Oversize tires? (higher sidewalls, but still as skinny as possible)
 

Have a couple questions about stepping up tire size:

1) Yes, I know I want to reduce weight, but (I'm driving around a lot of weight already) isn't there some advantage to getting the most distance out of a given engine crank? Driving my truck, I have engine to spare and low gearing so getting everything moving isn't going to be a problem.

2) My truck,when loaded, is full by volume not by weight, so I have lots of space in my fenders I don't need for suspension travel I can potentially fill with tire without clearance issues. Would filling more of the wheel arch with tire have any of the benfits of installing skirts?

I'd say you're right on both concepts. That is, reducing engine rpms and getting a bit of a wheel skirt effect. You'll see some small fuel-thrifty cars where they line up the tire sidewall with the body work in an attempt to get a smoothed air flow.

You could consider changing the tranny or rear end gearing instead of changing the tire diameter. I don't know what the options are for your truck but there are usually several gear combinations out there to be had. The cost might be in the same range as a set of tires.

Haven't done any A-B-A testing but I didn't see it with my 95 Toyota PU. Stock tires are 195/75/R14. First time I changed to 205/75/14. So I went a little wider and a little taller. Did not see a change in fuel. Then 40K later I went 215/75/r15. So I bought new steel rims. Now my Spedo is off by 8% so I have to remember that when going through the auto camera speed traps. Again no noticable change in mpg. I can say that the interstate here in AZ is 75 and now I can go 65 on spedo which is really 70 and not feel like the engine is too high RPM. Also 70 in 75 zone doesn't piss off too many people.
I think the real answer is trying to find taller tires that don't also increase width. If you are already at 75 ratio then you can't really go any taller without also going wider and heavier.

Not trying to be a nit-picker, just trying to help you avaoid a ticket...Your speedo is off by 8.5% with the 215/75/15s..an indicated 60 would really be 65.1.....my question is, when you figured your mpg did you also figured in that you drove 8.5% more miles than your odometer said? Rolling resistance ant wt will affect all vehicles, less so on big, heavy models. On my 3 banger Geo I went from the stock 13s (max psi 35) to 195/50/15s (max 51 psi) AND GAINED 8 lbs per corner! I had also removed an extra 32 lbs of interior wt so overall wt was the same. The best mpg still stayed at 57 due to higher psi and less rolling resistance. Now I put on a new set of 155/80/13s (OE size) but with higher psi AND lighter rims to knock off 40 lbs of rotating wt and my last 4 tank fulls have all been in the low 60 mpg range!:eek:

I noticed an increase of 1 MPG when I went to tires that are 2.3% taller than stock, but they also had less rolling resistance than my other tires, although these aren't labeled as LRR.

With a GPS confirmed adjustment factor to the odometer, my overall on-road mpg went up nicely on a 2002 I-6 Jeep Wrangler when I swapped my stock tires for some taller and wider mud tires. I fully expected the opposite (due to weight, rolling resistance, frontal area), but apparently the effective change to the final drive ratio superceded the rest. My 2003 Jeep also saw improved mpg even though the tires were a lot taller and wider, and this one had a 5.5" suspension lift as well.