A friend gave me a classic IKU speedo the other day, just like I used to run in the late 70s with the wheel that runs on the tyre. I wore mine down at the time and a teacher at school made me a metal one !

Anyhow, we've put it on my sons bike but the calibration is way out. As a temporary measure we made some labels to stick on the glass with the correct speeds, but i wondered is the speedo/drive wheel made for a particular wheel size.

From memory on the front of the speedo is says 260/266 in small letters, I wonder if this is significant.

The speedo over reads alot, 15mph on the speedo is a true 10mph. I could make a larger wheel for the drive to compensate but i wondered if anyone knew if these were made for a particular size wheel in period ?

Anyhow, my sons likes it, prefers it to the clinical digital speedos around today.

I had one BITD, and it was quite accurate after a bit of tweaking and dialing it in. IIRC I calibrated it against the mile markers on a county road I used to train on. I think I ended up sanding the wheel down just slightly because it was calibrated for 27 x 1 1/4" rather than the sewups I was using then. You can also adjust them slightly buy moving the point where the little red wheel touches the tire.

If your IKU is really that far out, I suspect you guessed correctly, and the model you have is meant for smaller wheels. I don't know if they did make them for different wheel sizes; that is a reasonable theory. I'd expect it to be a percent or two off out of the box, but not 50%. Dial overlay seems like the best bet.

The theory behind the IKU was that they could be run with any size wheel since they operated directly on the tyre. One revolution of the (speedometer) wheel was the same tyre rollout distance, regardless of the size. You could calibrate them somewhat, depending where on the tyre sidewall the contact was made.

I recall Bicycling doing a speedometer comparison test in the mid-1970s, after the first electronic speedometers started to appear. IIRC, the IKU was accurate in a very narrow range around 15 mph. It under read at lower speeds and over read at higher speeds, with the deviation increasing the farther you got from 15 mph. They tested three samples and there was some significant variation between samples.

Oh, duh, you are right of course. I blame low blood sugar...

Mine was fairly accurate, IIRC. I didn't test the extremes. I probably used it for 6 months before the Cat Eye electronic speedos became available and I got one of those instead.

I have several IKUs that I have had since the late 1970s. All are on 27 inch Motobecane's. I thought that the IKUs were designed to be used on 26 and 27 inch wheels. My 1977 Motobecane with IKU.