i have an 87 quadracer w/ 18 inch tires, it only goes 60!!!! how much of a speed increse if i went to say a 20 inch rear tire?

 

you may gain around 8 mph and you will loose low end torque

 

Rather than go up tire size, change your gearing. It's much cheaper to buy a new front/rear sprocket than tires. Plus, your kind of stuck with the tires (they don't fit well in the tool box) if you don't like the outcome. It's fun to play with the gearing set up and find what ya dig...BUT, as Mucky mentioned there is a trade off. If you go for overall top-speed you will lose torque and pick up outta da hole.

 

Hey bump that front sprocket before switching tire a whole lot cheaper!!!!