Tech Q&A With Mar­lan Davis

Hot Rod - - Contents - 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 Bg­ or Mon­sterTrans­mis­­ter-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...

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