[Tonys9]

First time I've seen this thread. I have been running an mpguino in a 940 for a few years now. ( figure in imperial gallons)

Car: 1995 B230FT estate
Modifications: Mainly none, lowered to sport levels, Bosch super 4 plugs, stainless exhaust.
Max average: 40mpg using pulse and glide, 60-80mph
Max flat: 46mpg at 30mph

No alternator - the lower/dropping voltage will screw up the mpg readings, as well as reduce the efficiency and calibration of everything in the fuel and ignition. You need to use additional batteries and/or solar cells to keep the voltage stable at 14V.
It should be worth something, typical min load is 300W. Hightbeams add about 0.03 gal/hr to the fuel flow at tickover. so that is about 0.03 gal/hr per 100W, which means fuel flow should drop by 0.09, which is about a 30% drop at tickover. The benefit is less the faster you go, say 15% reduction at 30mph, but would be a 4-5mpg improvement.

I must try that, need a good 2V battery.

I think you could fit 600W of solar, but you don't need that amount. 100W would be plenty with suitable batteries depending on your daily milage.
It might be worth increasing the amount of electricity used say with a electric water pump, steering, turbo etc to make use of solar.

Another thing I've notice is gearbox, engine and diff oil. MPG takes a while to get up there in cold mornings even after engine has warmed up. Maybe benefits with thinner oil or heater pads.