[aardvarcus]

Some relevant facts to add to the discussion. As Neil alluded to tire size is a very small part of overall tire performance, things like tread pattern play a huge role that is often overlooked.

Every single part of your vehicle is storing kinetic energy as you drive down the road. Some objects are only storing translational kinetic energy (radio, passenger, etc.), while others that are spinning are storing translational and rotational kinetic energy (drivetrain). All of this is measurable based on the items mass, the cars translational velocity, and the items angular velocity if any.

If we assume for a moment that the mass of the entire drivetrain (wheels, tires, drums/rotors, bearings, axles, gears, driveshafts, clutch, flywheel, crank, etcetera) is zero, then the rotational kinetic energy of these items is zero. Under this fictitious scenario, the only important gear ratio is from the engine to the road. The specific individual values of transmission gear, differential ratio, auxiliary gearing, and tire diameter don’t matter; it is only the net effect of all of them in series that matters.

Cars do not have infinite transmission gear ratios however, so at the end of the day the selection of your tire diameter will impose fixed limits on the lowest attainable engine to road gear ratio (in first) and the highest attainable engine to road gear ratio (in max overdrive), assuming you cannot change the differential gear, transmission, etcetera. Depending on your specific engine, car weight, drag, driving conditions, etcetera it may be beneficial to move this ratio up or down. In my scenerio, I wanted to move this ratio up (more road per rotation) where Dave wants to move the ratio down (less road per rotation). Obviously there are many other factors being affected here (weight, aerodynamics, etcetera).

Back to the zero rotational energy idealized scenario, there isn’t any difference between 30” tires and 3.73 differential gearing and 33” tires and 4.10 differential gearing, since the net gearing from engine to road stays the same (30” x 1 / 3.73=33” x 1 / 4.10).

Adding back in the pesky little problem of rotational kinetic energy, there is a big difference though. In the stated 30” & 3.73 versus 33” & 4.10 scenario, if the weight of the 30” and 33” tires was the same, the 33” tires would be spinning slower to achieve the same road speed, (re-edit fixed this part) thus the effects of the weight of the drivetrain, wheel, and part of the sidewall would have lower rotational inertia but the effects of the now larger section of the sidewall and the tread which are now farther from the center would experience an increase in rotational inertia. The net effect of this move may be positive or negative, depending on the weight of the various components. The translational inertial would be the same. In the real world, where 33” tires weigh a lot more than 30” tires, the 33” tires would have higher rotational inertia and translational inertia. This is what is illustrated in my previously posted (greatly simplified) chart. (Edit: also don't forget the significant aerodynamic effects of the 33" vs 30" tires, the vehicle body is 1.5" taller, another 1.5" of tire height is exposed, and typically taller also means wider so additional tire width is present.)

So why the focus on wheels and tires since all of the drivetrain has rotational inertia? I believe this is mainly because ties and wheels are easy to and commonly changed out, whereas changing other components are less commonly changed out. In high performance applications it is now common to see aluminum and even carbon fiber drive shafts to help reduce that rotational inertia. I have also seen aluminum brake rotors for sale.

From a less theoretical perspective, having experimented with tire diameter/size/weight on five different cars/trucks now, I can tell you that the inertia effects of heavy versus light wheels and tires can be felt. The most obvious time was when I switched from 255/85R16s to 285/70R17s on my 2001 2500HD. The 255/85R16s were actually taller at 33.4” but only weighted about 60lbs per wheel and tire. The 285/70R17s where shorter at 32.8, but were significantly heavier at about 85lbs per wheel and tire. From a purely gearing perspective, the truck should have been faster with the 285/70R17s, since they were shorter and therefore geared lower. But it was not so, the truck was significantly and noticeable slower with the 285/70R17s, due to the extra 100lbs of wheel and tire the truck had to not only spin but to spin 2% faster for the same road speed due to the reduced diameter.