Quite often, I get questions from readers that warrant a quick email reply, because those questions require a simple answer and are not one I hear very often. Then, there are the questions I get that require a more thorough answer and could potentially help many enthusiasts. One such question involves an inaccurate speedometer. Many times, the fix is as easy as changing the speedometer-cable gear in the transmission. Sometimes, it might require a little more work.
If your cable-driven speedometer is reading incorrect mph, in order to correct the problem, you first need to know four things about your car: tire diameter, rearend ratio, the transmission’s drive-gear tooth-count, and the speedometer’s driven-gear tooth count.
Tire diameter can usually be obtained from the tire manufacturer’s website. If not, you can physically measure the tire diameter to get this number. 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. You can also get the measurement by using what is called the rolling circumference method.
To do that, you need to put a chalk mark on the bottom of the sidewall in the middle of the contact patch. Next, place a chalk mark on the pavement, directly opposite of the tire’s chalk mark. Once that is done, roll the car forward until the sidewall’s chalk mark is back at the bottom-center. Mark the pavement at the tire mark. Now, measure the distance between the two marks you placed on the ground. Let’s imagine we measured 81.99 inches. Divide the distance by π (3.1416), and this will give you the tire diameter. Our hypothetical tire diameter is 26.1 inches.
There are two gears in the transmission that make the speedometer work. The speedometer cable gear (driven gear) is mounted within a sleeve/housing that goes into the side of the transmission’s tailshaft housing. This is where the speedometer cable attaches to the transmission. The driven gear’s teeth contact another gear (drive gear) that is attached to the transmission’s output shaft. 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.
The drive gear on the output shaft is usually held in place by a snap ring, pin, or key. 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.
A Chevrolet Turbo 350 — and some other transmissions — can use the same series of drive gears. However, the gear’s outer diameters could be different. Both 1.76- and 1.84-inch-od drive gears have been used in Chevy’s Turbo 350. Unless you want the gears to wear out quickly, the driven and drive gears must be compatible. To properly choose the correct driven gear, you will need to know the outer diameter of the drive gear.
When testing, mechanical speedos are typically most accurate when at 60 mph. 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. If the odometer numbers measure accurately but the needle is not accurate for your speed, the problem is in the internal speedometer mechanism. If that’s the case, changing gears is not going to help. In this instance, your speedometer will need to go to a shop for repair/calibration. A properly calibrated GM mechanical speedometer should spin 1,001 revolutions in one mile, in 60 seconds.
As an example subject, let’s imagine we have a car with a Chevy Turbo 350 and a 3.55 rearend ratio. We’ll also use our hypothetical tire diameter of 26.1 inches. The speedometer gear is blue with 20 teeth, and the output shaft’s drive gear has 8 teeth. At this point, the drive-gear outer diameter does not matter. However, it will later.
First, determine the tire’s revolutions per mile. To do that, we need to know how many inches are in one mile (63,360). Next, we need to divide that by our tire circumference (81.99). In the equation; 63,360 ÷ 81.99, we learn the revolutions per mile are 772.78. Next, we need to multiply the drive gear (8-teeth), times the tire revolutions per mile (772.78), and the rearend ratio (3.55) to find the proper driven-gear tooth count (8 x 772.78 x 3.55 = 21,947). Now, divide that number by 1,001. Remember the properly calibrated speedometer? That’s where we get 1,001. (24947 ÷ 1,001 = 24.922)
As you can see in the chart, the closest driven gear has 25 teeth. It also shows it must mesh with a 1.84-inch-od drive gear. This is why we need to know the outer diameter of the drive gear. If you place a driven gear that requires a 1.76-inch-od into use with a drive gear that has a 1.84-inch-od, if you can even get them to mesh, they will wear out within a few miles.
The Turbo 350, Powerglide, and GM four-speeds have driven gears that have anywhere from 17 to 25 teeth. Speedometer driven gears in the Turbo 400 and 700R4 have between 34 and 45 teeth. In these transmissions, each tooth added or deleted changes your speedometer reading. “Each tooth on the driven gear is equivalent to 3 miles per hour,” says Ondra Terry of TCI Automotive. “If you need to slow down the speedometer, you need more teeth on the driven gear.”
For example, if your speedometer is showing 60 mph and it should be showing 52 mph, you will need to use a driven gear with more teeth to spin the speedometer cable slower. In this case, you need to add three teeth. But, a perfect combination is not always going to happen. Case in point: we need to make an 8-mph change, but each tooth is worth 3 mph. An even 8 mph is not an option, so we need to choose between adding two teeth for 6 mph or three teeth for 9 mph.
If your preferred gear falls between tooth counts — as it typically does — the best choice would be to pick the smaller gear so your speedometer will read slightly high. In fact, it’s been reported by many that General Motors would err on the side of caution, and pick a speedometer gear so that the gauge would read anywhere between 60 and 63 mph at a true 60 mph.
Hopefully, this short explanation about selecting speedometer gears sheds some light on the process of selecting gears, and now you can have an accurate speedometer. That’s a priority because we don’t know of any police officer that will accept an inaccurate speedometer as a logical excuse for your 100 mph trips down the interstate.
If you want to get even more online help to get your speedometer showing the correct speed, then click here to go to the TCI Automotive website for more formulas and actual gear options available.