Instructions: The calculator is preloaded with some typical values. To see the speed with other values, click buttons on the lists of common values provided, and watch the speed change. Or fill-in numbers directly in the blanks, and when the cursor leaves the input box, the speed will be recalculated.
The speed at which a vehicle travels can be calculated from the engine speed, the gear ratios, and tire size.
The input to a standard transmission is the engine speed, expressed in revolutions per minute or RPM. Safe engine RPM in stock engines is lower than many other engines. The stock engines are long-stroke, slow-rpm, engines with good torque at low speeds. Maximum sustainable RPM is about 2800. Short bursts up to 3100 or 3200 are alright, but running the engine this fast for very long will overheat the pistons and melt them. Speeds above 3200 can cause rapid damage. You may find some figures for horsepower for these engines that were run above these speeds. I suspect dynomometer tests were performed with test engines that were only run a few minutes at those speeds. If you want your stock engine to last, keep it around 2800 RPM tops. In contrast, a typical short-stroke V8 can run up to 4000 RPM with no problems.
With the transmission in high gear, the output of the transmission is the same as the input speed, or a ratio or 1:1. Lower gears have a higher numbers, meaning the input turns faster than the output. 3:1 means 3 turns of the input to 1 turn of the output. The transfer case is also 1:1 in normal range, but low range would have a ratio like 2:1, meaning the input to the transfer case (output of transmission) turns twice for each turn of the output (drive shaft).
Ring and pinion ratios (also called axle gears) are expressed at tooth counts (e.g., 43/8) or decimal ratio (e.g., 4.27:1). The tooth count ratio of 43/8 means 43 teeth on the ring gear and 8 teeth on the pinion gear. If we divide 43 by 8 we get 5.375, which is usually rounded to 5.38. The drive shaft turns the pinion, the pinion turns the ring, and the ring turns the axle. The drive shaft always turns faster than the axle. Larger numbered ratios for the ring and pinion mean slower axle speed relative to drive shaft speed. Slower axle speed is called a lower gear. Like the transmission and transfer case, this lead to higher numbers for lower gears, which may be slightly confusing at first.
Some 2WD Willys were equipped with an overdrive (OD). A common modification to 4WD Willys Jeeps is the addition of an OD which fits into the back of the Spicer 18 (Dana 18) transfer case, replacing the input gear of the transfer case with a planetary, that increases the input speed to the transfer case. This has the effect of higher travel speeds for a given engine RPM.
Stock tires were 7.00x16, 6.50x16, 6.50x15, or 6.00x15. These are approximately 28" to 30" in diameter.
The total gear ratio is a combination of the transmission, overdrive, transfer case and the axle ratios. Engine RPM divided by total gear ratio, multiplied by the tire diameter and converted for units of measurement, gives vehicle's speed of travel.
|© Richard B. Grover 1997 to 2006.||Last updated: Wednesday, December 19, 2001|