[Fat Charlie]

Tire type is more critical than minute differences in tire size, yes, but tire size also plays a role and shouldn't be ignored. Increasing the tire size will affect handling, but not really by much. If you're going to drive so close to the edge that 1/2" of sidewall is going to make the difference, you shouldn't be doing it on public roads (or in your DD).

For hypermiling purposes, especially with an automatic, it's about gearing. Increasing your tire size increases your final drive ratio. It'll also make your automatic downshift sooner, eliminating some of the potential gain and maybe making for worse overall FE. How does your car feel right now, Gone2? Does it shift a lot or do you spend a lot of time with the torque converter locked up? If you're doing a lot of driving close to the edge of downshifting, then taller tires might be exactly the wrong thing for you.

Minor changes in tire size can get you 3% or 4% more distance travelled per wheel revolution by only adding about 1/2" to your tire radius. If changing the gearing on your tires won't send your transmission into lower gears it could be a big help. I went with LRRs that added 0.4" to my radius, so now my car is going 103.4 miles every time it thinks it's gone 100 miles.