RH77:
Excellent post as usual MetroMPG. The effect of wet roads has to significantly decrease mileage, but that all depends on tire choice. A while back, a thread was started to discuss the effect of tire resistance and fuel economy. The lower rolling resistance of tires (perhaps like the Goodyear Brand "AquaTread") might be pretty good at pushing water out of the way, but might be crummy on dry pavement (and how much energy does that take to push the water). Long story short, where the rubber meets the road could significantly effect economy. A search was on, BUT...
Tire companies are not required, and do not test, the Coefficient of Resistance on wet or dry roads -- so no new data was gathered (and no government agency or University/Institution stepped up to the plate to perform new test). I say "new" because in early 2003, an indepenent agency, hired by a bill by the Great State of California, performed tests on tire resistance. GreenSeal, a not-for-profit environmental group has made the
data public on the Internet (a good article too, by the way).
My next set of tires will be based on this testing. Hopefully new data comes out, as some tire models are outdated. I think the only company that performs this testing is located in the Rubber City (Akron, Ohio -- where I'm from originally) at a premium
(Smithers Inc.)
I've even tried the TireRack, who has their own test track, but they have no scientific way to test resistance.
RH77