Tire size

[Snow_Fox]

So how would optimal performance be decided for a tire?

I do realize there are advantages and drawbacks to any tire before looking at size and then even more advantages and drawbacks to the size of the tire.

When a company gives the 0-60 time is it safe to assume that they were using the default tires that the vehicle comes with? Or are they just giving 0-60 with the optimal performance tires and selling it as "well it can go 0-60" and then after purchase saying "but, your gunna need these as well"..

I'm not a fan of oversized tires to begin with. I know that my vehicle had its tires upped an inch from its default size (17 to 18 i think). and it looks fine.. but, when I see a car that has to be raised off the ground to fit the 22's on it.. I get a little sick to think these people do that to their cars. I mean a car just wasn't meant to be the same height off the ground as an SUV....

Looks aside how much do the tires affect the vehicle performance and speed? Would it be best to go with the factory size? or slightly bigger? or maybe slightly smaller?

[peterleo]

Performance figures for a car (acceleration, top speed, etc) are based on the base version of the car with the standard equipment. You should be able to get the same figures without any additional enhancements. However, optional extras ( electric seats, TV's, etc) may add to the weight of the car and affect performance.
As far as tire sizes go, you should be fine as long as you stick to the rim&wheel sizes recommended by the manufacturer of the car.
From what I know, tires that are more narrow tend to favor top speed but are not that good for handling ( lower level of grip). Bigger rims tend to favor stability while cornering (smaller tire wall reduces the roll of the car). The size of a tire is a compromise between all these factors. In general, the car makers already thought about all this stuff and came up with the best compromise for a particular car (their recommended rim&tire combination).

[Snow_Fox]

Ah.. Much appreciated. So the extra's are just for more fun and performance . Now to acquire a vehicle of the performance calibre.

[coolraoul]

A narrow tyre has less grip, but also less roll-resistance. It will save fuel, improve the overall acceleration, but have less grip (more wheelspin, decreased cornering performances).
They tend to have less aquaplaning, too, and perform better on snow (narrow means more weight per inch, so more pression, so more traction on slippery conditions)

A big alloy means low-profile tyres. They decrease the comfort but increase the cornering and braking performances (less wall means less deformations of the tyres under load).
A tall alloy allow a bigger disc-brake, and can improve the brake-cooling.

Generally, the overall diameter of the wheel (alloy + tyre) increases (despite the lower profile of the tyre) when you take additional inches of alloys.
That allows a better max-speed, but decreases the acceleration (increased diameter means harder to get moved, but also that the wheel travels more with the same rotation speed).

If you change the alloys, weight can be an issue. Lighter means better cornering perfs and comportment of the car (less non-suspended weight, very important aspect).

The shape and type of the tyre is important too.
Sporty tyres generally are soft: excellent grip, but wears out fast. Harder tyres are less adherent but last longer.

A sporty profile also ensures more grip, but can be sensitive to rain.

Some tyres are specificly designed to save fuel, but generally are not very good on wet conditions.

And then, there's the design aspect..