty stuff. In France I expect to be around the 420 - 440 miles. I can also get more than 50Lt when I fill up even though MG claim 11 gals/50Lt - how much does that huge filler tract hold? I have a fuel receipt here that shows 53.22Lt!!

A bit like my bike consumption; normally 52 mpg day in/day out; 2-up with luggage touring France and Spain jumps to over 60 mpg; on my own to Cadwell Park at six in the morning it can drop to below 35 mpg but my pulse goes up in sympathy.
sprinter @home

Phil, what airfilter have you got fitted and when was it last replaced/ serviced/ cleaned?

K&N 57i filters clog severely after about 10k miles/12 months (depending on road conditions) - and the things you notice when this happens is deteriorating fuel economy and deteriorating engine performance...
Rob Bell

Did you change the tyres (one type for another) or just the pressures ?
Mike (@home)

Aaah, could be the filter. I have a K&N and have had for app 8-10k. What with the recent weather I bet it is getting a bit dirty. Anything specific to the cleaning Rob? or is it just a case of run it under the kitchen tap and hope Lorna doesn't spot me ;-)

Will still increase the tyre pressue too I think.

Phil Brindley (Work)

Mike, swapped from Avon ZV1 to Toyo Proxes S1.
Phil Brindley (Work)

Phil, I'm sure the K&N requires something a bit more sophisticated for cleaning and may even require a light oiling afterwards.

I'm sure Rob will be along soon to inform and teach. Until then you could look on the manufacturers website?

sprinter @home

Damn, I've just swapped to TOYOs: does this mean that my consumption will now plummet
sprinter @home

I thought it was a 50 litre tank + 5 litre reserve = 55 litres total.

I've put in 54 litres (bleedin' lucky, the fuel guage got stuck, i gave it a tap and watched in horror as it dropped right to the bottom - must have been sucking fumes when i got to the next garage!)

Normal mileage from a tank full is around 350 miles, most i have ever had is 390 miles, but i chickened out early and i reckon there was another 10-20 miles worth in the tank. The 54 litres refuel was only 370 miles ish - i do very few motorway runs, 90% of my driving is B roads and lanes.

Scarlet Fever

Phil, Sprinter's right - a little bit more involved than just running under a tap - but not much!

You'll need a re-oiling kit from K&N. To clean the filter, you do need to run it under a cold tap and clean the filter in water. The part of the filter that actually does the filtering is the oil, not the cotton! Unfortunately, this will be washed away with the dirt. So once clean, you just need to penetrate the cotton matrix once again with the characteristic red oil :o)

Job done.

I bet that when you look at the filter that one side will be completely caked in dirt... Just shows how well those cooling ducts work! LOL
Rob Bell

If a dirty filter is not your cause (and it very well might be) the rolling resistance of you new tyres could (I only say could) be a factor in the increased fuel consumption.

Mike (@home)

OK. I'll try the tyre pressure first (as have just filled up again) and see if that makes much difference. If not I'll get the filter cleaned.

Thanks for the input guys, I'll keep you posted.
Phil Brindley (Work)


>I've put in 54 litres (bleedin' lucky, the fuel guage got stuck, i gave it a tap and watched in horror as it dropped right to the bottom - must have been sucking fumes when i got to the next garage!)

Methinks you've seen too many old films where they give the fuel gauge in air-craft a tap and it goes up a bit. Normally of course it goes back down a few moments later. ;-)
Paul Lathwell

OK, so the tyres are now up to 30psi all round. I'll see if that makes much difference.

While I was out pumping up the tyres I had a good look down the inspection cover and spotted what looks to me like a far too shiney thread to have been empty for long. Can anyone tell me if this is right or if not what should be in there.
Phil Brindley

Well, my journey is normally around 80 miles a day....most of it on the M4 and around 80mph.

I usually get between 390 and 410.....but the most so far is 430. Not sure how many litres I put in, but I will check next time.

I have to say that the F does seem very economical on fuel.....and I have a rather heavy right foot....:-)
Steve Green - KX51 NUH


When you clean the filter make sure you allow enough time to allow it to dry before you oil it. Don't attempt it late on a Sunday night.

As for your consumption figures they sound very good anyway. I normally acheive about 35 to 40mpg in the F. As for the MINI I have to use daily now I usually get around 25mpg:-( If I drive it very slowly I can get 28mpg!

Neil Stothert

You either need someone to go out and look at there motor to see what's what re: the photo/shiny thread, or someone with a very good memory. I'm in London at present, and won't be able to look at mine till tonight.
Mike (@home)


Just been out for a look.......mine's the same.. shiny threaded hole ( not a stud..right ? )


Phew, that's ok then. Thanks Grahame.
Phil Brindley (Work)


either it's ok, or you and I both have something vital missing from our cars......!

Anyone else looked yet ?


Phil, Did you get chance to follow up on Tims idea of a dragging brake?
R Baker

Everybody, surely the only to measure any vehicles fuel consumption is to keep an accurate record when purchasing fuel, mIleage covered etc and work it out at home after say at least three purchases
This will give an ACCURATE resultin MPG. litres/100lms or Kms/litre what ever turns you on
All this , how much I get out of a tank full is not accurate enough,and just a load of codswallop to base any line of rectiction on.
Eric Hayes


You are correct of course. Unfortunateley, I am a Virgo and an Accountant with all of the obsessive record keeping that is involved.

I have fuel/milage records for all my motorbikes from 1985 onwards (average4 for nearly 20 years = 54.27 mpg).

I also have the records for my MGF although I am only just approaching 2 years with this one.

I keep a rough and ready eye on my vehicles most days: is it running as well as yesterday, is it a bit noisier, did I seem to use fuel a bit quickly, has the oil level dropped?

However, as this bbs can be a bit like a night in the pub, a straight "how big is yours" over a pint or two is always welcome.

I don't believe that anybody on here would really go out and spend money installing or changing something just because of a casual comment.


I have been keeping an eye of my fuel consuption ever since I got the car this time last year. In all the months I have been driving around I have consistantly gotten well over 400 miles per re-fuel and each re-fuel is between 47 and 50 litres. Suddenly (last few weeks) I have been experiencing 350-370 miles for the same amount of fuel. This, to me, does not seem like a whim and suggests something is wrong. I am not just going to go out and spend money on suggestions of other people. As such I am trying the tyre pressure first (free) then cleaning the filter as per Robs suggestion (>10) then if nothing has changed I will have to get the brakes looked at again (this could well be into the hundreds).


See above :-D



How about the big fuel companies taking something out of the fuel (or putting something in) so we use more of it :-0
wouldn't be the first time big companies have added a little something to their products !
Mike :-)
Mike (@home)

Well when I said the most I had ever got was 430, this has been beaten today.

Just checked, and I have done 450 miles....with still a little left. The journeys have been all motorway, but still, this is still pretty good going.
Steve Green - KX51 NUH

This thread was discussed between 23/03/2004 and 26/03/2004

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