Speedometer gear help please

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My Car
-Former 70 & 72 Mustang coupes owner.
-69 Torino GT, Formal Roof, 351W, Edelbrock 600 cfm, dual plane high rise intake, long tube headers, AOD trans, dual flowmasters, Pertronix II Billet, 3.5 TRACLOC g/r, 15" Magnum 500s, 100A alt, mini-starter
-70 Mach 1, 351W, FMX
Gentlemen,

Can someone help me out? I upgraded my 72 coupe's 8 in Ford rear end from a 2.79 gear ratio to a 3.25 gear ratio (TRACLOC). I have a C4 transmission and my tires are all 21570R14.

Can someone help me out on which speedometer gear to get? Hell, I'll even order from Ohio Mustang.

Oh by the way, the upgrade to 3.25 is pretty nice. It certainly won't knock my socks off like a 3.5 and above but it's a decent trade off for power and economy.

KR

 
Thanks CZ-75,

My speedo gear has 18 teeth on it so doing to the math I'd get 20.97. Sounds like I need a 21 tooth speedometer gear.

Just before I bought it though, I wanted to calculate it since new tires are slightly different than the original in size. Anyway, two separate calculators recommended 20 tooth speedometer gears.

Additionally, I used this formula: (drive gear teeth)x(Axle Ratio)x(Tire revolutions per minute) / 1000. I assumed my 1972 C4 has 8 drive gear teeth.

8 x 3.25 x 780.19 (I looked this up) / 1000 = 20.28

Thus, I'm going with the 20 tooth speedometer gear. I added the math in above in case folks check out this thread later on.

KR

 
Last edited by a moderator:
Yep, if you also changed tire size then the original speedo gear may have needed to be replaced even without a change in gears
Exactly. Even though the modern tire size is as close as possible with the original tire and rim (14") it will still be off a tad. That tad might be just enough with the original speedometer gear to throw off the "new g/r divided by old g/r" calculation and round up or down to a number that is 1 off. 1/2 here and there adds up.

I'll report back when my order comes in if I can pace with someone.

 
Follow up from threat start - a 20 tooth speedometer gear works perfectly. While pacing a car at 60 mph I'm at 60 mph or just a slight tad under it.

 
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