Pumping up the tires can be a big deal as most recommended settings are based on giving a good ride and not being able to generate enough cornering force to roll the vehicle over. If your tires are below the recommended settings, they are a long way from being optimal for fuel economy. It may only be a few percent effect, but it is available for FREE. Be careful though. (;-0>
Re your #186:
I run an old '91 Plymouth Voyager with 137K on the clock, and it still runs like a top.
And I typically run about 45 PSI in the front tires, since most of the weight of the vehicle is on them.
It seems to handle better, and the tread does not wear out on the outside as much as it will when I run the reccomended 35 PSI - which I do in the rears.
Now all of these theories are very interesting, and I don't claim to know that much about it; all I do know is that "Old Red" gets significantly better milage at around 60 MPH than it will at 70 - 75 which one pretty much has to maintain in order to keep up with traffic on the highways here in Maine. Old Red is perfectly capable of cruising at 80+, but it will cost me at the pumps if we do.
So lately, if I'm not in a particular hurry, I'll cruise around 65 - which is the posted speed limit.
Of course, everybody passes us as if we were parked, riding at anchor, or hitched to a tree.
And they seem none too pleased about old red and I getting in their way.
I'm considering making up a bumper sticker (duct tape and magic marker) to stick on the tailgate;
"Sorry if I'm in your way; When Gas is back < $2 a gallon, I'll speed back up".
As for Diesel; I love it. Our 27' Class C Motor home has a big 450 CI V-8 Deisel with automatic electric overdrive, and it's great. But Deisel is running around $2.50 a gallon up this way (gas is down to $2.19), which hardly makes it an economical alternative!
I'm hoping that this "Biodeisel" made out of cooking fat and vegetable oil will become economically competitive, so we'll at least have something to run on! Apparently it creates much less air pollution and smells a lot better than your regular bus - breath.