metric 160 speedo in a 100+ truck

blackagatha · Post by **blackagatha** » April 16, 2008, 6:45 pm

anybody here, esp the guys with faster trucks, ever adapt one of those canadian speedos that go up to 160 KPH, with a gear reduction or something so that it actually reads mph? I think that would be cool...

I would think a 1.6 to 1 reduction box would make it work slick!

1944-1961 · Post by **1944-1961** » April 16, 2008, 8:20 pm

It reads mph now. except at for instance 60mph, it reads 80k or what ever on the speedo. Just expiriment to see what reads what. I dont know how you would ever get it geared right for what you want. You can not recalibrate the speedo for what you want. Also, if you start messing with the driven gear or put in a reducer ect. then your odometer wont be correct. Hope this helps.

Rosati · Post by **Rosati** » April 16, 2008, 9:11 pm

You will never get the calibration right because the ratio of sweep on the guage is exponential to the speed. Gear it higher, the guage will be off at higher speed. gear it lower, and the speed will be way under.

Z-MAN · Post by **Z-MAN** » April 16, 2008, 10:25 pm

Hey Rosati u get my message

blackagatha · Post by **blackagatha** » April 17, 2008, 2:37 pm

well, it just seems to me like the total sweep is about the same. Meaning 160 KPH is about equal to 100 MPH. If you reduce the speed at which it turns, I think it would be right. We dont want to count kilometers... seems like the 1.6 reduction would drop it down just right.

I've heard of gearboxes that go between the trans and speedo cable to adapt for pumpkins that are outside the range of the tranny's intended use (ie 3.89 pumpkin with a car trans) I think they may even be user-adjustable. I think they may have two tapered wheels or something.

Alan Mclennan · Post by **Alan Mclennan** » April 17, 2008, 5:24 pm

Mine does

but Lenore doesnt!
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Alan,

cdherman · Post by **cdherman** » April 17, 2008, 10:40 pm

I think it would be a whole helluva lot easier to find a correct speedo, or repair yours.

These items are often intact in junkyards -- I have a couple in boxes that I scrounged.

In terms of speedo technology, since mid 40's, speedos/odometers have been designed to have 1000 rotations of the speedo cable per mile.

I strongly suspect that the metric odo has 1000 rotations per 1.6 km.

Speedos have a magnet that moves the needle so and so far with various rpm. For mph, 1000 rpm of the speedo cable equates to 60 mph. One minute per mile.

To get that geared down so that it reflected miles, you would obviously need a tremendous reducer. You MIGHT be able to swap out a differnt spiral speedometer drive gear on the tail shaft, along with a much larger tooth count speedo driven gear on the end of the cable. But you would be pushing extremes. There are practical limits in the tooth counts of both.

I think the speedo assembly that shows Km is probably identical to the mph assembly, just with numbers painted elsewhere. Going to be a bugger to overcome that.......

blackagatha · Post by **blackagatha** » April 17, 2008, 10:50 pm

well, see, I have a perfectly good head, and 2-3 spares, I was just wondering about the possibility of a stock speedo that would indicate above 100.

the 1000 rev/mile argument is a good point... I never thought of that. it would only have 625 revs per mile... that would likely screw it all up. Maybe not though.

I think it would definitely be outside the capability of the trans gear, cable gear to correct for. That's where an auxiliary reduction box would come into play.

When I have the 3.89 in my truck, my C6 (with a car-type tail shaft, with machined steel gear, not replacable plastic, the closest I could get it was 8 mph faster than actual. SO I put it back at an even 10 so it was easy. And yeah, the miles went by too quick also.

I think I would like to get one of those boxes for that reason alone... although I havent used that pumpkin in 2 years anyway.