Towards the end, Walker is in a helicopter with a shotgun and supposedly shoots the tyres of the trucks. A shotgun doesn't have the range, and even if it did the shot would splatter all over the place. He would need a long range rifle for the purpose.
The trucks that Walker was chasing are headed east on Highway 80 to Louisiana. One of the characters mention stopping at the Four States Fairground. The nearest fairground with a similar name is in Texarkana, but Texarkana is about 70 miles north away from Highway 80