A company called A.C.I. Engineering, Inc., in Portage, Michigan, PH. (616) 327-1991 provides ratio adaptors, which are small aluminum-housed gear boxes that screw directly onto the speedometer cable spud on the transmission. They are small and compact and can be mounted in about the same time it takes to connect up a speedometer cable. On ordering, they will request that you make a five mile run in your vehicle and tell them the difference between your odometer reading and the actual distance (5 miles). For more accuracy, you might want to check the odometer error in 10 miles, or, better yet, 100 miles. From this, they determine the gear ratio required in the ratio adaptor. They will also need to know the make and model of the transmission so they can provide the correct threads to screw onto your transmission and accept your speedometer cable.
I bought two of these, one for my 36 Buick Special that had a Century rear end installed, the other for a 55 Chev pickup with a TH400 automatic transmission and the stock rear end. In both cases, the adaptors worked great, bringing the speedometer/odometer back to accuracy, and both were simple, non-obtrusive installations. I believe they were in the $50-$60 price range.
