TH400/3.70 speedo gear help?
Thread Starter
Burning Brakes
Joined: Apr 2001
Posts: 752
Likes: 1
From: Springfield Oregon
TH400/3.70 speedo gear help?
I need to know which speedo gear to use with a TH400 and 3.70:1 gears. Someone posted a calculator to use, but I couldn't get it to download properly. Thanks.
Troy
Troy
Race Director
Joined: Nov 2000
Posts: 11,872
Likes: 12
From: Waterloo ontario Canada
Re: TH400/3.70 speedo gear help? (ORVette)
When I had the 400 and 4.11 gears I couldn't get the speedo right so I went to a local 4x4 place and bought a gear reducer. They have them in different ratios like 20% slower, 30% slower etc etc. They speed the speedo up for large tires. I took the thing apart and reversed the gears so it slowed the speedo down. I paid $5 for a used one. The speedo cabel in my 75 was 2 piece so I just installed it between the two lenghts. I got it right on.
Burning Brakes
Joined: Aug 2000
Posts: 925
Likes: 1
From: LT1 and LS5 Coupes... New Orleans, LA
Re: TH400/3.70 speedo gear help? (norvalwilhelm)
I'm showing that you need driven gear #1362049 which is a 42 tooth, color code Green. The drive gear that goes with this is #8623310, but it looks like all ratios in 73 used that drive gear, so you should be able to just swap out the driven. Still available from your dealer, $8.60 list.
Norval, the book does show that using a 4.11 or lower ratio would require an adapter, and the the 3.70 is the lowest you can go without one.
Norval, the book does show that using a 4.11 or lower ratio would require an adapter, and the the 3.70 is the lowest you can go without one.
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
Re: TH400/3.70 speedo gear help? (Allan71)
And while your at it, Allen, what's the driven gear number for a TH400 with a 3.55? I suppose that I could do the steps in my 3.08 to 3.55 switch, pull the driven gear, do the math and struggle with a bonehead kid behind the Chevy parts counter for the gear, but if you already got your book open...:)
Burning Brakes
Joined: Aug 2000
Posts: 925
Likes: 1
From: LT1 and LS5 Coupes... New Orleans, LA
Re: TH400/3.70 speedo gear help? (gerry72)
Andre, yes it shows the same setup for the Turbo 350 with the 3.70 axle.
Gerry, your driven gear should be #1362048, a 40 tooth color code Black, still available same price as above.
Gerry, your driven gear should be #1362048, a 40 tooth color code Black, still available same price as above.
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Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
Re: TH400/3.70 speedo gear help? (ORVette)
Just posting a follow-up to this old topic. For the THM400 driven gears, you may also have to replace your gear sleeve depending on the ratio your going to. The break is at 3.55. If you're going from a numerically lower ratio (like a 3.08 with a 42-tooth driven gear) to the 3.55 (40-tooth driven gear), you can keep your original sleeve, part # 1362293 which spans driven gear tooth counts of 40 through 45. If you are going from a 3.55 and numerically lower gear to anything above the 3.55 (like a 3.70) you will need to also replace your gear sleeve with the part # 1362294, which carries driven gears with 34 through 39 teeth.
Just something else to add to your research materials...
Edited to add: The sleeve costs about $21.00 and is, of course, still available over your Chevrolet parts counter.
[Modified by gerry72, 7:46 AM 1/31/2002]
Just something else to add to your research materials...
Edited to add: The sleeve costs about $21.00 and is, of course, still available over your Chevrolet parts counter.
[Modified by gerry72, 7:46 AM 1/31/2002]
