Just found out that some older, medium-sized RVs are getting 7mpg and newer, big bus-like RVs are getting 11mpg. I think I'm on the right track converting a Sienna minivan into an RV, which gets 20+mpg. I'm confident I can do even better than that, too.

I just googled the Sienna as we don't get it here. Looks similar to a Dodge/Chrysler minivan, we hired at least 4 of those in the US. For some reason minivans or MPVs don't sell well in this country. You could find a diesel one with a manual box maybe and get better mileage? A diesel would be better for idling too, if you wanted to charge the battery or put the cars heating on.

Thanks, DraigFlag. Speaking of diesel... Why do diesel truck drivers prefer to idle instead of shutting down their engines?

Diesels use very little fuel when idling, think of generators etc. My brother got stuck some snow last year for about 15 hours, van was running the entire time but the fuel gauge barely moved. Another reason why people who do alot of city commutes prefer diesels, so thier fuel costs don't rise during traffic jams and red light stops.

With old diesels, shutting down and then back up with a warm engine could result in difficulty starting, but that, like 3000 mile oil change intervals, is a thing of the past that people are just in the habit of doing.

With long haul trucks with a sleeper, the truck can be idling for heat, A/C, and power for the cabin. So trucks will have a more efficient generator for this purpose instead of wasting the fuel idling the huge engine of the truck. A Volvo announced a diesel powered fuel cell that they have been running to eventually replace the generator.

I have also heard that some recovery trucks and all emergency vehicles will leave engines running, this in case they won't start again. They are trading the small amount of fuel for safety.

I've always wondered about small delivery vans leaving their engines running as well, but if you think that a starter motor might have to start the engine 20-30 times a day, there is probably a trade off in the small amount of fuel used vs having to fit a new starter motor every couple of years.

This is where hybrids like the Prius come into their own when they start on battery power and then when the vehicle is rolling use a clutch to slowly turn the engine as well and when the engine is at normal running speed introduce the fuel to keep it running, this is one of the reasons that Priuses (?) last so long, the engines don't have to go through the stress normally associated with starting them up.

Oliver.

Bigger vehicles require greater amounts of fuel to start, so it's more efficient to leave them running too. Not the case with small passenger cars. You'll also notice cars with stop/start have larger starter motors (mine included) which during lab tests managed 175,000 starting procedures before wear was detected.

When I was driving a semi, they told us the engine burned about a gallon per hour idling. It was a 12.7 liter engine. Technically, it wasn't idling overnight. It would idle at its normal 650 rpm for about five minutes, then the computer would shut it down. To idle overnight (as mentioned, for lights, heat and or A/C) one used the cruise control to run the engine at 1000 rpm. I didn't leave the engine running when I filled the tanks or went inside to pay, eat, or shower, unless the weather was very cold and I was going to be out of the truck for several hours.

Interestingly, UPS drivers appear to be driving gas powered delivery trucks and I've never seen them leave their engines running; even for a quick drop off at my folk's home.

Even if they used diesel, UPS would likely have their drivers turn off the engine on stops. Just no technical reason to leave them idling anymore.