Speedometer gear question...

Hey guys;

I have replaced the dash on my '82 XJ750 with an Acewell ACE-3973. Everything is good, except the speedometer is off. The Acewell guys told me that I need to enter a number that is "the rolling circumference of the tire in mm divided by the number of turns of the speedo cable per turn of the wheel". Thats all fine and dandy, but the new speedo cable adapter can't be seen turning as the other end is an electronic connector. Since I dont have my old speedo cable to check, and aside from sticking a similarily sized rod in the speedo gear to count the turns, does anyone know what this ratio might be?
just thought I'd ask first. The rolling circumference in mm is 2075.

 

The ratio is 2240rpm=60mph, IIRC.

Which I realize is of no use to you. I'd stick a pointer in the old unit and spin the wheel.

 

Is it an adapter that attaches to the factory speedo gear that is mounted on the wheel?

If so, just take the adapter off, take a fine point Sharpie, make a mark on the gear output port, and spin the wheel one full revolution. Count how many revs the gear output spins, and voila.

 

Well, I did what had to be done and stuck a pointer in there. In case you were wondering, it's about 1:3 (one full wheel rotation = 3 speedo cable rotations.) So if I round up, I'm gonna go with 692. That kind of makes sense because it was set at 650 and I was 10- 15 KMs off. I'll let you know how it goes.

 

So, as it turns out, after doing all that math, it's still about 10-15 under. So I grabbed my trusty GPS unit and went for a ride. After a few stops continually increasing the number each time, I have dialed it in. In case anyone else needs to know, the number is 1150. That's on a stock front wheel (26" diameter with tire).