TH400 speedometer gears - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**JQ-72** · 03-12-2026, 10:33 PM

I know there are online calculators where you can figure speedometer driven gears, but does anyone happen to know the teeth counts on a 3/4 ton with 3.54 and 4.10 gears?

**PbFut** · 03-14-2026, 11:46 AM

Tire size is needed. Also, there are 2 size and multi count drive gears that go on the output shaft. Without knowing all the variables, impossible to figure.

howzzzit · 03-14-2026, 01:29 PM

Quote:

Originally Posted by JQ-72

I know there are online calculators where you can figure speedometer driven gears, but does anyone happen to know the teeth counts on a 3/4 ton with 3.54 and 4.10 gears?

Your axle gear ratio determines how many times your driveshaft rotates for every full tire rotation. For example, a 3.73 gear ratio requires 3.73 driveshaft rotations to turn your tires once around. You should be able to find your gear ratio listed on the axle tag, however, if that has gone missing, you can simply count the number of teeth on each gear and divide the number of ring gear teeth by the number of pinion gear teeth.

[ Ring Gear Teeth / Pinion Gear Teeth ] = [ 41 / 11 ] = [ 3.73 ]

TCI® Auto https://share.google/R8IwduoEKFAxBm8Be

This site has drive, driven, and tire size calculators

**JQ-72** · 03-14-2026, 03:50 PM

Quote:

Originally Posted by howzzzit

Your axle gear ratio determines how many times your driveshaft rotates for every full tire rotation. For example, a 3.73 gear ratio requires 3.73 driveshaft rotations to turn your tires once around. You should be able to find your gear ratio listed on the axle tag, however, if that has gone missing, you can simply count the number of teeth on each gear and divide the number of ring gear teeth by the number of pinion gear teeth.

[ Ring Gear Teeth / Pinion Gear Teeth ] = [ 41 / 11 ] = [ 3.73 ]

TCI® Auto https://share.google/R8IwduoEKFAxBm8Be

This site has drive, driven, and tire size calculators

I know my axle ratios - one is a 3.54 Dana and the other is a 4.10 Chevrolet (Eaton, as some call it). I just bought a TH400 transmission local and the tag shows it to be out of a 1972 truck. It has a 41 tooth driven gear. Just wondering if it might fit right in with one of these gear ratios or if I need to swap the gear out

Orange Crate · 03-14-2026, 09:35 PM

Where you gonna get the new gear?

03-12-2026, 10:33 PM	#1
JQ-72 Senior Member Join Date: Feb 2001 Location: Killen, Alabama USA Posts: 3,746	TH400 speedometer gears I know there are online calculators where you can figure speedometer driven gears, but does anyone happen to know the teeth counts on a 3/4 ton with 3.54 and 4.10 gears? __________________ 72 Cheyenne Super swb. Factory 402/400,tach,posi,AM/FM, A/C. Adding cruise, 8-track 72 Cheyenne Super swb. 350/350 A/C, bucket seats 72 Cheyenne swb. Factory 402/400, tilt, posi, A/C 72 Cheyenne Super K-10. 350/4 spd., posi. 72 Suburban 350/350 tilt, tach, AM/FM, A/C, luggage rack 72 GMC Sierra lwb. 350/350, A/C. medium green/olive interior

03-14-2026, 11:46 AM	#2
PbFut Senior Member Join Date: Feb 2022 Location: Ca Posts: 1,024	Re: TH400 speedometer gears Tire size is needed. Also, there are 2 size and multi count drive gears that go on the output shaft. Without knowing all the variables, impossible to figure.

03-14-2026, 09:35 PM	#5
Orange Crate Registered User Join Date: Feb 2011 Location: Under the Truck Dreaming Posts: 888	Re: TH400 speedometer gears Where you gonna get the new gear? __________________ 72 K20 Custom Camper, D60, 14BFF, full roller 406 TPI w custom built ECU and yes..it has tow hooks, a 5 speed AND a front hitch!



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