We'll in the format it's presented there I'm sorry to say I can't be bothered to decipher it! Saying that is it just the tyre choice that differs? Do other spec changes not affect overall weight etc which may alter the calculated mpg? Someone more wise and knowledgable will be along soon to clarify I'm sure, but I do believe the calculated mpg values are based on set formulas plus lab test results on "stock" engines

Yes in fact the 'real world' fuel economy penalty is much greater than the official govt figures suggest, as shown by the What Car Magazine True MPG Comparison Tool. The penalty is around 14% going from the 1.25 litre 1 Air 5dr to the 1.25 litre 2 5dr with the former (cheaper) 185/65 R15 shod model offering overall average mpg of 46.4 mpg - much better than the 39.9 mpg average calculated by the What Car road tests for the grade 2 Rio running on 195/55 R16 tyres.

Yes, but was the tyre size difference the only change? I'd assume the actual difference would be forces acting against the larger circumference wheel and the transmission. But my physics was school boy level and rather long ago, but there a couple in the know on here

Apart from maybe a minor (less than 1 %) increase in kerb weight going from the Grade 1 to the fully loaded Grade 2 car, all other factors such as gear box ratios, engine specs, 0 to 60 mph , top speed etc appear to be identical. But then conversely, the Grade 3 1.4 litre CRDI on its 17 inch 205/45 alloys claims better fuel economy across the board than its arguably lighter Grade 2 cousin on narrower 16 inch 195/55 alloys !

Edited by Firmbutfair on 08/01/2014 at 18:33