speed-O help

I got a new 700r4 in my '49 rat truck and the speedometer is slow . What do I do now to get it regestering in the ballpark ? Also , before - at highway speeds (I am guessing around 70mph ) the speed-o showed around 50-55 but I drove it up the highway a bit last night (at around 70 ) and it showed only 30 and seemed to drop off to just above 20mph , even though I feel I was going the same speed . I got this new speed-O from LMC 2-3 years ago , could IT be going bad on me already also ? It shows only around 3000-4000 miles on it ( still kinda new) I think !

Welcome to the site Bears38....it does sound like the speedometer is your problem. But check the cable to be sure it isn't breaking up or possibly too short and not staying fully ingaged.

Doesn't sound like the speedo. Like mentioned before check your cable and to get it to register right you need a speedo new gear in the tranny. Any Tranny shop can help you out. They will need to know what gear ratio and tire diameter.

I'm using a stock F1 speedo in my 51 and I just changed the gear in my TH350 to get it to read right.

Now it...

seems to have gone completely crazy , the needle sets on 70mph when the trucks not moving and just wiggles and dangles when in motion. I was hoping to drive it to the Frog Follies next week but soo many little bugs are popping up faster than I can fix 'em ! I guess I didn't rely on the speedometer that much before anyway , now to figure out whats going on with that danged gas gauge !!!

I did find a problem with my speedo, the needle was loose on the shaft and I just spun it back on until snug again and back on 0 mph . Lubed up the cable while I had it apart and it seems to have settled down (less bounce) at lower speeds .