speedo gears

[dewert]

I have a 40 Ford that I bought. It has 15 inch wheels on it. Not sure of the rear end. But the speedo is about 20 mph off (faster than actural). It has a #19 gear in it. Is there one that will get me closer to the actual speed and is it available anywhere. Will it fit in the housing easily or should I just forget it.

Dewert

[Flathead Fever]

If your speedo is registering to fast you need a larger diameter speedo gear to slow it down. Here is a speedo chart from the Ford parts book. All of the speedo adaptors from 1928 to 1948 are interchangeable. These speedo adaptors show up on ebay.

[JM 35 Sedan]

Because of the speedometer gear diameter difference for each rear end gear ratio, the turtle housings on some are also different. So long story short, you can't just switch gears in any turtle housing to make it work. I'm sure you can find a gear combo that will bring you in closer than 20 mph. One way to determine how far your speedometer is off, is to run with a GPS and compare its speed reading to your speedometer.

[rotorwrench]

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Stewart Warner also made ratio adapters that can be installed at either end or somewhere in the middle with modifications. You just have to calculate the ratio to get closer to the actual speed. It seems like there is a ratio calculator somewhere on line.

Kerby