Put aftermarket wheels on my 1988 F250. Has 460 engine and C6 auto. 4.10 rearend gears

Changed the factory 235/85/16 to 265/75/16. Speedometer seems to be about 7mph slow. I have driven it like this for 50000 miles and now i ripped the speedo cable off so i am deciding to fix it and calibrate the speedo at the same time.

Also give a store where i can purchase the gear.

Thanks