Thank you Mike and Carl for the replies. I've heard back from the seller who has provided me with pictures of the back of the mechanism which allows for easy adjustment of the spring without having to take everything apart. I'll attempt my adjustment in a couple of days and let you know how it comes out.

Research in the Master Parts Catalogue is always fun! For the 1948 model year there are 7 different part numbers for the speedo as used in the 1/2. 3/4, 1, & 1-1/2 Ton models, with 7 different 'speedometer gear teeth' used. The gears range from 4-12 to 6-20, which I guess are like a ring/pinion relationship, though not presented as 4:12 or 6:20. If in fact there is such a range in the 'speedometer gear teeth', it's no wonder that many gauges aren't very accurate if they came from a different model truck than they're being installed in. By the way some particular models also use adapters attached to the back of the gauge. Lots of possible combinations for gauges that all look the same!