[Big Dave]

Comments from the Grinch:

1. I do agree that the minimum aspect ratio for an efficient tire is 55% and that might be pushing it.
2. Empirical experience about tire diameter is at variance to the assertion that bigger tires are more efficient. Wheel-tire assemblies are big flywheels that the engine must accelerate along with the vehicle every time the vehicle speeds up. Rotational moment of inertia goes up with the square of diameter. If a vehicle were operated like Class 8 trucks with long periods of operating at more or less constant road speed, this is not much of an issue. Lighter-duty vehicles have to operate in an environment of stop-and-go or at least one of deeper variations in speed. Hence there such vehicles have more scope for having to speed up these “flywheels.” Pickup trucks operate with very large wheel-tire diameters and invariably the bigger the outside diameter, the worse the MPG. I’ve never seen any real world variation in that generalization.
3. The air resistance of bigger diameter tires is subject to debate. With lifted pickups, the air resistance goes up greatly. Maybe, as with the Aptera, sitting taller may get a very streamlined body out of what aeronautical engineers call “ground effect.” Ground effect is the high lift/high drag regime of a wing flying close to the ground. When the plane climbs out of ground effect the rate of climb decreases as lift is lost but air speed increases as drag drops away.
4. High diameter, narrow width “pizza cutter” wheel-tire assemblies have some definite non-efficiency problems. They make the vehicle more top-heavy, therefore for safety’s sake, the designer has to increase track width. “Pizza cutters” are murder on wheel bearings.
5. Moving on to engines, the best way to increase engine efficiency without major expense is to eliminate throttling. Throttling imposes high pumping loads on the engine which reduces thermodynamic efficiency. Two ways around the throttling problem. The first is the diesel engine. It modulates power with the fuel injection and always operates at maximum volumetric efficiency. The EPA despises the diesel engine so using one may be problematic in the US. The second way is to use a true series hybrid. The tramming motors operate off the propulsion battery, and the engine only acts to charge the battery. Thus the engine can be an “on-off” engine that runs at full throttle when it is operating. The efficiency of this setup will approach that of the diesel but will require extra weight, complexity, and expense.
6. To make a high-MPG car, maybe the best approach is to simply lengthen out a 2F1R tricycle like the HyperRocket. Have four tandem seats rather than two. You wind up with a long, but narrow vehicle. It may be necessary to widen out the track and maybe go to a narrow two-wheel rear axle. This longer vehicle will be easier to achieve a very low Cd with. A tandem arrangement vehicle will probably encounter stiff sales resistance from families with small children. You would have to stop the car to tend to the kids. Myself, I’d see that as a feature rather than a bug. (I nearly met my Maker one night when some tom-fool woman reached over to mess with the brat and swerved in front of me) People with kids probably will not see it that way.
7. The four-seat car is a necessity and may become more so. If people cannot afford but one car, it has to meet the family mission. As cars become more complex and made of exotic materials, they will become more costly. Higher price means lower volume. So the four-seat car is not going to go away.
8. As I stated before, you can mandate all you like, but the car has to be designed to meet the needs of the customer at a reasonable price or they simply will not buy it and view whatever agency that foisted an unacceptable car on them as a latter-day version of Jimmy Carter’s sweater.