You dont often get the chance to drive two near-identical cars back-to-back, where the only noteworthy difference is low-friction tyres. The car in question is the Seat Mii. The two best Miis right now are the FR Line, a sporty little number with dark grey body graphics and a dinky roof spoiler, and the plum-trimmed Mii by Mango, which for a short time you can get in this Limited Edition guise. Both have the 74bhp 1.0-litre engine, and both of our test cars have covered around 800 miles from new, both allegedly weigh the same (despite having different door counts) and both have the removable Garmin-based infotainment system, complete with comprehensive trip computer readouts (including fuel economy). But while the sporty FR Line runs on 16-inch wheels and Bridgestone Turanza T001 tyres, the Mango uses funky 15-inch purple rims wearing low-rolling-resistance Bridgestone Ecopia rubber. Both tyres plant a 185mm-wide footprint on the road. In theory, the difference in the way they drive should be minimal. But all is not as it seems, as I quickly realise after sending the FR Line back home and settling into a few miles in the Mango. The FR Line, with its red-trimmed black seats and matching dashboard, is a little firmer over bumps. Bigger wheels plus the same suspension equals a smidgeon of added harshness. On the other hand the little red charger grips like a mountain goat, its lower-profile (and stiffer-sidewalled) Turanzas giving such a solid hold on the road that to un-stick it youd have to be unhinged. Even in the wet you can throw it around like a rag doll. On its eco-boots the Mango gives up the ghost sooner, although still not until youre pushing it hard. Worth mentioning but, fair enough, its rarely relevant since both grip well. The biggest difference is fuel economy - and the split is massive. Driving the same route in both, in near-identical weather and traffic conditions, sees just under 55mpg in the FR Line but a few tenths over 65mpg in the Mango. Thats close to a 20 per cent boost. Remember, the only relevant difference is the wheel and tyre combination. Tyre pressures were correct on all eight corners. Its technically possible that other factors are involved, if you like clutching at straws. Maybe the FR Lines engine isnt running quite as sweetly as the Mangos. Maybe a few tolerances are at their limit and it needs more miles to loosen up fully. Maybe the Miis diddy size and weight accentuates the eye-opening difference the tyres make. But, from this real-world evidence, perhaps buying efficiency-minded tyres might do more for your wallet than youd have thought.