OBD, while being as plug and play as you can get from a user perspective, is also a moving target from a development perspective. The standards are still evolving and the hodgepodge of protocols is going to grow or morph into a new (and incompatable with everything) protocol, which can also vary by region. Whereas injectors signals are a pretty stable thing by comparison.
Anyone who can install (or find someone to install) a tachometer in their car can install an injector/vss device. Also, there may be injector leads in the cabin on some cars, I dunno.
MAF is not universally available. More like 2000 something and up, not 96 and up. MPG computation is TRIVIAL with a MAF. Like MPG = (VSS * 710.7) / MAF according
to this vpw pic mpg gauge.
The obd mpg question comes up frequently, here's my notes on it without maf, what it might look like (I'm probably being too exacting, some factors won't make a diff):
http://forum.ecomodder.com/showthrea...1.html?p=18665 but only needed for 96 to 2000 something vehicles...
Still there is a lot of obd mpg gauge interest in the public domain, preferably without the expense of an elm chip. Someone should teach an arduino to talk to any sort of obd protocol, but that is not a small order, so it is no suprise that no-one has done that out of the goodness of their heart yet.