Do what I do when I change tire sizes. Drive by one of those portable radar speed signs at a steady speed and see how far off it is. You can check the odometer with mile marker signs. or set it on 60 and see how far it goes in 60 seconds (1 mile per minute).
My D2 with the stock tires had the speedometer reading fast. I put 245 75's on it and the speedometer was right on the money.
My D1 had 255 65 16's. Speedo read about 4 MPH faster than I was actually going. I put 245 75's on and now the speedo is about 2 mph fast instead of 4 mph. I guess I need 265 75's then it will be right on the money.
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