tollgirlz,
Typically, the energy content of 100% biodiesel is about 10-15% less than that of regular diesel (D2). Although, some argue that the lubricity of BD offsets it by 2-5%. But for simple math, if you normally get 15 mpgs on D2, you should expect around 13-14 with pure biodiesel (or B99)....if normally 20, then 17-18 with B99.
I have found some of my best mpgs in both my vehicles (05 Dodge Ram & 06 Jetta TDI) when running B20 (20% biodiesel). Most studies show that the energy loss when running 5-20% biodiesel blend is negligable, but the benefit of the added lubricity may put you in the positive direction on mpgs.
The first time you run BD through a vehicle (especially an old one), it will loosen up and build-up of crud in your lines. It's normally recommended to ease into running pure biodiesel so it doesn't jar all the crap up and cause blockage (in the lines or within your fuel filter). You may want to think about changing the fuel filter anyway. BD is a natural solvent (which is normally good), but with 150,000 miles of regular diesel running through it....it may be hindering performance.
It may just be coincedence, but did you fill your tank in the same fashion as previous times with regular diesel? The reason I say that is when I'm patient, I can squeeze 2-3 gallons of fuel in my truck, if I wait for it to 'bubble down'. But if I'm in a hurry (or someone else is pumping), I don't get as much in the tank. So if it was a quick fill-up, you may not have actually 'filled up'....hence the quick retreat to the 3/4 mark on your gauge.
...then again, it could be a combination of all the above (less enegy, dirty fuel filter, short fill).
Hope this helps.
