Fuel economy and petrol tank size mystery

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?

k.o.d

The most comprehensive way to check is to brim the tank, drive a couple of hundred miles, brim it again, and do the same a couple of times to get a respective test.
That way you get exac tly what you have used rather than all the faffing about you are currently trying to do by working on estimates rather than actuals.

Hintza

You are still messing around the edges OP. Don't worry how much the tank holds that is irrelevant.

Fill to brim and note mileage. Drive until tank close to empty and fill again note mileage and calculate fuel used. Then just do the sums.

iamana1ias

thor wrote: »

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.

You use a *lot* more petrol at 80mph than at 70mph

maninthestreet

35-36 mpg for a 2 litre car on a motorway run isnt bad, but if you slow down to 65 to 70mph, your journey won't be that much longer and you'll get better fuel consumption. FYI, petrol gauges aren't guaranteed to be linear across their entire range.

LandyAndy

Why are there two threads asking the same question?

Cool_Username

LandyAndy wrote: »

Why are there two threads asking the same question?

Double post...