Leon wrote:

By any chance did you drive through the mountains? Oddly I have always
gotten better gas mileage when going through the mountains. Thinner air
does not require as high of octane from the fuel to prevent valve clatter
and if you are burning regular elevation fuel 87 or better your mileage
could increase also. Typically gas octane in high elevation regions has an
85 or lower rating.

Thinner air requires a leaner mixture, but overall horsepower is reduced.

This is demonstrated to me every time I fly. G