Need tire recomendations

[Old Rust Bucket]

The first thing I do when I buy a truck is put new tires on the back, regardless of what the previuos owner tells me about them. Anyway I need new tire for a C20. The truck had 16.5 rims on it from the factory. It has 245/75R16s on it now. I'm thinking of going to 265/75R16. How much would this change the speedo from stock? and would it read high or low. High? (it's too late at night and my mind has went to bed) I want at least a 6Ply tire with good A/T tread I don't mind if there noisy on the road so long as they got good tread

TIA,
ORB