Tech Q&A With Marlan Davis
Marlan, I am hoping you can help me. I’m running a TH350 tranny in my 1955 Chevy. It has a stock 3.55:1 rear gear, the plastic drive gear has 20 teeth, and I’m running 15-inch tires. The speedo has also been rebuilt. At approximately 30 mph, the speedo is showing 15 mph. I am in Alvin, Texas, and have spoken to all the transmission rebuilders in town without getting an answer.
To correct the error on a cable-driven speedometer, you need to know four factors: the tire revolutions per mile, the rear-axle ratio, the driven-gear tooth-count, and the drive-gear tooth count. Don, unless you have a go-kart, I doubt you have “15-inch tires.” You’re probably confusing wheel diameter with tire diameter. Likewise, TH350 20-tooth speedo drive gears don’t exist, so I think you mean the driven gear. Let’s get you up to speedo.
Tire revs/mile: This data can usually be obtained from your tire manufacturer’s website. Failing that, physically measure (in inches) the tire diameter (the edge-to-edge distance across the entire tire face at its widest point as viewed from the side, not the tread width). That’s less accurate than straight published revolutions/mile data because the tire diameter “grows” at speed or as it heats up; it also “squats” at the bottom where it statically contacts the pavement. You can decrease physical static measurement error using the “rolling circumference” method. Put chalk marks on the bottom center of the sidewall and on its pavement contact point. Roll the car forward until the sidewall chalk mark is again at bottom center and mark the pavement. Measure the distance between the two pavement marks. That distance divided by ( pi, about 3.1416 in the absence of a dedicated pi key on your calculator) gives you the tire diameter, which form part of the error-correction equations we’ll be using.
Rear-axle ratio: The ratio of the ring-and-pinion in the rearend (in your case, 3.55:1).
Driven gear: The “pencil”-like gear inserts through a sleeve that goes into the side of the extension housing. The tooth end contacts the internal drive gear on the output shaft and the shaft end connects to the speedometer drive cable. Disconnecting the cable and removing the retaining clip permits pulling the gear/sleeve assembly to count the teeth. You can also go by the plastic gear’s color, but that’s not 100-percent foolproof. Chevy TH350 driven gears and sleeves interchange with most Powerglides and many classicera GM manual transmissions. One sleeve fits all driven gears, but not all driven gears mesh well with all drive gears.
Drive-gear tooth count: The round drive gear fits over the trans output shaft and is usually retained by a snap ring, pin, or key. Drive gears are generally specific to the trans model. You may be able to see the drivegear color or count its teeth without removing the housing, but removal is mandatory to change the gear. In a Chevy-pattern TH350 and other transmissions using the same series of driven gears, another reason to remove the extension housing is to measure the drive gear’s outer diameter (od). Both 1.76- and 1.84-inchod drive gears have been used on Chevy TH350s. To minimize wear, the driven and drive gears must be compatible. Currently, it’s easiest to find 17- to 22-tooth driven gears for 1.84-inch drive gears (GM still stocks many of them), and (from aftermarket sources) 22- to 25-tooth driven gears for 1.76-inch drive gears.
Chevy vs. BOP housings: Notice we keep saying “Chevy-pattern” TH350. Running a Chevy motor in a Chevy, we assume that’s what you have. But not everything is as it’s supposed to be. We know Chevy’s trans-case-to-engine-block mounting pattern differs from the “BOP” (aka Buick-Olds-Pontiac, but also Cadillac) pattern. Nevertheless, at the back end, you can physically swap BOP and Chevy TH350 rear extension housings of the same length. BOP TH350 sleeves and the driven and drive gears are larger with different tooth counts than Chevy TH350 gears, so any housing swap requires changing all three parts.
Gear availability: The accompanying tables, compiled with help from Ken Casey at Elway Chevrolet and Norm Rollings at Rollings Automotive, list the driven and drive gears once offered for popular GM performance transmissions. Where we can
find them, both GM and “universal” transmission shop part numbers are shown. Even if discontinued by GM, many of these gears still exist in the aftermarket or on eBay. Google is your friend!
Calibration: Mechanical speedos are most accurate at 60 mph. A properly calibrated GM mechanical speedometer will turn exactly 1,001 revolutions in 1 mile in 60 seconds.
It’s best to eyeball at 60 mph using a GPS device or by driving the car over a measured mile (for example, between freeway mile markers). Also watch for a discrepancy between the odometer and the speedo-head needle: If the odometer numbers check accurate but the needle is off, the problem is in the internal speedo mechanism. In that case, swapping gears won’t help.
Using the numbers you did provide, let’s run through a hypothetical correction scenario. We’ll assume you have a “normal” Chevy TH350 with the small extension-housing hole, a 3.55:1 rear-gear ratio, and a 20-tooth blue driven gear that’s designed to mesh with 1.84-inch-od drive gears. You didn’t supply the correct tire diameter or the number of drive-gear teeth, so let’s suppose the rear tire diameter comes out to 26.1 inches and the existing drive gear has 9 teeth. First, determine the tire’s revolutions/mile:
Next, plug the drive gear, the tire
revs/mile, and the rear-gear ratio into this driven-gear equation to find the proper driven-gear tooth count:
The closest driven gear has 25 teeth, but from the tables, you see it must mesh with a 1.76-inch-od drive gear. The only available 1.76-inch drive gear had 8 teeth, so now you need to recalculate the driven-gear tooth count for use with that 8-tooth drive gear. This time, the equation’s product is “21.92”; 22-tooth driven gears were available for both 1.76 and 1.84 drive gears.
Alternatively, you can shuffle the equation to solve for a drive-gear change while leaving the existing 20-tooth driven gear alone.
This result shows that installing a 1.84-inch-od 7-tooth drive gear lets you keep the existing 20-tooth driven gear and still end up with a reasonably accurate speedometer calibration.
Too much work? There are a number of online calibration tools where you just type in the previously discussed data factors. Check out Bgsoflex.com/speedo1.html or MonsterTransmission.com/Speedometer-Gear-Calculator_ep_122-1.html.
If available drive and driven gears can’t resolve the calibration, an old-school custom speedometer shop can build a miniature ratio-adapter box that bolts to the outside of the trans where the cable normally hooks up. The cable would now attach to the output side of the ratio box; 90-degree boxes are even offered for tight quarters. One source for custom ratio boxes is Lance Martin Automotive.
[ A single Chevy TH350/manual trans “bullet” sleeve ( left) accepts all driven gears designed for these trannys. The BOP TH350/GM 700R4 uses two different big sleeves: one ( shown, right) fits 34 through 39-tooth driven gears; the other fits 40- to...
[ TH350 extension housings and speedo sleeves: Chevys ( left) have a small 7⁄8- inch-od driven-gear hole and use the same sleeve as many GM manual trannys. Replace wear-prone aluminum sleeves ( A) with later steel versions ( B). BOP housings ( right)...
[ This BOP-pattern TH350 extension housing cutaway at a local Transtar branch shows the relationship between the speedo drive ( A) and driven ( B) gears. Increasing the number of drive-gear teeth speeds up the speedo; decreasing the tooth count slows...
[ A TH350-BOP/700-R4/ TH400 uses the same large driven gears ( right). Smaller Chevy TH350 driven gears ( left) interchange with many GM manual trannys. Chevy TH350s could have come with two subtly different tooth-end diameters: 0.810 inch properly...
[ Driven gears are color coded and also (if you look closely enough) stamped with the tooth count. Colors aren’t always a reliable indicator, because GM may have changed colors over the years or the colors can fade in service. This is a blue, 38-tooth,...
[ Although driven gears usually have broad application across various GM transmissions, round drive gears are specific to the trans model or model variation. They’re color-coded like the driven gears—or count the “points” on the side or the flutes edge...