How much I'm willing to spend to replace is ultimately what I'm trying to solve for. But, I don't want to start with I'll spend up to $10,000 for a replacement. (which is actually the most I'd ever consider spending. Really seems crazy high)

Thinking this through more. based on improvements to MPG, there should be a savings attached to each MPG.

So if 35 MPG = $275 per year which is a 9 MPG improvement each MPG above 26 is worth $30.55.

MPG/improvement in MPG/Savings for a year/Savings over 6 years

I'm assuming residual value after 6 years to be $0.

26/0/$0/$0

35/9/$275/$1,650

40/14/$428/$2,568

45/19/$580.45/$3,482.70

Here's my conclusion, someone let me know if it makes sense.

I should be willing to pay $6,650 for a car that today gets 35/MPG, $7,568 for a car that gets 40 MPG.*

*each price I'd be willing to pay would also need to factor in annual car tax and insurance differences which may or may not exist on a purchase by purchase basis. For simplistic purposes, I'm leaving that out of this calculation but trust me it would be part of the consideration given.