How much weight have you been putting on the gas pedal ? 7.8L compared to 8.5 is a marginal difference that'll really come down to driving habits I would think8.5 litres per 100km is equivalent to 32.1 miles per UK gallon (36.7 miles per US gallon). I had been getting around 35 mpg (UK) or 29 mpg (US) on a mix of local roads and highways, so 7.8 l/100km, and had been rather disappointed to be honest, but maybe that's OK?
ahhh, so that's it. Recently did a 2 hr road trip and going got 44 mpg and coming back 36 mpg. Couldn't figure out why the difference. But your post cleared it up...roof was up going and down coming back!Make sure you are comparing roof up/down. Official figures are roof up. I've done back to back roof up/down at 70mph and there is a 4mpg (uk) hit to performance