Disappointing fuel economy plus fuel tank size

thor

Right people in this forum have been saying the best way to measure fuel economy is by filling up a tank and seeing how much you get out of it.
Well my tank was just over a quarter full so I topped it up by another 45 quid @ £1.19/litre to give a total tank containing just over 11 gallons.
Now given my car is a 2004 Mazda 6 with a stated tank capacity of 64L=14 gallons, I realised I was going to have a confusing time ahead.
I was going to London Heathrow which was about a 400 mile round trip almost all of which is motorway driving. I tried to keep it legal but did manage to not go over 80 mph and barely needed to brake at all instead just letting the car decelerate. The tyres were at recommended pressure, two fairly small passengers with no other loads, hardly used air con or wound the window down and yet found that I had used just slightly over half a tank getting there. In other words if I went by the amount of petrol, bought my mpg was only about 35 or 36 to the gallon. If there was 14 gallons then the figures are even worse.
Is this ok for a 6 year old, 2 litre Mazda? I am quite a bit disappointed but not completely shocked as the car is very thirsty in urban driving but I was hoping that I would get a bit more out of it on a long journey. My brother accompanied me in his 3 litre diesel BMW and says he was getting 40 odd mpg.
Could my problm be a simple case of dirty fuel filter etc or is there something else I could do to make it cheaper to run? And doea anyone have an idea about the what the hell is going on with my car's fuel tank capacity?

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Sorry about the double post. It seemed to hang and then fail when I hit the submit button so I assumed wrongly it never made it and reposted again. Once I found out I had a double post i tried to remove it by hitting the edit button but there was no option to delete( I guess this ability has been removed from me)

motorguy

I think you're doing very well to get 36mpg out of a 2.0 petrol car.

Indout96

If there was 14 gallons then the figures are even worse

Did you refill it to see how much you actually used or are you saying half because the needle says half ?
36mpg is not great but not that bad, we could squeeze 40mph out of our Focus (4 door) 2.0l and can get just over that with our 1.8 Saab 93. I did 2 x 130 mile trips in my 1.8 Tigra this weekend nearly all on A1(M) and it averaged 42.8 as opposed to 27.2 around town (mostly top down LOL)

vikingaero

High speed motoring burns fuel. If you drive at 50mph you'll use roughly 15% less fuel than travelling at 70mph. 60mph uses about 10% less.

The most economical driving is often on clear A-roads following the bumbling 40mph drivers.

LandyAndy

It sounds like you haven't understood the brim to brim method of measuring consumption.

Nevertheless 36 mpg at motorway speeds doesn't sound unreasonable.

40+ mpg from a 3 litre diesel on the same trip also sounds Ok, though not brilliant. If your brother was relying on his trip computer then his real consumption could anywhere within 5 mpg either way. The computer on my 2009 Mazda6 is consistently 3-4 mpg optimistic.

Gloomendoom

It sounds totally reasonable to me. I had several 2.0L Rover 800's that would easily get 36mpg on a fast motorway run.

My old 2.0L TR7 DHC would get 36mpg in the same conditions.

LandyAndy

Gloomendoom wrote: »

It sounds totally reasonable to me. I had several 2.0L Rover 800's that would easily get 36mpg on a fast motorway run.

My old 2.0L TR7 DHC would get 36mpg in the same conditions.

I had one of those when they were new.:D I think I used to get around that, possibly slightly less. I got 27mpg consistently from my Rover SD1 3.5l V8.

blued

You need to brim the tank then do your trip (recording the miles with the trip counter in the car) and then brim the tank again. Then you work out from the amount of litres and miles what your MPG is. If you are guessing by half a tank you may as well have plucked 36mpg from thin air!

Gloomendoom

LandyAndy wrote: »

I had one of those when they were new.:D I think I used to get around that, possibly slightly less. I got 27mpg consistently from my Rover SD1 3.5l V8.

It only did 36 on motorway runs.

I bought it 25 years ago and I've actually still got it, but now it's got a 4.0L V8 in it. (28mpg)

LandyAndy

Gloomendoom wrote: »

It only did 36 on motorway runs.

I bought it 25 years ago and I've actually still got it, but now it's got a 4.0L V8 in it. (28mpg)

Grinnall or S&S or did you convert it yourself? Or does anyone else do these conversions now? Lost touch with the world of TRs after I sold my TR6. Many years ago I was on the national committee of the TR Drivers Club.

Gloomendoom

LandyAndy wrote: »

Grinnall or S&S or did you convert it yourself? Or does anyone else do these conversions now? Lost touch with the world of TRs after I sold my TR6. Many years ago I was on the national committee of the TR Drivers Club.

I did it myself about 20 years ago with a 3.5 carb. It has evolved over the years. Unfortunately, it is currently off the road while I fiddle with it some more.

/hijack