We live in Montgomery County, NE of Houston, TX. 40 cent difference was a bad example. That's what my wife reported that she paid wherever she filled up her Tahoe before our trip. I believe it's more like 15 cents for each upgrade in octane from what I have seen lately. We're currently paying around $3.29 per gallon for 87 octane. Outside temps vary of course but we were in mid-60's to mid-70's on our most recent trip. It's warmer now and the summer heat is just around the corner.Where are you buying gas that there is 40 cents per gallon difference between reg and mid grade. Out west at this time $3.59 for reg, $3.69 mid grade, $3.79 premium.
Besides what outside temps are you in ? When your in +100 degrees pulling grades you just might get ping.
Grades that are in the 6-8% range for several miles.
Besides if you want true actual mpg you really should top off the tank each time and hand calculate instead if going by the DIC.
I know not to rely on the trip computer for true gas mileage, was only using it for convenience to show the relative difference between normal driving and towing or results for 87 and 89 octane (limited test I know). I am logging my mileage for every tank in my daily driver and I see almost 10% error (on the high side) in what the trip computer indicates.