Tire size question

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Scott Hughes - 04 Apr 2006 22:36 GMT

I have stock 14" alloy wheels on my 94 Miata. According to the tire
calculator on Miata.net, the standard diameter & rev/mi are 22.7 and 887
respectively. The three tire options I'm looking at come in stock 185/60
(Yoko), 195/55 (Toyo) and 195/60 (Hankook). Going on the manufacturers
info, the measurements (dia, rev/mi, rev/mi %diff) are:
Stock: 22.7 887
Yoko: 22.7 914 +3.04%
Toyo: 22.4 926 +4.4%
Hank: 23.2 895 +0.9%

Why are the Yokos which are the stock 185/60 size, so far from the stock
numbers listed on the wheel calculator?

Should I be concerned with the apparent 4.4% increase in rev/mi w/ the
Toyo?

-Scott

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Chris D'Agnolo - 04 Apr 2006 23:06 GMT

I can't see why this would be significant, are you worried about accuracy of
your odo or speedo? Seems inconsequential to me, am I missing something?
I've driven with 195/60/14's on both my prev 92 and current 99, no problems.

Chris
99BBB

>I have stock 14" alloy wheels on my 94 Miata. According to the tire
> calculator on Miata.net, the standard diameter & rev/mi are 22.7 and 887
[quoted text clipped - 13 lines]
>
> -Scott

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Leon van Dommelen - 05 Apr 2006 00:24 GMT

>I have stock 14" alloy wheels on my 94 Miata. According to the tire
>calculator on Miata.net, the standard diameter & rev/mi are 22.7 and 887
>respectively.

That would depend on the standard size tires you put on there.

> The three tire options I'm looking at come in stock 185/60
>(Yoko), 195/55 (Toyo) and 195/60 (Hankook). Going on the manufacturers
[quoted text clipped - 6 lines]
>Why are the Yokos which are the stock 185/60 size, so far from the stock
>numbers listed on the wheel calculator?

I think because tires are not rigid, nor perfect cylinders. And the tire
width is rounded, as is the aspect ratio.

>Should I be concerned with the apparent 4.4% increase in rev/mi w/ the
>Toyo?

I guess, if you are racing someone to 55 mph, it may mean another shift.
If you are racing someone from a stop light, it is good. If you want
to be a law abiding citizen it helps you really obey the speed limit
with a vengeance and drive any white Miatas behind you crazy.

Leon

Signature

Leon van Dommelen :) Bess, the Miata :) Bozo, the Miata :)
rammm@dommelen.net http://www.dommelen.net/miata
The only thing better than a white Miata is two white Miatas

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Christopher Muto - 05 Apr 2006 01:01 GMT

using this calculator:
http://www.miata.net/garage/tirecalc.html
i don't see the smae results as you...
195/55/14 are 1.3% too fast (when speedo reads 60 actual is 59.2)
195/60/14 are 2.1% too slow (when speedo reads 60 actual is 61.2)
i don't know which models you are looking at but two great choices for all
seasons in 185/60/14 are falken ziex 512 and bf goodrich tracktion t/a h
rated. and they both happen to be very inexpensive.

>I have stock 14" alloy wheels on my 94 Miata. According to the tire
> calculator on Miata.net, the standard diameter & rev/mi are 22.7 and 887
[quoted text clipped - 13 lines]
>
> -Scott

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Lanny Chambers - 05 Apr 2006 05:41 GMT

> 195/55/14 are 1.3% too fast (when speedo reads 60 actual is 59.2)
> 195/60/14 are 2.1% too slow (when speedo reads 60 actual is 61.2)

You're basing your predictions on a mistaken assumption. With 185/60-14
tires, most NA speedometers read 3-4 mph fast at 60 anyway, so it simply
isn't very important. The 195/60-14 will be a little closer to correct,
perhaps indicating 60 at an actual 57. The only way to know for sure is
GPS, or a stopwatch between mileposts.

The odometer is a different story. With 185/60-14, it's pretty darned
accurate. It has to be, by law.

Signature

Lanny Chambers
'94C, St. Louis
http://www.hummingbirds.net/alignment.html

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Christopher Muto - 05 Apr 2006 15:14 GMT

>> 195/55/14 are 1.3% too fast (when speedo reads 60 actual is 59.2)
>> 195/60/14 are 2.1% too slow (when speedo reads 60 actual is 61.2)
[quoted text clipped - 7 lines]
> The odometer is a different story. With 185/60-14, it's pretty darned
> accurate. It has to be, by law.

wasn't making any assumptions or even forming an opinion. just pointing out
that the math that op had posted didn't agree with the calculator found on
miata.net. though your comments as usual are interesting... perhaps the na
speedometer reading 3-4mph over has kept me out of getting speeding tickets
when i had 205/50/15 on my car.

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Scott Hughes - 05 Apr 2006 18:39 GMT

>>I have stock 14" alloy wheels on my 94 Miata. According to the tire
>> calculator on Miata.net, the standard diameter & rev/mi are 22.7 and
[quoted text clipped - 5 lines]
>> Toyo: 22.4 926 +4.4%
>> Hank: 23.2 895 +0.9%

> using this calculator:
> http://www.miata.net/garage/tirecalc.html
> i don't see the smae results as you...
> 195/55/14 are 1.3% too fast (when speedo reads 60 actual is 59.2)
> 195/60/14 are 2.1% too slow (when speedo reads 60 actual is 61.2)

The calcualtions I got are based on the actual tire sizes listed by their
manufacturers. I took the "stock" values from Miata.net and then
calculated the percentages based on the manufacturer spec rev/mi as
compared to the "stock" rev/mi. Which is why the 185/60 of the Yoko is
no equal to the 185/60 of the "stock" figures.

-Scott

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Christopher Muto - 05 Apr 2006 20:08 GMT

>>>I have stock 14" alloy wheels on my 94 Miata. According to the tire
>>> calculator on Miata.net, the standard diameter & rev/mi are 22.7 and
[quoted text clipped - 19 lines]
>
> -Scott

if this is a big concern to you then perhaps you should call one or more of
the manufacturers to discuss it. specifically how can they call a tire
185/60/14 if it has a different diameter than another. i suspect the answer
lies in the what leon aptly said, "I think because tires are not rigid, nor
perfect cylinders." an inflated tire with the weight of a car on it is a
different shape than it is when it is not even mounted. if you do talk to
yokohama or the like please post whatever it is that they tell you. good
luck.

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XS11E - 05 Apr 2006 23:10 GMT

> if this is a big concern to you then perhaps you should call one
> or more of the manufacturers to discuss it. specifically how can
> they call a tire 185/60/14 if it has a different diameter than
> another.

Every manufacturer had their own molds and the sizes are NOT the same
from one manufacturer to another. Goodyear's 185/60/14 might be of
larger or smaller diameter than Michelin's or Toyo's of the "same"
size. Width can vary also. That's why it's possible to run into
clearance problems with various brands of supposedly identical sizes.
The problem is particularly well known with motorcycles where the
clearances may be much less than cars.

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M. Cantera - 06 Apr 2006 01:22 GMT

The outside diameter of the tire is not considered in determining the
tire size.

The tire sizes indicate the width of the thread (in mm), the ratio of
the widht of the tire cross section to the height of the tire cross
section (width/height * 100) and the diameter of the wheel (inches)

A tire with rigid sidewalls and a square cross seciton will be smaller
in outside diameter than a tire with a rounded section of the same
aspect ratio (the round tire will be wider cross section and
accomodates a greater tire diameter for the same ratio)

Add in differences if thread depth and rounding to the nearest 0 or 5
and you can have a real mess.

>> if this is a big concern to you then perhaps you should call one
>> or more of the manufacturers to discuss it. specifically how can
[quoted text clipped - 8 lines]
>The problem is particularly well known with motorcycles where the
>clearances may be much less than cars.

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