Id have to work out the physics of it, but if you are reporting thermal
efficiency in percent, then you can see
that something is missing.

There could be engines of similar thermal efficiency but having widely
varying fuel consumption per unit time.

You can however work backward from specific fuel consumption and relate it
to thermal efficiency by making a few assumptions.

As I say, I would have to work out the physics, and I hope I havent
overlooked the obvious and said something really stupid.

    For the common aircooled aircraft engine the thermal efficiency is
on the order of 25%. 15% is lost through the cooling fins, 10% through
the oil cooler, and 50% goes out the tailpipe. Modern auto engines will
be somewhat more efficient than that, using EFI and variable ignition
timing and the liquid cooling that keeps temps closer to ideal levels.
35% might be more normal in such an engine.
     Still, even a 100% efficient engine wouldn't give us a perfect
world. A long time ago when I was learning Physics during Aircraft
Mechanic's training we did some calculations. A 2300-lb Cessna 172,
with a 150-hp engine, accelerating from a stop to 60 mph takeoff speed
in an 800-foot run at sea level, was experiencing the equivalent thrust
of about 28 HP, IIRC. The rest was lost to things like rolling friction
and various forms of aerodynamic drag, and some power was missing due
to the fact that the engine, with its fixed-pitch prop, couldn't
achieve rated RPM and therefore rated HP in the takeoff run. A
constant-speed prop would fix that but would only bring the equivalent
power up to about 33 HP.
       The automobile won't be much better. HP lost in the drivetrain
is significant, and most cars are pretty draggy, in spite of the
apparently slippery designs. Just look at the chopped-off rear ends of
most; there's a lot of turbulence generated there, and that represents
wasted energy. The undersides of most cars are anything but smooth.
Some years ago I read that the Terraplane automobile of the 1920s had
the lowest drag coefficient of ANY automobile ever produced, but it was
"ugly" and gasoline was cheap, so it didn't catch on.

       Dan

Exactly. You can get any car's drag numbers from the EPA, or figure your own
by coast down. The only uncertainty is that you won't know the amount of
engine power that goes to power steering, charging or some other belt driven
thing.

Essentially, yes it is. Here's the quickie formula:

Eff = 100*LHV/SFC

Make sure you convert Lower Heating Value into the appropriate units
(i.e hp-hr/lb for sfc in lbs/hp-hr).