I have just changed the rear in my 1969 convertible with a Turbo 400 from a 3.08 to a 3.70. The current driven gear is green with 42 teeth. I am confused about what the replacement gear should be. Does it also have to be green? If after I check my speed with a GPS it requires 39 teeth or less, do I also have to replace the speedometer fitting? I appreciate any info that you can send my way.
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Re: Speedometer driven gear
Greg-----
You've got a bunch of problems, misconceptions, and "mysteries" here.
First of all, for accurate speedometer calibration with the information I'll provide, the tire diameter used on the car will need to be the same or close to the same as stock. If you use GPS to determine your accurate speed, then you can just use the information here as a general guide.
1) If you've changed ratios from 3.08 to 3.70 (which, incidentally, is not a change that I would ever recommend with a THM-400), then you need MORE teeth on the driven gear and not less;
2) Your car with 3.08:1 was originally fitted with a 37 tooth RED driven gear. It was also fitted with a speedo driven gear sleeve equivalent to the current GM #1362294. Obviously, if you currently have a 42 tooth green gear in there, then someone's changed it. Whether they changed the sleeve as they should have, I don't know;
3) Sleeve GM #1362294 is used with driven gears of 35 to 39 tooth count; sleeve GM #1362293 is used with driven gears of 40 to 45 tooth count; check the one you have for any indication of part number. The 1969 era part number for the 35-39 tooth sleeve was GM #1485025 and the 40-45 tooth sleeve was GM #1485026, so check for these numbers, too);
4) With a 3.70 rear gear ratio and assuming stock or near-stock tire diameter, you will require a 43 tooth PURPLE driven gear of GM #1362196 or a 44 tooth GRAY driven gear of GM #9780470 depending upon your actual tire diameter;
5) You will not need to be concerned about a speedometer DRIVE gear change. For 1969 THM-400 applications, the DRIVE gear was the same for all DRIVEN gear combinations.
6) In the event that you require a DRIVEN gear of greater than 45 tooth count due to tire diameter variation from stock, then you will have to use a speedometer adapter in combination with, possibly, a different DRIVEN gear.
7) All of the above-referenced parts (except, of course, the 1969-era sleeve part numbers which are replaced by the numbers mentioned) are still available from GM.
Give thanks for the NCRS Tech Board; it's one of the few places in the world that you could have gotten the above-referenced information.In Appreciation of John Hinckley -
Re: Speedometer driven gear
When it comes to "speedometer driven gear" questions (and part numbers), Joe usually has a GOOD response.... and that definitely sounded like a GOOD response to me!Comment
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Re: Speedometer driven gear
Greg,
Drive your car to 1410 S. Speer Blvd Denver, Co. ask for Russell. This is an old AC/DELCO facility and they calibrate the old fashioned way: they put rollers under your rear tires go a certain distance (a mile I think), read their inmstruments and then by "magic" (they have some sort of a chart) they know which gear you need. This is a great place. JimComment
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