I have a friend that has a 78 malibu with a turbo 350 trans & a 3:42 posi rear end he is running P245/60/15" tires that stand 26 & 1/2" high, what speedo gears does he need to run on the tail shaft & on the speedo cable? Any help would be greatly appreciated.He is currently running a 41 tooth gear on the speedo cable & he is about 20 miles a hour fast

This might help.

http://www.tciauto.com/tech_info/speedo_gears.htm

http://novaresource.org/speedo.htm
http://novaresource.org/speedo2.htm

There's a formula that uses tire diameter & axle ratio to find speedo gear ratio. Then pick the appropriate gear combo from the charts.