Gearing for Autocross ?

[Raffman]

I know rear gearing can come down to a personal preference but I keep hearing there is somewhat of a formula for Autocross . Take your peak rpm before a shift say 6000 RPM and in second gear the mph should top out at 80 MPH . This is done on a gearing chart. Any input on this ? I've crunched some numbers for my car and seems about right.....

[Bugzilla]

Here is what mine does for comparison. I can run most courses in 100% 2nd which is nice.

[dontlifttoshift]

65-70 mph is what I am looking for in 2nd.

[Chad-1stGen]

Course types will be location and venue specific. But for anyone who is targeting running a large variety of courses then I recommend being able to hit at least 70 mph in 2nd gear. Anything less than 65 and you will have to use 3rd gear occasionally. Anything under 60 and you are probably using 3rd gear frequently.

My car tops out around 73/74 mph and I've only used 3rd gear 4 times in over 100 different courses the past few years. And only 1 of those was 3rd gear not at the end but somewhere in the middle that required shifting back into 2nd.

[nokones]

With my 89 Corvette I am running an all aluminum Lingenfelter 427 SBC lots of horsepower and torque that will spin up to 7800 rpms. I have my rev limiter set at 7,000. Also, I am using a Tremec TKO 600 tranny with a 1.89 second gear and 3.54 rear Diff gear ratio. I am running 335-30-18 tires on the rear and I can peak at about 78-80 in second before I hit the rev limiter. That works great for me. The only time I need third gear is mostly at the American Autocross Series events. Once, I did see 104 MPH using third gear at an event in Southern California.

I know the C5 ZO6 & C6 Grand Sport Corvettes have a second gear ratio at 2.02 and a 3.42 rear diff. That will work great for coming off of a tight turn but, I think you'll find yourself looking for third most of the time on most SCCA courses. You definitely want to find the right combination for most courses you run and that will be trial & error. You should try to find a setup that will get you something between 70-80 mph in second and not be boggy in second coming off any turn.