Quote Originally Posted by RuralTowner View Post
Holy old thread (sorta).

Would be nice if it could be calibrated, but hardly worth the effort IMO. Coming up with the numbers hasn't been too difficult. I had to do it when using another temporary speedo...and to make adjustment to the odo (due to slightly oversize tires) when I still had the original 4sp in it (for every 20miles add 1 mile). The numbers I come up with for adjustment are typically about 5-15 miles different from the guesstimated miles traveled between fill ups which gives me a rough low//high mpg range.

The only other thing I see happening to fix the problem was if I was able to figure out exactly what year the tranny is from...and find a donor vehicle with the appropriate instrument panel. I THINK the tranny is from a mid 80s which more than likely means by then the speedo was an integral part of the instrument panel. As a result I'd have to hope the gas gauge in it would work with my sending unit (that is even if I was successful in finding the right wires to hook up).
Yeah, it's an old thread, but it's good to follow up on these things. I figure it's significant because I have a hunch that the speedometer/odometer issue is relatively common for those who do a number of modifications on their truck. Sounds like you just kind of have to make do though, as it's not easy to remedy this issue.