widest wheels

[gonzomania]

On 16x6 wheels, Base model Pt what would be the suggestion of the biggest fatest tire I could fit? They had 205/55/16 on them

[Jacked Up PT]

hi i have 225/60-16 on the back with the stock 205/55-16 on the front[8)]

[gonzomania]

Hey jackedupPt,
Welcome to the forum. I'm trying to get the same tire all the way around, then again maybe I'll change my mind. Does that effect your handling or noise.

[snakepitt1]

When you went to the 60 series tire, how much did it throw off your speedometer? Going wider does not affect the speedo, but changing overall height does.

[MichaelDay]

For those that may not know, I will attempt to clarify the relationship between width, sidewall height and speedometer error.

The width of the tire (the first number, for example the 205 in a 205/55-16 tire) determines how much rubber is put on the ground (this can vary somewhat depending on rim size, inflation, etc). The second number or profile (55 in our example) is the ratio of the sidewall height to the width. So a 205/55 tire would have a sidewall height of 112.75 mm (or 55% of the 205 mm width).

The rolling diameter of the tire determines how many times the tire will turn per mile. The rolling diameter is the sidewall height times 2 plus the diameter of the wheel (the last number in our example - 16). In the case of our example, the tire width (converted to inches) is 8.07 inches. The sidewall height is 4.44 inches. The tire diameter is 24.88 inches (4.44 + 16 + 4.44). The circumference is 78.16 inches (tire diameter of 24.88 times pi which is 3.1417). Since a mile is 5280 feet and there are 12 inches per foot, a mile is 63,360 inches per mile. Dividing 63,360 inches/mile by 78.16 inches of tire circumference gives 810.68 tire revolutions per mile. If you increase the width to 225 mm (8.86 inches) and don't change the profile (the 55 series) the sidewall height increases to 4.87 inches. That increases the tire diameter to 25.74 inches and increases the circumference to 80.88 inches. This converts to 783.41 tire revolutions per mile. The difference between the 810.68 tire revolutions/mile for the 205/55-16 and the 783.41 tire revolutions per mile for the 225/55-16 is 27.27 revolutions. The difference is almost 3.5% decrease in the number of times the tire turns. Thus the speedometer will read 58 mph instead of 60 mph. The PCM thinks the car is going faster than it actually is. Put a 205/55-16 on one side of the front of the PT and a 225/55-16 on the other side of the front and, if you have anti-lock brakes and traction control, the computer will be on the verge of having a fit thinking that one tire is spinning during acceleration or one tire is locking up during braking. (Actually, it probably takes greater than a 5% difference but 3% is the recommended maximum difference).

That is why you have to go to a lower profile (45 vs 50) if you increase the tire width if you don't want to change the speedometer reading. A 225/50-16 tire has a circumference of 78.09 inches and 811.32 revolutions per mile making it appear almost exactly the same as the 205/55-16 to the speedometer and the PT's PCM.

Of course the difference is smaller (about 1.7%) if you only go up one size in width (say from a 205/55-16 to a 215/55-16). The 225/60-16 has a circumference of 83.66 inches and 757.35 revs/mile making it over 7% variance from the 205/55-16.

Now aren't you glad you know that?

[Jacked Up PT]

gonzomania thanks for the welcome handling in proved with btg strut bar ride was grate we want same tires for the front toyo tires from les schwab 800 ultra premium touring.snakepitt1 it did not im jacked up in the back only.

[PTRON]

Just go here - http://www.miata.net/garage/tirecalc.html

[AlmondTurbo]

Gonzomania, I just on put 205x60x16 Yokohama Avid H4s tires (at 8500 miles). This is the tallest tire you can put on this wheel. The clearence between the tire and the lower spring perch (support for the bottom of the front spring) was reduced from 5/8 inch to slightly more than 1/8, maybe 3/16 inch. After a high speed run, there is no evidence of rubbing or tire contact. The 1 inch taller tire will 'slow down' your speedometer. With stock tires, my speedometer was reading 1 mile fast for each 20 traveled (5% fast). I expect to be very close to dead on now. The tires look great (a little more chubby),feel even greater, and have a 130 mph speed rating.

[AlmondTurbo]

Snakepitt1
My previous post was to answer your post. Darrell

[gonzomania]

Mike, got anything in English!!![:0]