getting speedo and odo right

[barryc]

I have a '72 C10 street rod I bought last summer - now I'd like to make some changes. I don't know a lot about this stuff, so hang in there with me... The truck has a 4.11 rear (I'm told by the seller), and I believe the wheels are a non-stock size. So, the speedo and odo don't read right (actual 50 mph reads as 65 or so). The transmission is a TH350, and the gauges look stock. I found on the internet that I can get a ratio adapter, but only after some calculating. Can anybody advise on the best way to get the gauges to read right?