Ticopowell has some good info but I'm going to add to it.
The 78 mph limit might have been because of the wind resistance brought on by the boat. They aren't the most aerodynamic thing you can pull on the street at high speeds. The speed limiter is probably still set at 95+ but the truck just can't get going any faster with the trailer. I think Bill goes as high as 120. You could probably test it after you unhook the boat, just dont change tunes and see if you can make it past the 78.
When you put a different sized tire than what you ordered you go into the custom options menu and enter your tire size. Then check the gear ratio to make sure that is still accurate. After that you wont have to change any of the values you entered. You will, however, have to use the custom options menu next time you reprogram to a different tune. This time all you need to do is use "Load Previous Settings" and click "Done".
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