What pitch chain?

[HoosierDeereMan]

My 340/S has the stock gearing with a 17 upper and 39 lower sprocket and 74 pitch chain. My top speed is about 65 MPH. With my present set up I can spin the track out from under it on low end. I would like to try a 21 tooth upper sprocket as that is what the 340 Dator, Cyclone, and Liquifire used to increase my top end speed. My question is how do I determine what pitch chain I need? If I increase my top gear by 4 teeth do I need a chain that is 4 pins longer? Would a 78 pitch chain work? Confused and at a loss!

Troy

[lwb140]

i have a 19/35 which is a 1.84 final drive i think its a 74 link chain ,i bought a 72 and a 74 i think its got the long one in it , [put the cover on it a few days ago and forget already lol ] the 21/39 is a 1.85 and have no idea what chain it would take i bought a 84 link chain to try and use a 24/40 and it was waaay to long hope this helps wade