Plus, the Gear Vendors unit can be used to split the TH400 gears, which will generally improve e.t. If we plug in 3.20:1 to the formula, it works out to 2,688 rpm as our new cruise rpm, a reduction of over 750 rpm. What's that worth? One way to do that calculation is simply to multiply the Gear Vendors' overdrive ratio of 0.78:1 by our gear ratio to create our effective ratio: 4.10 x 0.78 = 3.198. One way to improve this without having to completely change over to an overdrive automatic might be a Gear Vendors overdrive bolted to the back of our TH400. This rpm is a little steep for highway cruising. RPM = (MPH x Gear Ratio x 336) Tire Diameter.The simple formula for figuring out our cruise rpm at 70 mph looks like this: To keep this discussion about gear ratios and not about torque converters, we'll ignore slippage. For street driving, we want to know our highway cruising rpm. Now let's take our theoretical 6.0-liter Chevelle with its aforementioned 4.10:1 gears and put it on the street with a set of 28-inch tall street tires. If we multiply 400 lb-ft of engine torque by this overall first-gear ratio of 10.17, we'll see that we're applying 4,068 lb-ft of torque to the two rear axles, or over 2,000 lb-ft of torque per axle-that's why the car launches so hard, and why parts like axles and driveshafts sometimes twist or break. Determining the vehicle's overall ratio is as simple as multiplying the first-gear ratio by the rear-axle ratio (e.g., 2.48 x 4.10 = 10.17:1).

We'll use our 4.10 rear-axle ratio and the 2.48 first-gear ratio of the popular GM TH400 automatic (ignoring the multiplication factor of a typical torque converter to keep things simple), to demonstrate. The Note-So-Simple Side of Gear Ratiosīut that's just the rear-axle ratio-we also have multiplication of engine torque by the transmission ratio to consider. This means that rear gear ratios are a compromise. A gear with a lower numerical ratio, like 3.08:1, does not multiply torque as much, but, all else being equal, will produce a higher axle or tire speed.

This also multiplies the input torque by 4.1, which is the reason the car jumps when you hit the throttle. If we have a 10-tooth pinion gear and a 41-tooth ring gear and do the math (41 / 10 = 4.10) that means for every revolution of the ring gear, the pinion will spin 4.1 times.

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