What's your idea of better performance? Asking about top speeds in regards to transmissions makes me think you're after cruiseability.
Also, do you mean a 3spd vice a 4spd?
Presuming this is what you mean, the 3 spd's and 4 spd's both have a 1:1 final drive ratio. The SM420 4 spd may as well be a 3 spd as you'll probably never use the 7.06 granny 1st gear.
Check your tire size and your rear end gearing, then use this rpm calculator.
rpm calc. For sh!ts and grins, type in an overdrive ratio of .72 or .63. Then your next question will undoubtedly be, what OD tranny should I use!
-David
Kustoms-and-Sleds
