[Diesel_Dave]

In my opinion, neither option is worth a hill of beans, particulalry if you're hoping to use the FE info as feedback for driving modification.

As UFO has correctly pointed out, the first option assumes that air-fuel ratio is constant--a major error for a diesel. A constant air fuel ratio of 14.7 is a reasonable assumption on most gasoline cars, but AFR on a diesel can vary from neary 100 at idle to down near 15 or so (depending on speed, load, and engine). On transient cycles, the engine can vary from 100 to 15, back to 100 in a matter of a few seconds. Without any way of knowing whether you're at 15 or 100 AFR, there's no good way to get fuel flow from air flow. Gassers are a differnent story, because they are controlled to a nearly-constant AFR.

The second method is little more than an educated guess. It basically assumes any vehicle of a certain size with a certain weight and a certain engine will burn a certain amount of fuel. They're obviously huge problems with that. Lots of ecomodders have doubled they're fuel economy with driving mods--and they're size, weight, and engine are pretty much unchanged.

Unfortunately, I'm not aware of any effective, affordable ways to get diesel FE on pre-OBDII engines. If you'd like the details, I do know of a way to do it with 1 flowmeter as opposed to 2 as UFO suggested, but it's by no means cheap or simple. There's also a way to do it with air flow info and a wideband O2 sensor, but that's not cheap or simple either.