just for info, it is not the actual wheel diameter that keeps the speedometer the same, it is the outside diameter of the tire that determines the speedometer speed. the speedometer is basically driven off a small gear set up on the output of the transmission. if you know the original size tire that the truck came with then the speedometer will stay the same with different wheels and tires as long as you keep the outside diameter of the tires the same. you can use the comparison chart in this link to search for a tire with a different wheel diameter but the same outside diameter. a smaller wheel with a tire that has a taller side wall and the same outside diameter of tire will make the speedometer read the same.
https://tiresize.com/comparison/
here is a link to the coker site that shows the specs for the 6.70 x 16 tire. it shows that tire is 28.6 inches tall/overall diameter. if youu find another wheel size, like a more common 15 inch, with a tire the same height, like a 207/70R15 at 26.3 inches tall or a 215/65r15 at 26.8 inches tall, your speedo should read very close to what it should read.
https://www.cokertire.com/tires/670-...blackwall.html