I don't think the emphasis on saving is correct. You're picking an arbitrary origin which is not zero. If you go from 10 MPG to 30 MPG you are now using 1/3 a

I don't think the emphasis on saving is correct. You're picking an arbitrary origin which is not zero. If you go from 10 MPG to 30 MPG you are now using 1/3 as much gas. If you go from 30 MPG to 90 MPG you are once again using 1/3 as much gas.