Of course you are correct about the difference between the "D" and the "J" --my bad--it had been too long since I've examined that area of my formely M40 '69 with it's M41J and 4.10 rear. When I first put the car on the road in '89 I installed an M41D and made the gear switch. Then at some point I installed the "J". I've had lots of wheel/tire combinations--15's and 14's--the measurement of the circumference is generally within 2-3" difference, meaning approx. 2-4% difference. I don't recall the combo used the last time I drove the car a long way but the difference between actual distance and odo reading was 10-12% based on my ten mile mileage checks with the odo reading high. In other words I'd driven ten miles, odo registers 11 point something. I do think you have to take into account the fact that your 15 mile (or 13?) commute is not very far. You are spending a great percentage of your trips in the warming up mode--coolant temp comes up right away but the mechanical parts of the drivetrain and the oils they carry take much longer to full running temp. I think you should not be too concerned with comparing your mileage with that claimed by others, but rather in relative terms to what you are getting and the improvements made along the way.