Going to 3.89 gears..speedo gear?

[Mike85000]

Hey Guys,

I have a set of 3.89 richmond gears for my 87 GT. I see speedo calibration charts which show correction gears for 3.73s, but not 3.89s.

I am running 245/60/15s on the back. Since 3.89 is only about 4% greater ratio than 3.73s, and I have taller than stock tires, do you think using 21tooth red speedo gear would work fairly accurate?

I think the stock tire size is 225/60/15 and I'm running 245s which are about 9% taller than stock.

If I were to run the red gear, wouldn't that leave me with only about 5% error?

I did search and read a bunch of posts, but couldn't find much info for 3.90 gears.

Thanks,
Mike

 

[91 HATCH]

3.90 gear with 225/60/vr15 which is 815 revolutions per mile

83-89 automatic requires a 7 tooth drive gear and a 23 tooth speedometer gear
90-98 automatic requires a 7 tooth drive gear and a 23 tooth speedometer gear
83-89 5 speed t-5 requires a 7 tooth drive gear and a 23 tooth speedometer gear
90-98 5 speed t-5 requires a 8 tooth drive gear and a 23 tooth speedometer gear

this is as close as it gets
here is the formula for figuring your speedometer gears


(drive gear teeth) x (axle ratio) x (tire revolutions per mile)
divide this number by 1000 and the sum is the # of teeth required


sample for your 3.90 gear set

7 x 3.90 x 815 = 22.2495
1000

you would normally select the closest whole number which would be 22 in your case (with stock tires) but there is no 22 only 21 and 23 also your tires are taller which means less revolutions per mile but i dont think it calls for going to 21 and i would use a 23 the difference would be very very close

 

[jrichker]

The stock range of speedo gears work correctly with a limited range of tire sizes. The 23 tooth white gear works good for some guys, but self destructs in a few months for others.

See http://www.corral.net/tech/gearcalc.html for more help

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.

 

[Mike85000]

Thanks for the input guys.

jrichker : That would be the ideal solution, but I'm not worried about the stock speedo being calibrated that precisely. And for the price, its not in the cards for me (student).

91 HATCH: Thanks for providing the formula. With your formula, my setup wold be

7 x 3.90 x 759 / 1000 = 20.667

I think i will stick with my plan for going with the 21t gear. Thanks for the help though guys, i appreciate it.