Here is a nifty link for those of you needing info about plus sizing and effects on speedo etc.
http://www.miata.net/garage/tirecalc.html
http://www.miata.net/garage/tirecalc.html
Tire Size Calculator
- Thread starter shortyb
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I did a plus size conversion from 155/80-13 to 195/50-15 using these calculators.
miata.net says the plus size combo will run at 59.8 mph instead of 60 mph.
The Galaba site says 60.23 mph instead of 60 mph.
My math tells me that since the 195/50-15 combo has a slightly larger overall diameter then it should be a little faster (as Galaba states).
Who's right?
miata.net says the plus size combo will run at 59.8 mph instead of 60 mph.
The Galaba site says 60.23 mph instead of 60 mph.
My math tells me that since the 195/50-15 combo has a slightly larger overall diameter then it should be a little faster (as Galaba states).
Who's right?
For the 155/80/13's: 155*.80*2=248mm:/25.4 = 9.7637 in; + 13 = 22.7637 inches nominal diameter.(Oughta be exact enough for the fussiest among us!)
the 195/50/15's: 195*.5*2 = 195 mm:/25.4 = 7.6771 in; + 15 = 22.6771 inches nominal diameter.
The 195/50/15 is a bit shorter than the 155/80/13, according to the numbers. But by less than 0.4%. That's less than one-half of one per cent. So, if both tires were their exact nominal heights(unlikely in the real world), & the 155/80/13's were 100% accurate on the speedometer, when you run the 195/50/15's & your speedo says 100 mph, you'll actually be doing only a measly ~99.62 mph.
In short, you chose as close to a matching height tire as anyone could expect.
And to your original question: if speedo was 100% correct at 60 mph with the 155-13, when it reads 60 mph with the 195-15's, you'll be going ~59.78 mph. Too small to worry about. Enjoy your new wheels & tires.
the 195/50/15's: 195*.5*2 = 195 mm:/25.4 = 7.6771 in; + 15 = 22.6771 inches nominal diameter.
The 195/50/15 is a bit shorter than the 155/80/13, according to the numbers. But by less than 0.4%. That's less than one-half of one per cent. So, if both tires were their exact nominal heights(unlikely in the real world), & the 155/80/13's were 100% accurate on the speedometer, when you run the 195/50/15's & your speedo says 100 mph, you'll actually be doing only a measly ~99.62 mph.
In short, you chose as close to a matching height tire as anyone could expect.
And to your original question: if speedo was 100% correct at 60 mph with the 155-13, when it reads 60 mph with the 195-15's, you'll be going ~59.78 mph. Too small to worry about. Enjoy your new wheels & tires.
They both are!
The miata calculator states:
"When your speedo reads 60mph, you are actually traveling 59.8mph."
The Galaba calculator says:
"New Speedometer Reading = What speed the car will need to go (with the after market tires on) to be traveling at the same speed if the car had the orginal tires on."
So miata tells you what your true speed is when you're speedometer says 60mph, while Galaba tells you what reading you'll need to see on your speedometer in order to really be at 60mph.
Keep in mind that most speedometers are off by a couple of MPH with the OEM tires...
A word of caution:
Using these calculators to compare tire sizes is great.
However, they hardly ever take into account that the rolling diameter of a tire is about 3% less than the overall diameter. So if the calculations continue to go into gearing and actual speed, the figures will not be accurate.
Hope this helps.
Using these calculators to compare tire sizes is great.
However, they hardly ever take into account that the rolling diameter of a tire is about 3% less than the overall diameter. So if the calculations continue to go into gearing and actual speed, the figures will not be accurate.
Hope this helps.
heres another simular toy
http://www.bigcountryoff-road.com/Extras/Extras Home.htm
http://www.bigcountryoff-road.com/Extras/Extras Home.htm
The actual size of tires of the same specification can vary about +/-.2" from brand to brand. Check the actual size from the manufacturer's data. The calculators only give you an estimate.
I also believe the rolling diameter and O.D. are the same thing and can be used to calculate speedometer correction.
I also believe the rolling diameter and O.D. are the same thing and can be used to calculate speedometer correction.
Unless you are going to recalibrate your system everytime you chnge brands of tires, or know you will never change brands of tires, going by about 97% of calculated tire diameter makes the most sense.
Capriracer was correct about the 3% difference in rolling diameter and outside diameter. At least, the numbers on Pirellis webpage indicate that the diffrence runs about 3%.
A tire also changes about 0.5 inch in diameter as it wears out.
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