My speedo in my 69 Chevelle shows about 15 MPH faster than i'm actually going. I figure its because the previous owner possibly swiched rear gears to a much lower ratio. Maybe swaped 3:31 for 4:10?? How can i correct the speedo to the correct MPH? Randy

69 Chevelle SS Lemans Blue. Big block, 12 bolt, 400 turbo.

figure out the final drive ratio- essentially your true cruise rpm at 60 in 3rd gear divided by 1000- and get a drive gear/driven gear that comes out to that ratio.
for example, if you have 3000 rpm at 60, then your final drive ratio is 3:1, and a drive/driven gear combo of 30 teeth and 10 teeth would make your speedo read correct.