Get out on one of the CA freeways, and when you pass a mile marker, as the sign passes the right hand windshield edge, note the odometer. Pass the next one, and note the odometer, and to really get a good picture, drive for 8 more til you have gone by ten mile markers. Note the mileage. If you odometer shows 8.5 miles when you have gone 10, you are off by 15%, and so is the miles per gallon using the odometer for calculations.
I would also check the fuel pressure regulator. It will knock the mileage down significantly if it is not working, but you'd likely notice black exhaust smoke.
tom

 

That's old school, today you use GPS or a GPS enabled smartphone.

Josh

 

So, I am old. What are you gonna do about it?
tom

 

Point being, today's day and age you can get pinpoint and instant accuracy using GPS.

And having a friend that works for DOT... those mile markers aren't that accurate, so at best you will have a very arbitrary understanding of how far off your speedo really is.

Josh

 

Bullitt390:
The reliability of DOT is ... what it is. That's why I said to check for ten miles. Odds are that the wandering mile markers would add up to near ten miles, even if the individual markers were not exactly placed.
Some people don't have GPS, and what I suggested would work for them. Why don't you put up some directions on how to use a GPS to know when you've gone ten miles, for example? I would not have a clue as to how to figure actual distance traveled vs the odometer reading, which is why I posted in the 1st place.
tom