Originally Posted by Biffmeistro
It's a definite that the narrower tire is better for FE. Not only do they generally have a lower rolling resistance, they definitely have a smaller silhouette for aerodynamics.
Not entirely true. The aerodynamic part is true, and a narrower tire will be lighter weight. Rolling resistance, OTOH, works opposite.
Given that everything else is equal (construction, materials, tread pattern, air pressure), the wider tire will have LESS rolling resistance. Rolling resistance comes mainly from sidewall deformation -- look at where the tire meets the road, it's partially flattened instead of perfectly round, and the sidewall bulges to make that happen. Comparing most car
tires, tread squirm should not make much difference in RR, though knobby off-road truck tires might have more.
Now, on to why wider tires have less RR. The contact patch size is determined by the weight on the tire and the pressure in it. At 50psi (pounds per square inch) with a 500 pound load, that's a 10 square inch contact patch. If that tire is 5 inches wide, it will have a 2 inch long contact patch -- so only 2 inches of sidewall must deform. If the tire is 2 inches wide, it will have a 5 inch long contact patch -- so 5 inches of sidewall must deform. The narrower tire in that extreme example will have far worse RR.
The other advantage for wider tires is better cornering, so you can carry more of your momentum through the corner.
In Hateful's case, talking about 195/70R14 vs 185/75R14 there's just not much difference all around. The 195 is 10mm wider than the 185, that's slightly less than 1/2 inch difference.
According to http://www.miata.net/garage/tirecalc.html
the 185 is .6 inches taller resulting in a difference of 6 revolutions per mile and a speedometer error of .4mph at 60mph. Again, a tiny tiny difference. It's all pretty much a wash, and if it were my car I'd go with the wider tire.