i've thought of this before, very though question.

a. 195/55R15, total diameter = 595.5mm, circumference = 1870.82mm
b. 185/65R15, total diameter = 621.5mm, circumference = 1952.50mm
185/65R15 is bigger by 4.37%, which means for each revolution, the 185/65R15 is covering more actual distance than 195/55R15 (won't show up in odometer unless you really do calibration for odometer)

if look just on the distance covered, b. seems like can archive better FC by 4.37%. However being bigger diameter also means bigger moments of inertia (I) even if they have the exact same total weight, meaning FC will worst if got looks of stop go movement. Can someone help to calculate the I?? If the I of b. is less 4.37% of a., then i think b. will have better FC? If you do highway a lot, then I should be less of factor.

correct, it (theoretically) means better FC ONLY IF you do mostly constant speed. If you do stop go a lot, means that more energy is needed to accelerate the bigger moment of inertia, even if both diff size tyre weight exactly the same KG.

Not long ago i was planning to change my stock 165/60R14 GY NCT5 tyre. And i want a good FC tyre, and did some forum crawling and found out that most ppl talks about Michelin XM2. But alas XM2 closest size is 175/65R14. I'm worried that bigger tyre will cost more FC even if XM2 is a fuel efficient tyre. that's why thought of this topic... and got no answer so far lols

This post has been edited by efaceninja: Sep 24 2013, 06:15 PM

oh btw, it also means 4.37% less servicing.. cause your odometer still thought you've moved 5000km, but in fact, you've travelled 5218.5km