The nominal tire sizes will all calculate to a difference of less than 1/2" I'd imagine. In reality they're all going to be very close when mounted on the truck and inflated to spec. There are only 2 rear end gear ratios available for the Hemi's, so it should be relatively simple. I don't have the number's in front of me at the moment for the trans gears, but calculating the overall final drive ratio including tires would still just come down to the rear axle ratio's.
For example, the 20's on my Laramie are 275/55R20, so that means 275mm wide * 55% * 2 / 25.4 + 20 = 31.9" diameter.
Your Rebel has 275/70R18 which calculates out to 33.15", which means radially, there's a little over a half inch difference (I spoke too soon a few lines ago huh?), which would translate into a less than 4% difference in circumference (which is what will translate to speedo error, if any), IF all other metrics were the same, and we wouldn't need to consider wheel width, tire pressure, and vehicle weight to determine true rolling diameter (or circumference, whichever we use to calculate).
As this math unfolds as I type here, that 4% difference is 2.4mph difference baselined at 60mph, so I imagine we're missing something with the actual mounted diameter of the wheels. Otherwise, each truck would need to be calibrated to its tire size at the factory, and from what I understand, that's not the case with how most manufacturers do things.
