tire size????????????????????what is it really

[bassman]

145 80 12 on the front of my 93 auto.
speedo is 100 k and never wrong gps says 93 and i am always getting passed.
155 80 12 is next up biggar .
what should i do.
i have to go 120 to go 105 so i am thinking mpg is suffering .
maybe auto wrecker has 13's cheap??
shayne

[2ndmouse]

we had to special order 155/12s for my Gf festiva.

it rides a lot nicer now and i dont see yet where the mpgs are down.

[bassman]

:shock:
so i go to the tire shop and the guy says that all car manufactures are doing this to make there cars get better on paper mpg???
he tells a good story but today i also drove 3 new cars and took my gps and all were right on as far as speedo .
it just sucks that 145 80 12 are $26 and 155 80 12 is $42

i still will look for used 13's

[Tim]

The tires on my Festiva were so bald they looked like racing slicks. I went to the junkyard and got a set of 155s off of a totalled Festiva that were practically brand new - I found the documentation for the car inside and the receipt said they were bought 3 months before I found them. $50 later they were mine...


Regards,

Tim

[25Horseplay]

I bought tires at Walmart for $17.95 each. 155R12, they had them in stock! All 4 with mounting, balancing, and road hazard were $113 out the door.

[mechanicaldj]

Yokahama 165/70/R12 is the best for improved handling and brings the speedo alot closer or you can try 175/70/R13 Goodyear Aquatred III after the Aspire brake swap, My speedo seems to be on the nose, bye bye hydroplaneing.

[Safety Guy]

145/80-12s and 165/70-12s are essentially the same ride height:

145
.80
116.00

165
.70
115.50

However, the 175/70-13 is much higher than stock ride height:

Let's say for simplicity that 116 is stock ride height (hub center to tire bottom=total radius=mathematically calculated ride height, but here I'm using just the tire height so we can compare height changes).

Add 12.7 mm to that to make up for the 1/2 inch increased wheel radius of the 13 inch wheel:

116.00
012.70
128.70

Round that down to a ride height of 128mm, which is our ride height on a 13: wheel if we kept the 145/80 tire on it. But we have a 175/70 tire, so do that aspect ratio:

175
.70
122.50

Now round that one down to 122mm and subtract the height of the 145/80 rubber, which is already included in our previous total :

122 minus
116
006

On any given wheel diameter, a 175/70 tire will be 6mm higher than a 145/80 tire. And that's rounding down, which is just being conservative to minimize the difference. Actually, the difference is probably over 7mm.

So we now add that increased rubber height of 6mm to our 128mm (adjusted 13 inch wheel size) and get a total of 134mm.

Compare that with a 116 for a stock Festiva size.

Mathematically calculated, our 175/70-13 is a whopping 18mm taller than stock. That's about a 3/4" increase in ride height.

Of course it's easier to go plug that into one of the net's tire size/speedo correction calculators to see what the estimated speedo variance would be. My 165/70-13s that I run on my '89L will show about a 2 mph slower speed at around 60mph compared to stock.

However let's remember that this is merely all calculated on paper. Tires wear, different manufacturers may fudge on their tire sizes, some may be unintentionally out of spec, etc. So it is probably more of an estimate than a hard figure, even with new tires. If you're comparing worn tires, subtract a couple mms rubber to get a bit closer to a real figure.

Now let me hit the "post" button and check my figures again! (Again, be advised that my "ride height" figures are not actual, just comparative, since I'm not adding up the actual wheel height. Stay tuned for next post, where I will simplify all this.)

Karl

[Safety Guy]

Crap, I just edited my previous post about five times to try to eliminate confusion. I doubt it helped that much, so let's really simplify. In this post is a more "down and dirty" way to estimate ride height changes.

Festiva stock tires are 145/80 and 165/70 12s. Essentially, they're the same height, within .5mm of each other.

What if you go to a 13" wheel with 165/70s, like I did on Muttstiva? How will your ride height change?

Well, the 165/70 tire sidewall height will not change, but your wheel radius (hub center to tire bottom) will. So we just add half an inch, which is roughly 12mm, to compensate for the switch to the large wheel.

With the larger wheel, I am riding 12mm higher than stock. Simple.

When you go up an inch in wheel diameter, add half of that (the radius) to your ride height.

Going from a 12" to a 13", you add 12mm.

Going from a 12" to a 14", add a whole inch, or 25 mm (remember, we're rounding here to keep it simpler).



What if you want to change tire sizes though?

Think what the most common tire size change is on Festivas: going from the increasingly hard to find original equipment 145-12s to the more commonly found 155-12s. I bet 70 to 90 percent of Festiva owners who bought new tires in the early nineties probably did this. Both my '89L and my '93GL had 155s on them when I bought them, even though the original equipment 12" tire sizes were 145/80 and 165/70, respectively.

The aspect ratio of 145s and 155s is probably the same. (Sometimes there is no stated aspect ratio on the 155/12s in dealer inventories, so I'm ASSuming that they are 80 series tires.)

For those who don't follow, "aspect ratio" is simply the percentage of height compared to width of the tire. The width is in the tire's sizing which is given to you: a 155/80-12 tire is 155mm wide.

The aspect ratio would be 80 percent of 155mm.

Work it out yourself if you want to. That's how I started on this many years ago. I sat and polluted my environment with scads of multiplication, addition, subtraction, etc. problems on paper! Actually, I HATE MATH, but this exercise was oddly interesting to me, since it involved CARS.

Here, we are just doing rough comparisons, so actual ride heights are not important.

Anyway, going from that 145/80 to a 155/80 obviously increases your tire width 10mm. And the aspect ratio is the same: eighty percent of the width. What's eighty percent of 10mm? Eight millimeters.

Going to the 155/80-12 from the stock 145/80-12 (or from the equally hard to find, and more expensive same-ride-height 165/70-12s) will raise your car 8mm.

Every time you go up a width size (10mm) on a tire, your height increases the same percent of aspect ratio.

On an eighty series tire, that is 8mm.

On a seventy series tire, it is 7mm.

On a sixty series tire, it's 6mm. And so on.

Going from a 195/60-14 to a 185/60-14 is now a simple matter of working it out in your head. You're going to a 10mm narrower tire in a sixty series aspect ratio. That's a six millimeter difference in height. Your car will now (theoretically/mathmatically anyway), be sitting six millimeters lower than before.

One more:

From reading the FMS project car stuff, we know that the 175/60-13 is about the stock ride height of a Festiva, with little or no speedo change to worry about. But what if you want to go to a 185/60-13? (Falken made, or at least made, both sizes.)

Well, if 175/60-13 is stock height, then the 185/60-13 is 10mm wider, and 6mm higher (both sixty series aspect ratios) than stock height.

You'll ride 6mm higher with the 185/60-13s.

In my next post we'll get a bit more complicated.

Karl

[Safety Guy]

Maybe your'e sitting in your high school or college algebra class, trying to follow the teacher as he drones on and on about the square root of Pythagoras' backyard doghouse roof surface area and it's implications for Leonhard Euler's mistress and her tax returns.

Well, that would be getting more interesting than the average math class, I guess, so scratch that.

All you hear is "blah blah blah blah blah" as you zone out and gaze through the window.

You see a Festiva drive by.

Hmmm. Wonder how much lower I would be with some stock Miata wheels on my Festiva?

So you work it out in your head:

You now have 155/80-12s on your stock wheels. From my previous post, you know that's 8mm higher than stock, and you would like to get your car lower, not higher. Would some 185/60-14s from a Miata do the trick?

You'd be going to a 14" wheel from your 12" wheel, so figure that out first:

A 2" larger wheel diameter means a 1 inch increase in your wheel radius, which is what we need to know. If you somehow managed to find a 155/80-14 tire to go on that Miata wheel, you'd wind up with a one inch higher ride height. But while that makes for an easy math problem, that tire size has probably never existed. Hmmm.

Go back to what you know now: that the 175/60-13 is the stock ride height for a Festiva. If you used a 185/60 tire on that 13" rim, you'd still be with a sixty series tire, so just add six millimeters and you know you'll be that much higher up.

But you have a 14" rim!

So add half an inch to six millimeters and you'll have your actual ride height change from your current set up.

Here it is in short form:

You know your 155/80-12 is the same height as a FMS project car's 175/60-13.

You easily figure that going to a 185/60-13 will raise your car 6mm.

You know you are going to a 14" rim from that 13" rim figure, which in metric is about 12mm.

Six mm plus 12 mm equals a total change of 18mm. Higher.

You'd not be lowering your car, you'd be raising it.



Now you're in your office, daydreaming of a different wheel/tire set up. You currently have an Aspire with 175/70-13s, which incidentily, are one size up from the stock 165/70-13s (which are getting harder to find and aren't as cheap as they used to be, which is why you went with the larger, more common, replacement 175/70s). Just to review, your current tires raised your ride 7mm.

You want 15" wheels. The smallest tires you can locally find to go on them are 195/50s. If you went with this combo, how much higher would your Aspire be?

Figure it out:

First, the easy part is changing the wheel diameter from 13 to 15 inches.

That's a two inch increase, so just add one inch of wheel radius, or approximately 25mm.

Now you're gonna have to do some MATH, because you can't think of any tire sizes you can easily compare between the two.

Figure for the tire sidewall height of a 175/70:

70 percent of a 175mm wide tire is 122.5mm tire sidewall height.

Now figure for a 195/50 tire:

50 percent of a 195mm wide tire gives you a 97.5mm tire sidewall height.

For simplicity, round 'em off. I'll just round up to 123 and 98mm.

Subtract 98 from 123 and you get a difference of 25mm.

On the same sized rim, a 195/50 tire will be about 25mm lower than a 175/70 tire.

But you went with a two inch larger wheel, which is one inch taller (in radius) than you currently have. That's about 25mm.

A 195/50-15 tire is about the same ride height as your current 175/70-13. (Remember it's also 7mm higher than the stock 165/70-13s would be.)

No appreciable change.

Questions?

Karl

[Tim]

On that note, is there a 12" wheel that fits the Aspire rotors?

Regards,

Tim

[krustyguitar]

I've got 195 60 R14 's on my aspire rims from a rwd BMW 325I Looks sick, and every body thought they wouldn't fit. Paid $120 canadian for 5 rims with decent tires. (i work at a junk yard i mean "auto recyclers"

[HappyMom]

What is the tallest tire and widest tire that will fit the STOCK Festiva. I guess the widest would be offset dependant.

Can I run 175/70/13 without clearance issues, (speedo should be 7% off)

[fastivaca]

I'm running 175/60 13's with no clearance problems and the speedo is bang on.

They are an oddball size but Falken has them as does Nexen. I've run both and prefer the Nexen N2000... they are awesome in the wet.
Check out this link

http://www.nexentire.co.kr/english/product_tec/sub2.asp?idx=11&name=N2000&expend=1

Good luck

Ian

[gomer]

Just thought I'd point out that the actual tire size can be as much as 6% off in width (and therefore also in sidewall height as discussed extensively in this thread) either the plus or minus direction... and still meet industry standards... not all 155/80/12's are the same size ... and to make it even more confusing remember that the standard rim width used to measure a tire varies with the tire width and the tires aspect ratio,,, wider tires get measured on wider wheels,,, lower aspect ratio tires get measured on wider wheels also ... manufacturers datasheets are your best friends...

I would think that a 185/60/13 on a 5.5 or 6 inch wheel would be the hot ticket for a Festy... bigger tires (14" +2 or 15" +3 fitments) would require tires that are engineered for heavier cars and likely wouldn't ever get up to a good operating temp on a 1770# Festy...

Same overall height ,, more rubber on the ground ,, slightly less sidewall but still enough to get a good turnin response (lets face it ,, the suspension stinks and the tire sidewall is a big part of the comfort factor)...

Current production KIA Prides use 165/65/13 tires...

http://www.kia-smg.com/kia.html

[qaking]

I got lucky awhile back and picked up some BF Goodrich Comp T/A's that are 185/55 R13 and according to this tire size calculator, it is pretty damn close to the perfect size: http://www.miata.net/garage/tirecalc.html :wink: