[aerohead]

Some of the members have thrown out some really good points.Off the aero forum there's discussion about rolling resistance and inflation pressures,and I've been accumulating stuff to share but not ready yet.When I saw your post,I'd been reading a few nights ago and here's a few notes from the proprietary GM concept tires for the Ultralite 100-mpg car.An overall larger diameter ( 18-inch )tire has reduced R-R due to the fact that each tread element "see's" the road surface fewer times for a given road speed.Surface speed is constant,but greater circumference reduces revolutions per mile ( kilometer ) and each tread block and it's accompanied sidewall elements bends and recovers fewer times per revolution.Inflation pressure is 65-psi to pump up the "air-spring" which a tire is,which helps hold the tread flat against the road ( traction,braking ,and less deformation).The sidewall is very short ( to minimize deflection under load ) and has a designed-in curvature to help it deform,when striking road irregularities and hazards so as to absorb vibration which would otherwise be transmitted into the cabin of the car.The tread is kept narrow to minimize aerodynamic loading on the car,and each tread block is "hinged" so as to deform as freely as possisible when it strikes the road surface ( this also makes for a" noisy" tire,now regulated in the US by EPA ).Rubber compounds are balanced to allow as much hysteresis ( the ability to absorb,then release energy without heating-up ) and still allow wet and dry traction.Michelin used silica in it's "Green-X" tires for reduced rolling resistance and they ended up shocking people with static electricity and had to be re-formulated.The whole business is fraught with trade-offs.By the way,while under-inflation can remarkably lower mpg and shorten tire life,over-inflating,beyond the car-maker's recommendation shows no appreciable gain in mpg and you run the risk of reduced contact-patch and trouble associated with all that.Magnesium is out-lawed in the U.S. by DOT as it breaks when struck.Forged-aluminum is best bet for mass reduction.Polar-moment-of-inertia is an issue,and you want mass away from the extremities of the wheel/tire unit.The steel wheel can take wheel-covers,which is good for aero.Cast aluminum wheels can be as heavy as steel.Take a scale with you.I do! Hope this doesn't muddy the waters for you.I spent $1,000 to put larger aluminum wheels on my CRX so I could run the Michelin tires,and they made no improvement in mpg.They did have excellent wet and dry traction,so the experiment wasn't a total loss,however no mpg for "one-large" really hurt! Good luck!