I WAS WONDERING IF ANYBODY KNOWS IF YOU PUT 20' WHEELS IN THE REAR.. DO U LOOSE HORSEPOWER? A TIRE SHOP TOLD ME SINCE THE TIRE HAS TO SPIN MORE IT SLOWS IT DOWN DRASTICLY...BUT IT RIDES NICER. WHAT DO U GUYS THINK..ESPECIALLY BDDIDY?

 

The gearing is all based on the outside diameter of the tire. Go to http://www.miata.net/garage/tirecalc.html and input your stock tire set up. Then, input the 20" set up and keep it within 2% more or less of the stock diameter. It will tell you how much faster or slower your will will be. Remember, faster means more acceration but slower top end.

About aspect ratio of tires (what series tire you are getting). In gereral, you are keeping the same outside tire diameter thus if you go 20" wheels instead of 18", you are simply replacing the ht id the tire wall with more wheel which is heavier. If I remember correctly, 6lbs wt at the wheel decreases 1 hp. Many people are going 19" over 17" lets say because they like the look of the lower profile tire and more wheel. As far as performance goes, from what i have read, there is a point of no advantage to having a smaller side wall on a tire. The smaller the side wall (series tire), the less sway in the tire thus better handling in corners but after you go beyond a 40 series tire (compared to lets say 30 series), the improvement in handling is not significant and other negatives come into play such as worrying about damaging your wheels because there is so little tire to prevent damage and a rougher ride as well.

Remember, a 40 series tire is 40% of the tire width. IE a 245/40-18 is 40% of 245mm to determine the tire wall ht. So, if you have a 295 tire in a 35 series, it is a different ht then a 225/40 series. My first suggestion is to pick the tire width you like, then start playing with 17,18, 19,20 combinations. Some size tires are not really availible in certain brand tires. I really wanted Toyo's and they did not come in 19" size for the tire width i wanted.

I hope all this helps you in selecting a tire wheel combo that suits your needs. Let me know if i can assist you more..Good luck!

Jeff

 

JEFF
u have been more than helpful
thanks a million

 

Jeff,

You seem pretty on the ball with the tire breakdown. So let me see if I understand you correctly...

If I keep the same diameter wheel and tire width, but go with a lower profile tire (say from 45 to 40), I should have small acceleration gains at the expense of top speed in each gear?

 

Here's is a comparison that might help.

RoJa custom wheels by Motegi offer some models that are forged aluminum, the lightest available construction technique.

A five spoke 17" wheel, is approx. 14.5 lbs.

The same model in 18" is approx. 22.5lbs.

The same in 19" is close to 30lbs.

The 20" I imagine is between 35-45lbs.

And that's for forged aluminum. Way expensive. If you were to go with just a chrome plated two piece aluminum wheel, it would be cheaper, but heavier still. Going from stock, you are going to add at least 60lbs to your unsprung weight. You will feel everything and go even slower. Hit a pot hole, and your wheels are toast. Buy the 17" RoJa's or another 14.5" wheel, and you'll thank me later.

 

Mcduck:

Yes, if the wheel width and Dimeter meaning lets say an 18" wheel are the same but you only change the profile from 45 to 40, then what you have done is decreased the outside dimater of the tire thus it is spinning quicker or geared lower for faster acceleration. Since the dimeter is less, that means the circum. is less thus your tire is traveling less distance per revoluion. So to sum it up, quicker off the line but slower top speed.

Jeff