speedo gear question? November 12, 2008, 11:14:39 AM I was wondering if yall know what tooth count gear ill need for my 86 5.0 with aod going from a 205/70/14 tire to a 215/75/14 tire with a 2.73 rear gear ratio? i have several speedo's with gears from c5's, aods, and other misc 80's ford transmissions so... i just dont know which one is which anymore any clue's yall?Just incase its pertinant the 205/70/14 comes out to 25.3 inches tall and the 215/75/14 is 26.7 inches tall according to my calculationOk i just think i got it figured out... if ound a calculator here http://www.sccoa.com/faq/speedgr.htmlstock should be bout 17.4 so either a 17-18 tooth and with the new tires should be 16.5 so a 16-17 tooth would be good... this is based on 2.73 rear with 8 tooth drive gear in aod. does this sound right yall? Quote Selected
speedo gear question? Reply #1 – November 12, 2008, 07:17:54 PM Try this.http://www.fordracingparts.com/download/charts/129.pdf Quote Selected
speedo gear question? Reply #2 – November 12, 2008, 07:23:40 PM FWIW, I've seen an 8 tooth drive geared T-bird with 2.73's and 225 60/15's (stock tire on turbines) run an 18 tooth sppedo and register dead perfect indicated MPH when tested with a radar @ varying speeds.My own car; I run a 235 60/15 on 10 holes with 3.45's. The AOD is a newer unit with the 8 tooth gear. I run a 21 tooth speedo. It's close. 3MPH off throughout the tested range. Curious that the variance does not rise with speed, but that's what it is. I've tested it from speeds as low as 14 up to 70+. The difference is always 3. Quite strange. Quote Selected
speedo gear question? Reply #3 – November 12, 2008, 09:20:57 PM thanks for the input and cars just seem to be strange... but much more familiar with cars than women it seems Quote Selected
speedo gear question? Reply #4 – November 13, 2008, 05:20:44 PM Quote from: V8Demon;242765FWIW, I've seen an 8 tooth drive geared T-bird with 2.73's and 225 60/15's (stock tire on turbines) run an 18 tooth sppedo and register dead perfect indicated MPH when tested with a radar @ varying speeds.My own car I runn a 235 60/15 on 10 holes with 3.45's. The AOD is a newer unit with the 8 tooth gear. I run a 21 tooth speedo. It's close. 3MPH off throughout the tested range. Curious that the variance does not rise with speed, but that's what it is. I've tested it from speeds as low as 14 up to 70+. The difference is always 3. Quite strange.I'm running an 8 tooth AOD with 3.73s and a 23 tooth speedo gear. I have the same 3 mph variance all through the speed range too. Like you I'm puzzled as to why it is always 3 mph and doesn't go up.... Quote Selected
speedo gear question? Reply #5 – November 13, 2008, 10:20:36 PM wouldnt it be because the mesh of the gears is constant and when the drive gear turns at say 3000 rpms the driven gear spins at 4500 rpms (just for example) which is 1.5 times the rotation of the drive gear... therefor the drive gear at 6000 rpms would make the driven gear drive at 9000 rpms ( again a constant 1.5 times the rotation of the drive gear) thats how it makes sense to me Quote Selected
speedo gear question? Reply #6 – November 14, 2008, 08:44:45 PM The error -- if any -- SHOULD be proportional. That is to say it should be a constant PERCENTAGE different i.e. 5%.As true speed increased so should the indicated, but at a greater rate due to the actual percentage difference.FWIW I took note that the teeth on my new speedo gear are actually shaped differently than the ones on my old one when looking at the side face. Quote Selected
speedo gear question? Reply #7 – November 15, 2008, 02:26:42 PM Yeah I held a 21 tooth and the 23 tooth I'm using now side by side and the teeth look different. They both look different than the stock one as well. Quote Selected
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