http://www.consumerreports.org/main/...East_id=333137

 

Ok I got to reading the post and decided to give some ideas that may be helpful.

Let's say that you currently have 31 inch tires and 3.55 gear ratio rear end.

Given this formula:
MPH*Ratio*(Time*PI*Distance)
---------------------------------
Tire Diameter

We can know what the RPM will be at any given MPH.

Now the Time*PI*Distance can be converted into a constant like 336 (This is calculated by using 1.07 for Time and 100 for Distance)

So now we can say

60*3.55*336
-------------- = 2308 RPM
31

So if your tire size went up to 33 inches then we would see this:

60*3.55*336
------------- = 2168 RPM
33

At 2168 RPM(33 inch tires) your motor may be struggling up hills more than it would with the 2308 RPM(31 inch tires).

However, If you changed the gear ratio up to the next level which is 3.73 then you would see this RPM with the 33 inch tires.

60*3.73*336
------------- = 2278 RPM
33