![]() ![]() My the specs on my cam say that I should expect to start producing power between 1800 RPM through 6500 RPM. For example: the 2.73:1 in my 76 is behind a 350 with a final drive ratio of 1:1. Hot rodders are mainly concerned with getting the engine into its power band as soon as possible and keeping it there as long as possible. If you change tires then that changes the dynamics of the rear end gear ratios.įactory sets the gears in accordance with some formula that sets a ratio of a certain amount for each gear change. Give or take a few RPM this is going to be a constant. A 2.73, for example behind a 350, at 60 Mph with 26" dia tires your engine should be turning about 2100 RPM. So anytime you select a set of gears you should or it would be my advise to consider the ratios in the transmission. With today's overdrives you can actually have the best of both worlds. In the past you could have a vehicle that had good acceleration off the line or you could have a vehicle that got acceptable fuel economy but not really both. There is some formula to this, off hand I don't have it or have it memorized, most people don't really use it. Generally the thought is to select a set of gears that will get your vehicle into it power band (the RPM level where the engine is producing it best power) as soon as possible through all the gears. The gears are actually a torque multiplier. A ring gear with 37 teeth to a pinion with 9 teeth is 3.73:1. For example a Ring gear with 41 teeth and an pinion with 11 teeth is generally a 4.11:1 ratio in Final Drive or 3rd gear if your talking about a 350 transmission. The ratio of the Ring and Pinion is determined by the number of teeth on the Ring Gear and the number of teeth in the Pinion. Carrier is the portion of the rear end that the Ring Gear is attached to. This portion of the rear end has a Carrier, Ring and Pinion Gears and bearings with some shims. Several years ago, I had a 65 El Camino with a similar set up to what's in the Nova Wagon (mild 350, 700R4, 3.31 gears and 235/70x15s) - it turned barely 2000 rpm at 75, and got 20 mpg at that speed.The rear end center section is called the 3rd member or Differential. 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 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). 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. ![]()
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