You have 3 options, I looked into this on my 2017 yukon
1) waste a lot of money and have a dealer do it. They have to pay GM around 100-150 to send in a new system software file. Plus whatever they charge for labor. If the place you went to won't do it, try another place.
2) buy a tuner to update your tire revolution/diameter settings. You can also mess with a lot of neat other things.
3) don't do anything. At most your speed at 70 will be 1 mph off. GPS it. You will realize it isn't enough to do option 1 and 2
1) waste a lot of money and have a dealer do it. They have to pay GM around 100-150 to send in a new system software file. Plus whatever they charge for labor. If the place you went to won't do it, try another place.
2) buy a tuner to update your tire revolution/diameter settings. You can also mess with a lot of neat other things.
3) don't do anything. At most your speed at 70 will be 1 mph off. GPS it. You will realize it isn't enough to do option 1 and 2