IA WTB TH400 Speedo Drive Gear: 7 or 8 Tooth - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**ThreeQuarter** · 10-26-2018, 03:58 PM

I have a 3.07 (3.08?) rear gear ratio so my transmission builder needs a 7- or 8-tooth drive gear for my TH400 speedometer. This is the gear that goes on the shaft. Not sure if it matters but this is a short tail shaft.

Builder says his supplier has told him the part is obsolete, no new ones to be had.

**WorkinLonghorn** · 10-26-2018, 05:14 PM

Can't help with finding one but what size tire do you have? 15/35 or 15/36 for a 26" tire.
17/34 for 31" tires.

http://www.tciauto.com/tc/speedometer-gear-calculator/

Make sure to scroll down to TH400 -BA

**ThreeQuarter** · 10-26-2018, 07:01 PM

He didn’t pass along what his calculations are, but here’s what I’m figuring:
Tire size 27.7”
Gear ratio 3.076
He must be thinking 8/18 but it looks like 17/38 will be even closer.

I assume I can ignore that some driven gears in the TH400 table are labeled (truck)? Is that just showing OE, and not an indicator that I should limit my selection to only those gears?

10-26-2018, 03:58 PM	#1
ThreeQuarter Senior Member Join Date: Jun 2000 Location: Texan in Iowa Posts: 2,522	WTB TH400 Speedo Drive Gear: 7 or 8 Tooth I have a 3.07 (3.08?) rear gear ratio so my transmission builder needs a 7- or 8-tooth drive gear for my TH400 speedometer. This is the gear that goes on the shaft. Not sure if it matters but this is a short tail shaft. Builder says his supplier has told him the part is obsolete, no new ones to be had. __________________ Ben '68 Chevy C10 Custom LWB 327/TH400 2nd owner '16 GMC Sierra 1500 SLE Dblcab 4wd 5.3

10-26-2018, 05:14 PM	#2
WorkinLonghorn Senior Member Join Date: Dec 2006 Location: Studio City, Calif. Posts: 2,886	Re: WTB TH400 Speedo Drive Gear: 7 or 8 Tooth Can't help with finding one but what size tire do you have? 15/35 or 15/36 for a 26" tire. 17/34 for 31" tires. http://www.tciauto.com/tc/speedometer-gear-calculator/ Make sure to scroll down to TH400 -BA __________________ '69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, stereo speakers, 2nd owner, Work-Truck supreme.

10-26-2018, 07:01 PM	#3
ThreeQuarter Senior Member Join Date: Jun 2000 Location: Texan in Iowa Posts: 2,522	Re: WTB TH400 Speedo Drive Gear: 7 or 8 Tooth He didn’t pass along what his calculations are, but here’s what I’m figuring: Tire size 27.7” Gear ratio 3.076 He must be thinking 8/18 but it looks like 17/38 will be even closer. I assume I can ignore that some driven gears in the TH400 table are labeled (truck)? Is that just showing OE, and not an indicator that I should limit my selection to only those gears? __________________ Ben '68 Chevy C10 Custom LWB 327/TH400 2nd owner '16 GMC Sierra 1500 SLE Dblcab 4wd 5.3



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