I'm about to buy a set of low rolling resistance tires (Michelin energy savers) and I'm torn between the two size choices, 185/65/15 vs 195/60/15, and I'm trying to figure out the differences in rotaional mass/intertia, rolling resistance, and fuel economy. The 195/60/15 tires have a 117mm sidewall, while the taller skinner 185/65/15 tires have a 120.25mm sidewall (calculating 65% of 185 width). If I am calcuating this right, this means the the 195 tire has a 6.5mm smaller overall diameter than the 185 tire. Is there any way to calculate (approximately) which of these is likely to be more fuel efficient? Although the 195 tire has 10mm more rubber width on the road, its diameter is smaller, giving it less rotational inertia to get moving.