There are lots of ways to measure efficiency and progress. My son Nelson just wrote a freshman English paper about DBI for a class at NAU. He's particularly impressed that by running homebrew biodiesel it's now cheaper for me to drive my 3/4 ton Duramax pickup truck on my 75 mile work commute than my 37 mpg Toyota Corolla.
Another angle on this is something I've been thinking about: Petroleum MPG, or PMPG (yes, I made that up! "That just happened!"). Since environmentally it is a heck of a lot better to be running biodiesel from WVO than gasoline from petroleum, for a number of reasons explained elsewhere, I was interested in what my effective MPG was for the truck if I measure based on the amount of petroleum diesel I use.
Since the beginning of the year, when my truck had an oil change (Jan 8), I've driven, as of this morning, 6883 miles. If I counted correctly, that's 107 days, a little over 64 miles a day. Looking at my fuel receipts, and using $3.50/gal as a rough number for that period, I've purchased 86.9 gallons of petroleum diesel. The rest has been either B99 from Western States Petroleum , B100 from Dynamite Biofuels Co-op, or my own individually produced biodiesel.
My PMPG is 79.2 miles per gallon.
If I factor in the methanol needed to produce the biodiesel  used to increase my PMPG so dramatically, that is another 44.3 gallons of "fossil fuel" products (most methanol is produced from natural gas). That brings my "petroleum" fuel total up to 131.2 gallons for the period.
My PMPG is 52.5 miles per gallon.
Something interesting to think about.
 WSP's B99 is not WVO-based, but soy biodiesel from Iowa. I'm giving it a pass here for simplicity. It's still better than petrol diesel!
 20% of the volume of oil for the reaction, but we'll factor in methanol recovery since we are environmentally and economically sensitive :-) , so let's call it 15%.