Gear ratio.

[Cshenker1121]

Hey guys. I'm gonna be running a 700r4/383 sbc (roughly 420hp) with a 18" rim and a 40 series tire. (Not sure the hiegth in inches, I'll check when I get home) truck will be street driven and autocrossed. Is a 373 gear too much or not enough? What's your opinion? Thanks

[bryant]

usually its ideal to have the trans in second gear for the whole course in most autocross. So a few assumptions had to be made for the info you provided. You didnt say the tire width. 40 series is a percentage of the tire width. I assumed 275 for the width. Next what is the engines red line. I assumed 6000. Most autocross wont get above 60 mph.
i plugged in all the relevant numbers into this calculator. http://www.engineersedge.com/mechani...calculator.htm
the 700r4 has a 3.06 1st gear, 1.63 2nd gear, 1.00 3rd gear, .070 4th gear
so with these numbers 4.56s would get you the most use of your power band if it pulls to 6000 rpm. You can fix all of my wrong assumptions in the calculator and figure your ideal gearing. You can decide on what compromises need to be made. Whether its optimized for auto cross or freeway cruising.