the speedo gear change was in 1985 when they went from 85 mph speedos to 120 mph speedos -gears are different. found it out the hard way - the tranny failed in my 85 ram 50 and I got a tranny from an 84 - the shop working on it scrapped the old tranny - drove the truck down the road at 55 and wondered why I was blowing by everything on the road - the speedo was 15 mph off low.