So I'm having this debate with a friend. I say that having wider tires = faster with everything the same and he says that it is not true. Any opinions?

not true, wider tires make more friction. it depends on what the car needs there's a happy median but bigger isnt always better. thats y most track guys run a 225 instead of the 275

i agree. everything else on the car being the same as well as speed being the same. it'll be slower. more rotational mass and more friction.

I assume you are referring to track speed and not backroad twisties speed?

The most important aspect is the tire compound. 225/45/15 is about as large as you can go on a low powered miata, but most spec miatas run 205/50/15's and they are very fast. On a stock miata 205's are probably going to always be quicker than 225s. Less unsprung weight, less friction. Double or triple the power and 225's start to feel too skinny.

Don't ignore the importance of LOW unsprung weight - lighter is ALWAYS better = more grip with the tires ON the pavement. Not all tires are created equally - some are substantially heavier in the same sizes. Wider = more weight = your rotational mass is bouncing higher and rebounding harder = rougher ride and less grip.

I have 225/45/15 RS2's on 15x8 6UL's on my bling-bling(pretty much stock powered) LE. While I can corner substantially faster than my other LE with 185/60/14 RS2's on 14 inch BBS's, I can accelerate a bit quicker on the smaller tires and have nearly the same grip, and even a bit better grip on rough roads due to the lighter unsprung weight.

For grip, as wide as possible. For straight line speed, as skinny as possible.

Most spec miata run 205's because of the rule book?

Craig

How much of that is perceived due to the extra sidewall though?

Honestly it depends on the target. I agree with Mike, my 225s won't budge on the street at any speed, but the accel difference between the 185s and 225s is so substantial that unless you're after a target number, like me, its pointless to have them on. Lighter is always better, but the key Is balance. Personally though, I'd start with 195s, learn how to use them and then move up to 205s. My $.02

Sure there's more rotational mass but the extra grip going through corners would out weigh that since you can maintain a higher speed out of the turn and use the momentum to exit. I'm not talking about crazy huge diameter just inch up.

Still depends on the overall balance of the setup, but you got the gist.

Craig is correct. Spec Miata's must run the 205-sized tires. On a non-forced induction miata, these are a good balance between cornering speed, suspension compliance (unsprung weight) and acceleration (rotational mass).

-B

Except not really. Saying "wider is better" is such a broad statement that any argument for or against it can be completely invalidated by changing the scenario. Given the conditions of going from 205 to a 215 or even a 225 your contact patch area isn't going to change much (if at all) meaning there wont be any noticeable gain in cornering power.

check out the latest Grassroots Motorsports Magazine. They have a great comparison test with wider and +1 wheels/tires on an NC Miata. In the end, the widest isn't the fastest.

-B