You're right, it will be close. Probably only a couple MPH off with stock tire size. If you go oversize, like most do, it's something to consider. My factory size is 275/70/18 and I'm running 35x12.5x18 and I was off almost 8mph.

I'm just sayin bigger wheels generally lead to bigger tires, that's all.

 

If anyone ends up near me, I can recal speedo's free of charge for ya.

 

Thanks it looks like standard 65 series 20's only add an inch to the overall diameter on standard on-off road tires with a max of 34.1 inches. I was looking at the Good Year Dura Trac Commercial. Looks like a quality on off road tire with a decent tread depth.