So I have a 2021 Chevy Tahoe LT. We have had it since November 2020 and we use it on 2WD. On the dashboard when I fill up with gas whether on a 1/4 or 1/2 tank i get different readings and we use 87/89. So what is the actual mpg we should be getting on this vehicle? I know 1 time it said like 314 but it has never gotten that high again. Can someone help with this?