Honestly it could be more towards 15 miles an hour over
Hey guys my speedo reads faster than I’m going by about 10 miles an hour just another thing someone put in this car incorrectly I know there’s a gear behind the speedometer cable on the transmission.
should I check it out and look at the color? The rear end seems to be open I’m not sure of the ratio

 

You need to know the tire diameter and gear ratio, then you can use this chart

https://www.tciauto.com/speedometer-gear-calculator

pull your existing speedo gear, count the teeth. look inside the transmission and see what the color Is of the existing drive gear. Hopefully it’s black, red, or some other color that won’t fade after long exposure to transmission fluid. Sometimes it’s hard to tell light green from yellow, they fade and start to look very similar. you should be able to come up with a combination of speedo gears to get it accurate. You may need to change just one, or both, and possibly the housing. Play around with different tooth counts. Try and get the combination as close as possible to a whole number. You can’t get a gear with say 35.5 teeth, you either have to round up to a 36, or round down to a 35.

 

You also need to know which trans and which drive gear is in that trans.

 

Enjoy the illusion of driving fast without the risk of getting a speeding ticket :-D

I have the same problem in my '71, which as far as I know is stock. 14' rims, tires might be a bit taller but not much, stock rear end (open diff) and no reason to suspect transmission has been out of the car for work at any point. I'll be interested to hear what you discover.

 

You reminded me of when I swapped the 2.56 rear for a 3.55 rear in my 70 Supreme. My Dad was riding with me and he was very concerned about me doing 85 on a 55 MPH highway and would not believe my printed out Excel conversion chart that showed we were really doing the speed limit.