Mike Brichta Posted March 3, 2007 Share Posted March 3, 2007 I'm sure someone has dealt with this question before but I have never done this myself. I recently changed my rear gears from 2.73 to 3.55 and now I need to change the trans gear to make the speedo look somewhat correct. Questions: 1) Do I have to pull the tailshaft to switch the gears? 2) How many teeth does the new gear have to have? 3) Are the TH350 gears and the TH400 gears the same? The reason I am asking some of these questions is because I have a new trans gear for a TH400 that has 38 teeth. It is brand new in the box and was purchased years ago to address the same issue in a TH400 but was never used. Maybe I can use this one? Thanks, Mike Quote Link to comment Share on other sites More sharing options...
montefrazer Posted March 3, 2007 Share Posted March 3, 2007 This site should help. http://www.tciauto.com/Products/TechInfo/speedo_gears.asp Quote Link to comment Share on other sites More sharing options...
Mike 57 Posted March 4, 2007 Share Posted March 4, 2007 The speedo gears drive and driven are different between TH350 and TH400 the TCI site is great for info on this. Quote Link to comment Share on other sites More sharing options...
Mike Brichta Posted March 12, 2007 Author Share Posted March 12, 2007 Ok guys. You got me into this, now help get me out. Here is the story: I have a 70 monte that has a TH350 from a 72 custom, which has the two-bolt pattern housing (fits Chevy & BOP) blocks. The 72 custom had a 2.73 rear in it and the speedo worked great. I now have the trans in a 70 monte with a 3.55 rear rear in it. I have to change drive and/or driven gears to make the speedo work correctly again. Before: 2.73 gear ratio, green 17 tooth drive gear, 37 tooth red driven gear Now: 3.55 gear ratio Question: what drive and/or driven gears do I need? HELP!! I am getting all sorts of answers from TCI and local Chevy dealers. Is this a Chevy tail housing or not? What do I need to change and to what? I have posted pics at: http://groups.msn.com/FGMCCPhotos/mikesmontes.msnw?Page=Last THANKS!! Mike Quote Link to comment Share on other sites More sharing options...
Mike 57 Posted March 13, 2007 Share Posted March 13, 2007 Mike, I was looking at the drive/driven gears available for that tranny and it does not look too good. Your best bet may be to find a speedo shop that can get a ratio adapter. It seems like the gears available for that tranny will only get you to around 3.40 with a 26 inch tire and you would have to get a new housing for the higher number driven gear. Quote Link to comment Share on other sites More sharing options...
Mike Brichta Posted March 13, 2007 Author Share Posted March 13, 2007 Hi Mike, Thanks for the reply. I am finding kind of the same results, not good. Apparently, back then it was easy to get all sorts of gear combinations for any kind of tailshaft. Not today. Vendors only stock what gears are most commonly used. The best suggestion that I am hearing so far (from TCI) is to find a "chevy" tailshaft because many vendors carry those gear combinations. I will see what I can find. Mike Quote Link to comment Share on other sites More sharing options...
monte70car Posted March 13, 2007 Share Posted March 13, 2007 i dont know what size tire u r runnin but if it a 26" tire u need drive 15 tooth and a 41 tooth driven. if u go to 700r4.com and click on the speedometer calculator. Quote Link to comment Share on other sites More sharing options...
Mike 57 Posted March 13, 2007 Share Posted March 13, 2007 Mike, Try doing a web search for 'speedometer ratio adapter' You might be able to get it done that way a lot easier. I think the keyway that holds the drive gear is different width between the chevy and the B.O.P. tailhousings Quote Link to comment Share on other sites More sharing options...
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