Just depends on how fast you want to go. You can gear for 160 on the open road or 120 on a closed circuit. Use this formula :
axle=rpm*W*1/88 /mph, where W equals tire circumference in feet. This is for 4th gear 1 to 1.
Mustang Mike
http://members.spree.com/SIP/svo86/M...4and86SVOs.htm