rpmracer70 Posted October 16, 2008 Share Posted October 16, 2008 i have a turbo 350 trans , i just changed the rearend i went from a 273 to a 410 and my speedo reads 70 when i'm going around 40 , i was wondering how to make it right is there a gear that i can change or do i go to a trans shop ? thanks Quote Link to comment Share on other sites More sharing options...
Mo's70MCs Posted October 16, 2008 Share Posted October 16, 2008 Bob, quickest and easiest way is to change your speedometer drive gear.Try this site.. TCI Rod Quote Link to comment Share on other sites More sharing options...
judge Posted October 16, 2008 Share Posted October 16, 2008 Heres a web site that I used I went from a 2:73 to a 3:42 This site will help you make the calculations and tell you what gears you need. and also you will likely need a new driven gear housing as i did because the gear will be to big to fit in the houseing. www.transmissioncenter.net Quote Link to comment Share on other sites More sharing options...
kc8oye Posted October 18, 2008 Share Posted October 18, 2008 i just did the same thing with my 700r4. do NOT buy the gears from TCI..they are _WAY_ over priced! check out this post here for more details. http://www.firstgenmc.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=162231#Post162231 Quote Link to comment Share on other sites More sharing options...
kc8oye Posted October 18, 2008 Share Posted October 18, 2008 What size rear tires do you have? Quote Link to comment Share on other sites More sharing options...
rpmracer70 Posted October 18, 2008 Author Share Posted October 18, 2008 tires are 255/60-15 Quote Link to comment Share on other sites More sharing options...
kc8oye Posted October 19, 2008 Share Posted October 19, 2008 okie dokie.. I ran the #'s thru TCI's calculator (this calc proved 100% accurate for my 700r4 speedo) the math goes like this (# of drive teeth) * (axle ratio) * (tire revs per mile) all of that divided by 1001 (I believe the 1001 comes from the fact that our speedo's want 1000 revs per mile to read acccuratly but I'm not for sure. tire rev's per mile is calc'd like this: 20168 (divided by) tire diamter in inches. I know that a 255/60-15 is 27.04" tall (that's what I use) the options for drive gears are 7, 8, 9,10, and 17 teeth. the drive gear is the gear on the output shaft.. most likely with 2.73's u have a red drive gear (that's what my th350 and my 700r4 both had and they were right with my 2.73's) that's 17 teeth.. when u do the math with 17 teeth, it comes out to 52 driven teeth.. they don't make gears that big. so your options are like this.. left # is drive teeth, right column is driven teeth. 7 - 21.3 8 - 24.439 9 - 27.494 10- 30.549 17- 51.934 looking at the list from the bottom up.. there is no 52, so that's out, there is a huge jump from 22 teeth to 34 teeth in the chart, so basicly, the only practical combination is a 7 tooth drive, and a 21 driven you will still be a couple mph off.. you might want to call that company I listed above and talk to them about a 'gear box' that can provide exactly the right correction for you. Quote Link to comment Share on other sites More sharing options...
rpmracer70 Posted October 20, 2008 Author Share Posted October 20, 2008 ok i will look into that and thanks alot for your help Quote Link to comment Share on other sites More sharing options...
kc8oye Posted October 21, 2008 Share Posted October 21, 2008 i forgot to mention, you could also consider changing your tire size a little bit to get it exactly right. I got lucky with my new 3.55 gears. it just happened that with the 255/60-15 tires I Was thinking about getting, I could get the speedo dead-on. if you go down to say a 245/60 that will bring your diameter down a little bit for example. I'm not sure which way you would actully need to go. Quote Link to comment Share on other sites More sharing options...
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