Here's a link to the download page of the Gear Ratio Calculator program.
A 29" with a 40 tooth acts just about like a 26" with a 36 tooth.
The difference can be thought of as 4 teeth. Simple, no?
On my stock engined bike, a 29" with a 40, I am geared to go 39 at 7500, but there's no way the engine can hit 7500 geared this way with me on it.
The 40 is the tallest gear usable, and it only hits about 30. The low RPM at cruise is very nice. I suspect the 36 is the tallest usable gear on a 26 with a stock engine.
I'd like to try a 48 tooth on my bike sometime, just to be able to putt real slow sometimes. I can slow to about 16/18 and have the engine pull smoothly.
Below that it's rough and lugging.