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vondy · 04-21-2002, 08:27 AM

First I want to thank all who helped with my drooping headlight door, it works great. Now my speedo reads about twice as fast as it should,I.E. 30mph it reads 60mph, and it gets worse the faster you go. I have a turbo 400, 3:90 geared 10 bolt, tire size is 235 60 r15. I guess I need an adaptor but I don't Know for sure. Any help would be great, thanks.

vondy · 04-21-2002, 08:27 AM

First I want to thank all who helped with my drooping headlight door, it works great. Now my speedo reads about twice as fast as it should,I.E. 30mph it reads 60mph, and it gets worse the faster you go. I have a turbo 400, 3:90 geared 10 bolt, tire size is 235 60 r15. I guess I need an adaptor but I don't Know for sure. Any help would be great, thanks.

goathead68 · 04-21-2002, 07:29 PM

vondy,

Try this link:

http://www.offline.org/~burch/firebi...er%20Gears.xls

You'll need to know how tall your tires are and the code of the side of your trans to figure out the number of teeth on your drive gear.

Check the post in the 'Street' section of this BB titled 'Speedo gear spreadsheet' or something like that.

vondy · 04-21-2002, 09:13 PM

Goathead68, I tried to open the link but I couldn't get it to open any tips on how to do it. Vondy

goathead68 · 04-21-2002, 09:57 PM

It won't open unless you have Excel installed on your computer. Get me the code off the trans, the brand of tire and I'll punch in the numbers into the spreadsheet and we'll go from there.

goathead68 · 04-21-2002, 10:27 PM

Here's what I found out. I'm assuming you're using BF Goodrich T/As, which have a height of 26.1 inches. This is what you'll need to correct the speedo.

Drive gear #8642620, color gray, 15 tooth
Driven gear # 9775187, light blue, 45 tooth
Driven gear sleeve # 1362293

These parts will get your speedo in line-60 mph indicated will be 60.27 actual. Check with your local dealer to see if these parts are still available. I'm pretty sure the rear of the trans will have to come apart to change the drive gear.

Colorado Bill · 04-22-2002, 12:12 AM

Vondy,

If you can find the 2-letter code on your trans, that will tell you what drive gear is on the output shaft. Then you can use links from the last speedo post in this forum to find what you need. When I was trying to correct my speedo, somebody mentioned that the drive gear selection from GM is limited. I don't know that for sure, but there doesn't seem to be a problem getting driven gears.

Colorado Bill · 04-22-2002, 12:13 AM

[ April 22, 2002, 12:16 PM: Message edited by: Colorado Bill ]

vondy · 04-22-2002, 08:35 AM

Thank you both for helping me out, I'm going out to check the code in a few minutes. I am pretty sure the trans is a 1977 model so i'm sure this will throw a wrench in the works. I wonder if dr. frankenstien had this many problems when he built his monster? Vondy

vondy · 04-22-2002, 08:48 AM

OK, here are the codes I found; top line 470, bottom line 77 BC, followed by what appeared to be a serial number. I hope these are the codes needed. again, thank you.

goathead68 · 04-23-2002, 07:46 PM

Vondy,

Try this link:

http://www.offline.org/~burch/firebird/speedo.mv

It's also posted in the Lobby under "Speedo Calculator". It will come up with five gear combos that will help to get your speedo in line. One of the combos is the one I posted earlier.

Glad to help, and let us know how things turn out.

[ April 23, 2002, 07:47 PM: Message edited by: goathead68 ]

vondy · 05-09-2002, 07:49 AM

Well, my gears finally came in and the i.d. of the 15 tooth drive gear is to small for my output shaft. So I took it back to the dealer, the parts guy ordered another one, and amazingly the I.d. was right but the gear was a metal gear with no provision for the clip to hold it on. Does GM make a gear for this? thanks again. Vondy

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