Gear ratio questions

Jimbo70 · Post by **Jimbo70** » Thu Jun 14, 2007 1:08 pm

ENGINE
I'm having a heck of a time trying to figure out what I should be looking at for gears and final drive ratios for a gearbox for my Fastback. The 1915 is going together now. The only information I have right now is that its based on a Web Cams #109 grind, mildly ported heads with slightly larger stainless valves and dual valve springs, Weber 44s and I plan to run a 23" tall tire. I'm thinking that with the T3 cooling fan (even welded and balanced) I don't want to go through the lights at much more than 6,000 RPM.

Thoughts?

Thanks,

Post by **Marc** » Thu Jun 14, 2007 4:53 pm

Will this be race-only or will you be driving it on the street? If so, how much highway travel will it see?

Post by **Tom Notch** » Thu Jun 14, 2007 5:51 pm

Here is the best gear/tire calculator I'm run across. scroll down and down load it, its really slick.

http://www.baja.com/kombi/bustech6.htm

Jimbo70 · Post by **Jimbo70** » Thu Jun 14, 2007 7:23 pm

Marc wrote:Will this be race-only or will you be driving it on the street? If so, how much highway travel will it see?

It is about 85 miles from my house to the local track, and its a mix of highway and seconday roads. Apart from going up to the track, I don't plan on driving much. This will not be a commuter car.

John S. · Post by **John S.** » Thu Jun 14, 2007 9:47 pm

4.12 R&P with stock ratio 1-4.
Maybe a 3.88 cuz of the short tires. JMO

Post by **Marc** » Thu Jun 14, 2007 10:59 pm

3.88 with a 1.125 4th and 23" tires would be 6371RPM @ 100MPH (3827 @ 60). You could probably live with that (and a 1.48 3rd) for your use...or bolt on some stock tires for the trip to the track, that'd drop the RPM at 60 down to ~3460.
http://www.shoptalkforums.com/viewtopic.php?t=93301

Jimbo70 · Post by **Jimbo70** » Tue Aug 21, 2007 7:31 pm

Thanks for the info. I was talking to my engine builder today about gearing, so finding this post again was timely.