Substitute your tire size (Tire Rack's "specs" section for each tire will give you the diameter and revs per mile for any size tire you want), diff ratio and top gear ratio and you can calculate engine speed at 60 mph for any combination you can come up with.īow Tie Overdrives has a good section on their web site on calculating rear wheel torque based on diff gearing and first gear ratios that's also useful for figuring out where you want to be on both ends of the gearing spectrum As a matter of fact ALL Sprint Firebirds for 1967 came with factory traction bars. Optional Rear Axle Ratio, Luggage Rack (station wagon). I have had several 1967 Sprints with traction bar rears. 1968 Chevrolet Chevelle In 1968, Chevrolet redesigned the Chevelle with a three inch shortened. Cars that had traction bars were any that had a Heavy duty rear end whether it was 326 HO or 400. 67 (4th gear OD ratio) = 2031 rpm 60 mphĨ13 x 3.43 = 2788 x. All 67s had the eyelets on top of pumpkin because they used the A body rear (Tempest,GTO) 1967. For simplicity, I generally calculate at 60 mph - here's how the tire size/gearing/rpm formula pencils out for my current set up, and for the ratio changes I'm considering.Ĩ13 (tire revs per mile) x 3.73 (diff ratio) = 3032 (driveshaft rpm) x. The formula for figuring engine speed for any given tire size and diff / trans ratio is tire revolutions per mile x differential gearing x transmission gear ratio = engine rpm. The 25.6 diameter rear tires (225/60x15's) turn 813 revs per mile, and are about the biggest that will fit in the wagon's rear wheel wells (this a street car that is never tracked and sees quite a bit of long-distance freeway driving - I'd like to drop rpms a bit further for freeway running). If your looking for more power or performance, you would switch to a lower. My 66 wagon currently runs a mild 350 with 3.73's in a 10 bolt with 25.6" diameter tires & a 2004R OD trans, and I'm considering going to either 3.43's or 3.23's. If youre looking for better gas mileage, you want to switch to a higher gear ratio.
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