[gordon]

The easy way is to out and buy a new reproduction jeep round speedo, which will interchange nicely. The US cost was about $60 last time I looked.

Stripping and rebuilding the two speedos is no real problem, even if the front galss breaks you can change it as it's just a flat sheet. The problem is you would have to fit the pointer and then road test it against a vehicle sitting at, say 30mph to see how far off it was, and whether you wanted to move the pointer and have another go at checking.

I don't know any reliable checking and calibrating procedure for them except comparison with another vehicle which has a reasonably accurate speedo, but since many of the small speedo drive gears on the transmission were commonly changed to get the right setting for axle ratios and tyre sizes a check at 30 isn't a bad idea. Less than 30 who cares if it's accurate, and if it is right at 30 it'll be close enough till one of these venerable trucks reaches the top end of what we laughingly call its' speed range.

Gordon