MTFCA Forum

Can anyone tell me what gear ratio the wheel drive on a 1926 Touring needs to have to display the correct miles per hour on the Stewart Warner 490?
Is it correct that it measures one mile after 1000 revolutions?

I'm getting a Stewart 490 and I'm wondering how I could drive its own gears so that it displays the speed in km/h.
I have 21" rims with Firestone 4.40/4.50-21 tires.

Greetings from Switzerland

When Australia went metric from miles to kilometers, there were al sorts of stickers, cards, etc produced to attach to existing speedo glasses with the new calibrations marked. The speedo was not changed, the dial was altered to reflect the new range. There were a few gear adaptors made which were installed at the speedo end, but these were expensive and only fitted some vehicles.

Allan from down under.

Stewart Warner produced speedometers that measured miles per hour or kilometers per hour. These units had different internal gears that detrmined the speed and distance. The gearing on the wheel (the road gear and pinion gear) did not determine whether the speed was in miles or kilometers. It was the internal gears in the speedometer head that made the difference. I hope this helps. Russ Furstnow

Thanks for the feedback.

Allan, with your version, the speedometer could be adjusted, but it still has the problem that the odometer counts in miles.

Russ Furstnow, thanks for your feedback as well. Yes, I'm aware that speedometers in kilometers per hour existed back then. However, such a model is probably difficult to find, especially with the small diameter of the Stewart Warner 490.

If I see correctly, the drive in the two pictures (which I found on MTFCA) has 16 teeth on the gear on the hub and 11 teeth on the gear on the speedometer cable. Correct me if I'm wrong; the pictures are blurry!

If I start with the rolling circumference of the tire and calculate the gear ratio, I get almost exactly 1000 revolutions per mile on the cable.

If I were to use a 56-tooth gear on the hub and a 24-tooth gear on the speedometer cable, that would result in approximately 1000 revolutions per kilometer.

Wouldn't my Stewart Warner then display km/h?

Greetings from Switzerland

I hope this information is appropriate: I also have a '26 touring which has 21" wood spoked wheels and I installed an early style 490 speedometer about a year ago. At the time I could not find original speedo drive parts. I bought a speedometer kit from Model T Ranch (which the vendors also sell). The cable had to be shortened. I had to make and crimp onto the cable a new square drive to fit the 490 speedo and fabricate a fork shape retainer to hold the cable housing to the early speedo I bought used. The Model T Ranch speedometer kit says their gearing rotates the cable 1000 revolutions per mile. I've not yet found any roadside radar to verify speed accuracy but have checked odometer accuracy against my modern daily driver and it is very close as best I can determine. Hope this info may help.
Peter Haid
West Falls, NY

You will need to buy a set of gears made especially for the 490. As you can see in the photos the gear ring that mounts on the wheel has spiral gears while the gear that mates up to it is a gear with regularly spaced teeth. If I am counting correctly, there are 11 teeth. The photo of the 490 in the cardboard box is a photo I took before mounting it on my1926 Coupe. I purchased it from Russ Furstow about 20 years ago.

I have 21” wheels and my 490 indicates mile per hour and miles driven. Being so small, the 490 is the only speedometer that will fit into the 1926 dashboard (with cowl mounted gas tank) without an adapter.