Georgia Flash Posted August 1, 2006 Share Posted August 1, 2006 Good morning all, I have a question regarding the correct speedometer calibration gear to use. I searched this topic and found a post from 2002 discussing this very topic, however, it was talking about swapping a Datsun 4-speed for a 5-speed. I need to know about my set up. As most of you know I have the Chevy 355 and GM 700R4 set up. I also have an aftermarket Autometer speedometer. Right now the speedometer is way off. It appears to be about 20 to 30 mph off. For example when cuising at about 50-60 mph the thing reads about 100mph! I know that the rear end willl determine what calibration gear to use but I do not know what the rear end ratio is. It is a R200 in a 1976 Datsun 280Z if that helps. Oh and also where would I find the rear end ratio? If anyone with this set up has gone to Chevrolet and got the calibration gear help would be appreciated. Thanks, Charlie Ball Quote Link to comment Share on other sites More sharing options...
Georgia Flash Posted August 1, 2006 Author Share Posted August 1, 2006 Did a little more searching and found a post from 2003 where Mat73GNZ posted a list of differential ratios. According to the list the 1975-76 280Z came with a 3.545 R200 from the factory. If I could get a second on this I will see if I can correct my speedometer problem. Also if there is a way to verify this on the R200 housing or elswhere that would be helpful. Quote Link to comment Share on other sites More sharing options...
Paz8 Posted August 1, 2006 Share Posted August 1, 2006 Start by pulling the speedo gear out of the trans to see what color it is, I am assuming you have a cable driven speedo and not electronic. 34 teeth Light Green, 35T pink, 36T white, 37T Red, 38T Blue, 39T brown, 40Teeth and up requires you to change the trans bullet that holds the gear into the tailshaft. Go to a Chevy dealer and get a 39 tooth gear (about $6.00) see if that helps. Quote Link to comment Share on other sites More sharing options...
Mike kZ Posted August 1, 2006 Share Posted August 1, 2006 It is a 3:54 ratio. I also heard that the brown gear ( 39 ) is closest. Quote Link to comment Share on other sites More sharing options...
Paz8 Posted August 1, 2006 Share Posted August 1, 2006 39 tooth gear is GM part# 1359273, thats for a cable drive system. Quote Link to comment Share on other sites More sharing options...
Georgia Flash Posted August 1, 2006 Author Share Posted August 1, 2006 Thanks Mike and Paz8, I will head that way as soon as I get my dash bolted back in and the wires hooked back together. I finally got around to pulling the dash the other night to secure the speedo and tidy up the wires to the other Autometers. I am working on some type of custom plate for the fuel gage and of course the matching autometer clock. (It just wouldn't be a Z-car without a clock you know) Thanks again Quote Link to comment Share on other sites More sharing options...
Georgia Flash Posted August 25, 2006 Author Share Posted August 25, 2006 39 tooth gear is GM part# 1359273, thats for a cable drive system. Update: I purchased the brown plastic 39 tooth “driven gear†from Chevrolet and installed it onto my transmission yesterday. However, it seems the odometer is still way off. Today I went for a ride and came upon an old pickup truck towing a homemade trailer full of trash. According to my speedometer, we were going 95mph. We were probably only going about 50 to 55mph at the most. The driven gear that I took out of my 700R4 was the blue one with 38 teeth. The brown driven gear only has one more tooth than did the blue gear so there in lies my problem. I am wondering now what size the drive gear is on the tail shaft of the transmission. I used a calculator on a transmission web site that took into account my rear end gearing (3.545) and tire size and it suggested that I use a 43 tooth driven gear and a 15-tooth drive gear to get my speedometer right. I guess that means that I will have to get the other housing from Chevy if they even have a 43 tooth driven gear. Oh well back to the drawing board. Quote Link to comment Share on other sites More sharing options...
Mike kZ Posted August 25, 2006 Share Posted August 25, 2006 Sorry, I hope they are not expencive. Quote Link to comment Share on other sites More sharing options...
Georgia Flash Posted August 25, 2006 Author Share Posted August 25, 2006 Mike, The plastic gear was only about $6 so no biggie. I think that the housing is going to run somewhere between $40 and $50. I will have to go looking again on my next off days. Quote Link to comment Share on other sites More sharing options...
Poundz9oh9 Posted August 25, 2006 Share Posted August 25, 2006 I'm a little unclear as to why you need to change the housing....???? I went through the same thing when I was in the planning phases of my swap with the 700R4. I too found a calculator on a transmission website that told me the gears that I needed. I swapped out the drive, and driven gear and my speedo was pretty close to being dead nuts. I purchased the gears through the same site that had the calculator. There was like a $20 minimum purchase though. So, I just bought a couple gaskets and seals as well. edit: This stuff might help http://www.bgsoflex.com/speedo1.html http://www.transmissioncenter.net/speedometer_calibration_______va.htm#700r4 Quote Link to comment Share on other sites More sharing options...
Georgia Flash Posted August 25, 2006 Author Share Posted August 25, 2006 I'm a little unclear as to why you need to change the housing....???? Because anything above 39 teeth which is the brown gear I purchased requires a different housing according to the TCI website. http://www.tciauto.com/tech_info/speedo_gears.htm According to their calculator I need a 43 tooth driven gear Quote Link to comment Share on other sites More sharing options...
Poundz9oh9 Posted August 25, 2006 Share Posted August 25, 2006 Are you running the JTR recommended Caprice style housing? Quote Link to comment Share on other sites More sharing options...
Georgia Flash Posted August 27, 2006 Author Share Posted August 27, 2006 Do not know anything about that but i would say no. The transmission came from a full size chevy or GMC van. Quote Link to comment Share on other sites More sharing options...
Paz8 Posted August 28, 2006 Share Posted August 28, 2006 Are you sure about the rear end ratio, the speedo shouldn't be that far off. I think the drive gear on the tail shaft of my trans is red in color 17 teeth. Quote Link to comment Share on other sites More sharing options...
Georgia Flash Posted August 28, 2006 Author Share Posted August 28, 2006 Paz, no I am not sure lol that is why I have asked the question "is there a way to find out what rear end gear I have by looking at the numbers on the outside of the rearend housing." My guess is that I have a different drive gear than I am supposed to have on the tail shaft of the tranny. I will get it right though and when I do I will update the thread to tell what I did, I promise Quote Link to comment Share on other sites More sharing options...
Paz8 Posted August 28, 2006 Share Posted August 28, 2006 Try doing a search about your rear end gear, I seem to remember someone explaining how to check this. The other way is to remove the rear cover on the differental and look at the ring gear, will have a number like this H39:11. 39 divided by 11 = 3.54. Look close the numbers are stamped small, and don't forget to drain the gear fluid first. Sorry about the mix up I thought you knew for sure about the ratio. You don't need to replace the tail shaft for a gear larger than 39 teeth, just the holder (bulllet) that the driven gear fastens into the trans tail shaft, the round part that your speedo cable fastens to. Quote Link to comment Share on other sites More sharing options...
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