Speedo Problem I think?

[Buckskin7T7V]

I installed a 1982 four speed, with a 3.08 rear-end. I know it a automatic rear end ratio.
My speedo get up to 40 and stay there. I am driving by the tach. I was going to start trying different gears in the tranny to get it close, but I am wondering if I need a new speedo or speedo cable first.
I had a 3.70 in there, but the mileage sucks for todays California gas prices.

 

[1979toy]

Does this mean that you're not at 3300 RPM on the highway and can even hear the radio?
Did you change the speedometer gear in the transmission? There is a plastic gear to drive the speedometer in the transmission that has to be changed when you make a rear gear or a major tire size change. It's easy to do, but getting the right gear is a guess.