The Ford Barn

Afordman31 · 05-29-2013, 12:24 PM

I have the round 30-31 speedometer which has a drive gear of 19 tooth, this is for the 3.78-1 axle ratio with 19 inch wheels. What I want to do is change the drive gear to a different tooth to make up the speed difference of putting on 35 Ford 16 inch wheels. I called Bert's they said to use a 20 tooth gear. Any thoughts on this appreciated.

CarlG · 05-29-2013, 01:30 PM

If Phil or Steve at Bert's said it, you can be certain that what they said is 100% correct.

Tom Wesenberg · 05-29-2013, 02:33 PM

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Bert's is right. If you install a smaller diameter tire then you won't be going as fast as indicated on the speedometer, so you need to slow it down a bit. Adding a tooth to the DRIVEN gear will slow it by about 5%.

I always check mileage for accuracy first, then the speed (if not correct) is adjusted inside the speedometer.

05-29-2013, 12:24 PM	#1
Afordman31 Senior Member Join Date: May 2010 Location: Michigan Posts: 593	Speedometer Drive Gear I have the round 30-31 speedometer which has a drive gear of 19 tooth, this is for the 3.78-1 axle ratio with 19 inch wheels. What I want to do is change the drive gear to a different tooth to make up the speed difference of putting on 35 Ford 16 inch wheels. I called Bert's they said to use a 20 tooth gear. Any thoughts on this appreciated.

05-29-2013, 01:30 PM	#2
CarlG Senior Member Join Date: Nov 2010 Location: Anchorage, Alaska Posts: 9,192	Re: Speedometer Drive Gear If Phil or Steve at Bert's said it, you can be certain that what they said is 100% correct. __________________ Alaskan A's Antique Auto Mushers of Alaska Model A Ford Club of America Model A Restorers Club Antique Automobile Club of America Mullins Owner's Club

05-29-2013, 02:33 PM	#3
Tom Wesenberg Senior Member Join Date: May 2010 Location: Mpls, MN Posts: 27,582	Re: Speedometer Drive Gear Sponsored Links (Register now to hide all advertisements) Bert's is right. If you install a smaller diameter tire then you won't be going as fast as indicated on the speedometer, so you need to slow it down a bit. Adding a tooth to the DRIVEN gear will slow it by about 5%. I always check mileage for accuracy first, then the speed (if not correct) is adjusted inside the speedometer.