I've run a 40T, and a 36T. My 80 motors seem to handle the 36T sprockets ok. Where I live it's fairly flat. All the hills near my house are low.
I like both size sprockets. The 40 offers faster acceleration, and + 30 mph speeds. Even a stock motor feels like a little moped with a 40T.
A 36 has less a lot less acceleration, but will top 35 mph. That's really only a few more mph than a 40T, but cruising at the same speeds with a 36T, the motor feels a little less stressed, and a little less vibration.
I don't like to run my motor over 7000 rpm much, and I don't think the motor appreciates it much either. So it comes down to what kind riding you do, for which is best. I live in a big city, full of Suv's. Sometimes I have to get on a big street in traffic. I try not to, but sometimes you just have to, to get where you're going. At times like that it's nice to be able to go 35, just to keep from becoming a hood ornament lol.
40T.... rear wheel sprocket...........36T sprocket
26x2.125 tire 81.8" circumference..26x1.95 tire 80.25" circumference
RPM MPH....................................RPM MPH
1000 4.7....................................1000 5.2
1200 5.7....................................1200 6.2
1400 6.6....................................1400 7.2
1600 7.6....................................1600 8.3
1800 8.5....................................1800 9.3
2000 9.4....................................2000 10.3
2200 10.4...................................2200 11.4
2400 11.3...................................2400 12.4
2600 12.3...................................2600 13.4
2800 13.2...................................2800 14.5
3000 14.2...................................3000 15.5
3200 15.1...................................3200 16.5
3400 16.1...................................3400 17.6
3600 17.0...................................3600 18.6
3800 17.9...................................3800 19.6
4000 18.9...................................4000 20.7
4200 19.8...................................4200 21.7
4400 20.8...................................4400 22.7
4600 21.7...................................4600 23.8
4800 22.7...................................4800 24.8
5000 23.6...................................5000 25.8
5200 24.6...................................5200 26.9
5400 25.5...................................5400 27.9
5600 26.5...................................5600 28.9
5800 27.4...................................5800 30.0
6000 28.3...................................6000 31.0
6200 29.3...................................6200 32.0
6400 30.2...................................6400 33.1
6600 31.2...................................6600 34.1
6800 32.1...................................6800 35.1
7000 33.1...................................7000 36.2