For rolling resistance data, I went to Tire Rack, and found 3 tests of passenger car tires that include fuel economy-based comparisons of rolling resistance.
The first showed the widest range, as it included a tire specifically not designed for LRR, as well our OEM Michelin Energy Savers. Thus the 7.3% range in fuel economy is much wider than one might expect. The other two tests show a more-reasonable 2% effect.
Tire Rack also has a series of article on why you should expect lower MPG whenever fitting new tires in place of worn, based solely on the diameter change, about 2.5%. Switch from OEM to the Goodyear ComforTred, the 7.3% pair, and add in tread depth and you might make 10%!
Finally, it depends on your driving mix, as RR is a much larger fraction of highway fuel usage than for city driving. Tire rack gets into this in the first link.
You mention using lower tire pressure (which reduces your car's load capacity) when you carry heavy stuff. Clearly a bad idea, but...
Not if you use oversize or reinforced or extended load tires. The OEM rating is a 93 load index, or 1433 lb. Use a tire of greater load capacity and it's safe to reduce pressure.
- Switching from the P-metric P225/50-17 to a Euro-spec 225/50-17 gets you a 94 load index and 44lb. more capacity per tire.
- Switching to an "extended load" 225/50-17 gets you as high as 98 load index, good for 220 lb. more capacity per tire (an available in LRR designs)
Food for thought, I hope!
PS My Michelin snow tires are XL, yielding a 99 load rating. I see no change in fuel economy when making the switch from summer to winter tires.
Edited by fbov, 05 January 2016 - 05:56 PM.