Posted 16 October 2007 - 11:22 PM
A popular question - problem is, there are too many variables that you are not directly in control to come to a good, precise result. I could spew into all kinds of equations, theories, and some simulations (by profession, I'm a scientist - physics, math, and computer sims are my specialities) - but I'll leave those details out for right now and just give a gross overview of what "could" happen. A short answer - most performance gains are picked up running a lightweight, larger contact area, stiff sidewalled, and traction oriented tire. But that same tire will also return the worst fuel efficiency and generally ask the most from the car's engine.
For the performance oriented group - reducing overall weight, is the Holy Grail to performance, as you don't change any of the existing powertrain - plus mods of more of less free to very low cost (at least initially). In that regard - having a smaller tire/wheel will help with achieving those goals. Good example is the GM Sunraycer (solar powered race vehicle) in the late 1980s. That had low rolling resistance, bicycles tires. Doesn't look like much, in the way of contact patch size and overall traction, but that vehicle could take a 90 degree turn - flatout at 70MPH, no braking at all, just the go pedal all the way. The keys were a lightweight vehicle, lightweight components. There is no arguing that given identical tires - the lighter one (generally the smaller one) will have more performance potential.
But that's not the whole story - other factors you have to consider. Smaller (diameter) tires generally means the tire will have to make more revolutions to cover a set distance. Assuming the gearing of the car will not change. What this means is that the car will have to do more work to cover the same distance. If it will make a difference in performance - depends on the car and the desired performance. Off the line acceleration may or may not be better with a smaller tire - depends on the powerband and the gearing. You might gain some with better acceleration, but if you need shift sooner (say a run from 0-60MPH) - you might end up on the losing side of that battle. Since gearing will not change, your top speed will definitely suffer, assuming that the engine is sufficiently powerful enough to get you up there. That's because the tire has to be turning more to cover the same distance, you may end up just "run out of revs".
Handling is also a tough one to approach. You have to balance traction, sidewall stiffness, rolling resistance, and contact patch size. If we look at the extremes - say a 14" tire vs a 18" tire, keeping the overall diameter the same, both filled to max cold tire pressure. Traction is more a function of tire design - assuming that they are of the same material and if possible, the same tire - the 18" tire will definitely be better that the 14" tire on the stiffness, low rolling resistance, and contact patch size. But does that make it a better tire - all depends. If the combination turned out to be the same weight, then maybe the bigger tire will be the better choice. But currently, I've yet to personally run a larger diameter tire that weights less than a smaller tire (assuming they are the same series, type) - eventually we may see them get closer in weight, but I've yet to run into that situation. Hard part is matching the tire to the car - just because a tire is better in certain aspects, doesn't mean it will translate to better performance on the car.
Moment of Inertia - I wouldn't even worry about this. Difference is so minute, given the dynamic road conditions that people run into everyday, I wouldn't waste too much of my time worrying about this. If your running it on a prescribed track against identical cars - then that is a different story.
Best way to approach this problem is to run two identical tire sets. Then experiment with parameters that you can control - like wheel types, tire pressure, tire temperature, tread depth, etc. Even buying tires of two different sizes from the same series and make of tire will not always mean an apples to apples comparison (tire construction can vary wildly with even slight tire size differences). A little extra belt material here, difference tire casting process, difference in QC could turn out a hugely different performing tire. Even tire testing machines - loading with identical tires from difference batchs and running the tire to failure can turn up vacillating results from tire to tire. I've seen tires that were so-so in performance turn into some decent tires, once you shaved a few tenths off of them. Others actually became worse the more then wore down. Some respond well to heat cycling, others don't care.
Personally, I saw nearly no differences between running a 205/45R16 tire vs a 195/60R14 tire - in terms of handling, cornering, and acceleration. Basically it boiled down to that the tire's capabilities entended much further than what the point in which the chassis was capable of pushing the tires. Fuel economy on those tires flipped flopped between them - initially I was getting better fuel economy with the 16" tires than the 14" tires - but after about the same distance traveled (~20K miles) on the sets - the 14" is starting to get better numbers than what the 16" tire was able to put up. Weight wasn't that much different between the two sets - maybe 5-10lbs per tire/wheel.