speedo pinion question

[maxine-70]

im getting ready to do a rear gear swap on my 86 GT. it has 3.27's now and im wanting to put in the 3.90 motive gears. the one little snag im finding is that since ford never made a 3.90 gear (unless you go way back to the 9" 3.89), im not finding speedo pinion gears to match this rear gear swap. is there somehting out there im not seeing that will just pop in place of my old one? my trans is a tremec TKO, and im using the speedo pinion in it that i took out of the original AOD. tire size is the stock diameter, but wider.

i want more than the 3.73 ratio but i dont think i really want to go with a 4.10. its a street cruiser convertible with a carbed explorer 5.0 with a trick flow stage 1 cam, weiand stealth intake and holley 1850 600 carb, heads shaved .030 and bowl ported only, new comp valve springs, and a TKO 5 speed.

 

[Mr Grim]

I think the gears you have now are 7/19. If so, I believe 7/23 will be the most accurate. You might want to confirm what gears you have in there now, and how accurate they are now.

 

[trbkrb]

This would probably be your best source for the info.

http://www.hanlonmotorsports.com/t5speedo.asp

It looks like if you have a 7 tooth drive gear on the output shaft, then a 21 tooth driven gear would be the closest.

 

[jrichker]

If removing and replacing the tailshaft housing to change the drive speedo gear isn't a good plan for you, here's an alternative:

Check to see if there is a speedometer shop in your area. If there is, ask about the Stewart-Warner speedometer gearbox. I believe it is either a 777 Series Drive Joint Kits and Parts or 666 Series Drive Joint Kits and Parts. It is a small gear box that fits between the speedo pickup gear on the transmission and the speedometer. It has quick change gears that allow you to choose almost any tire size and rear end gear ratio you want. This will allow you to get the accuracy within 1%-3%. The drawback is that it isn't cheap.
Please excuse the typo (their's, not mine) in the following picture - it should read Custom ratio adapters, not Custom radio adapters.

See http://www.gaugeguys.com/ratioadapt.htm for more info.

The stock range of speedo gears work correctly with a limited range of tire sizes.
See http://www.corral.net/tech/gearcalc.html for more help