Next, we need to divide that by our tire circumference (81.99). To do that, we need to know how many inches are in one mile (63,360). However, it will later.įirst, determine the tire’s revolutions per mile. At this point, the drive-gear outer diameter does not matter. The speedometer gear is blue with 20 teeth, and the output shaft’s drive gear has 8 teeth. We’ll also use our hypothetical tire diameter of 26.1 inches. A properly calibrated GM mechanical speedometer should spin 1,001 revolutions in one mile, in 60 seconds.Īs an example subject, let’s imagine we have a car with a Chevy Turbo 350 and a 3.55 rearend ratio.
In this instance, your speedometer will need to go to a shop for repair/calibration. If that’s the case, changing gears is not going to help. If the odometer numbers measure accurately but the needle is not accurate for your speed, the problem is in the internal speedometer mechanism.
While testing, you will not only be looking at mph, but you also need to look for a discrepancy between the odometer and the speedometer needle. When testing, mechanical speedos are typically most accurate when at 60 mph. To properly choose the correct driven gear, you will need to know the outer diameter of the drive gear. Unless you want the gears to wear out quickly, the driven and drive gears must be compatible. Both 1.76- and 1.84-inch-od drive gears have been used in Chevy’s Turbo 350. However, the gear’s outer diameters could be different.
SPEEDOMETER GEAR CALCULATOR TURBO 350 SERIES
The way it is mounted to the output shaft, you will probably be able to see it through the hole where the speedometer-cable sleeve has been removed, but removing the transmission-tailshaft housing is required in order to change the drive gear.Ī Chevrolet Turbo 350 - and some other transmissions - can use the same series of drive gears. The drive gear on the output shaft is usually held in place by a snap ring, pin, or key. Driven gears are also identifiable by counting teeth, or by the color of the gear. However, they do come in different outer diameters. By disconnecting the speedometer cable from the transmission and removing the retaining clip, you can remove the driven gear and the sleeve to count the gear teeth.ĭrive gears (left) are easily identifiable by their tooth count. The driven gear’s teeth contact another gear (drive gear) that is attached to the transmission’s output shaft. This is where the speedometer cable attaches to the transmission. The speedometer cable gear (driven gear) is mounted within a sleeve/housing that goes into the side of the transmission’s tailshaft housing. There are two gears in the transmission that make the speedometer work. Our hypothetical tire diameter is 26.1 inches. Divide the distance by π (3.1416), and this will give you the tire diameter. Now, measure the distance between the two marks you placed on the ground. Once that is done, roll the car forward until the sidewall’s chalk mark is back at the bottom-center. Next, place a chalk mark on the pavement, directly opposite of the tire’s chalk mark. To do that, you need to put a chalk mark on the bottom of the sidewall in the middle of the contact patch. You can also get the measurement by using what is called the rolling circumference method. Measuring the tire is less accurate than using the published revolutions/mile information because of variations in how the measuring is accomplished, but it is close enough. If not, you can physically measure the tire diameter to get this number. Tire diameter can usually be obtained from the tire manufacturer’s website. This cutaway of a Turbo 350 tailshaft housing shows the relationship between the speedometer drive gear (A) and driven gear (B).
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