I note how HJ rates the Mazda 2 very highly. My partner has a Mazda 2 90 bhp and really loves. She gets fantastic economy out of it - according to the onboard computer she has got in excess of 54 mpg over the last 2000 miles which is mostly local driving and in winter too.

As much as I like the car (I have a 3 [admitedly much older, but am a fan of Mazdas]) as I've drive one as a courtesy car, don't believe the trip computer, on ANY car. The most accurate way, other than with professional equipment (using the satnav is out, as it can't differentiate between the distance on a flat [top view] map and the REAL distance on undulating roadds, e.g. when going up/down hills which adds to the distance covered), is to use the 'brim-to-brim' method:

1. Fill up with fuel right up to the top of the fuel pipe or at least to a spot you can can remember every time (of course, making sure the level doesn't drop when filling due to the car getting rid of air as its filled with fuel), then make a note of the mileage on the odometer before moving off;
2. When you next need to refuel, repeat. Do so for a good few fill-ups to average out any errors and discrepancies;
3. Then add up all the fuel (not including the last fill up - you're still using it so it doesn't count yet) you've used since the first fill-up above, and take the mileage from the first fill-up from that from the latest. use the following formula to get the mpg:

(Distance travelled in miles / fuel used in litres) x 4.546 UK gallons per litre = miles per gallon.

The only 'error' will be in how accurate the odometer is, and is less so than using the car's trip computer, which uses a fuel flow meter to calculate the mpg (or equivalent, e.g. ltrs per 100km), which isn't that accurate in itself and has the error of the odometer as well.

I suspect you'll find that your Mazda2 (presumably the 1.5 petrol rather than the diesel) probably does nearer to 50-52mpg - still none too shabby for a nippy 1.5 ltr petrol engine with a 0-60 time of 9.5 sec. My first car, a mid 90s Nissan Micra 1.0 (petrol) could 'only' do 52mpg with my 'light footed' driving style (I can get 40.5mpg in my Mazda3 mk1 1.6petrol), about 10% more than the advertised combined mpg. The 2 in 90hp form would suit me nicely if I don't need the boot space of the 3 any more and is far cheaper than the (still very good) CX-3, which is, in my view overpriced and thus sells in low numbers.

At the risk of being labelled a complete anorak, I have a spreadsheet which I fill in every time I fuel the car, and it works out MPG over the last fill, last 10 fills, last 20 fills and over the entire life. It's useful to see any trend that might indicate a problem. Note that the odometer reading from the car may not be correct. In my last Hyundai, it under-read the true mileage by about 2.5%. I checked this on a long run on the M6, calibrating it against my sat-nav and the distance markers by the side of the carriageway.

I too have a spreadsheet to work out my car's ongoing and historical mpg (since 2006).

As regards the satnav being 'more accurate' - my argument from before stands - satnavs ONLY measure the distance as if it were looking on a map, so doesn't differentiate when you go up/downhill, which is often why you notice the 'speed' you are supposedly driving at shown on the satnav (normally lower than on the speedo) drop as you go up or down a hill, even if your actual speed remains constant. As such, DON'T ever rely on the satnav to just duck under the speed limit whilst passing a speed camera on a hill, especially a steep one.

Of course, a small inaccuracy (under-estimation of distance) will also come from worn tyres, as the rolling circumfrence will be lower than when they had the full 7-8mm of tread on them.