Quote:
Originally Posted by honestabe
Daox, the resistance difference between the 155/80 and 175/70 is significant. By my calculations, 4 175/70's are as wide as 5 155/80's. That's like driving around with a dual rear wheel setup on 1 side of the rear. And for someone who drives as much as I do, that's significant. IIRC I'm losing 3-5 MPG by having the 175/70 tires vs 155/80 tires.
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First, it would be hard for me to believe that you are losing 3-5 mpg by having a slightly wider tire... it just doesn't make sense. The analogy that you use of 4 tires vs 5 may be correct if you only consider the width of the contact patch on the ground, but that does not take into account the ACTUAL overall width of the tire, plus what really matters is all the stuff that happens around the tire (air rushing into cavities and the like).
If you are serious about doing aero mods, you will undoubtedly eventually get around to shielding your tires with spats or fairings. These will negate the additional width of the tire, and the lower rolling resistance will take over. Actually, the lower rolling resistance will probably take over immediately, and be compounded by the aero improvements. Just make sure you get tires that have a good rolling resistance, either by getting LRR tires, or at least by getting 44psi tires vs 35psi ones. When I need new tires I am going with wider, 44psi ones, regardless of the aerodynamic implications... I will just get some lawn edging and shield the tires.