Tyre Info

This information added and made available to all as per the instructions of its originator, Al Crosby.

 

**WARNING** THIS IS A FIRST DRAFT (THERE MAY BE ERRORS)

BUYING DIFFERENT SIZED TYRES.
My front tires were bald and I could feel the vibration in the steering wheel as the sidewalls were wearing out. I needed to buy new front tires on my '87 Toyota Tercel and I wanted to get the best tire I could for the weather conditions in my area of the world. The following should explain what I went through to find out what I could do, what I needed to consider, and what I did. Some helpful posts are at the end of this message.
===
TIRE CODES (And there are a lot): You have to know what the codes mean on your tires first.
eg. Metric P155/80HR13 P= Passenger LT=Light Truck 155= 155 cross sectional width in millimetres
(Not the tread width at the road surface)

/]------------[\-------------------------------
/-------------- \
(<---155--->) <-- the width here
\_________/_____________________________
^
To find Height (.80*155=124 mm)

80= Aspect Ratio (Height / cross sectional width)*100 An aspect ratio of 70 means the height of the tyre is 70% as high as it is wide. The aspect ratio is related to the Load Carrying capability which is in the next example. The Load Carrying capability is related to the amount of air in the tyre too.

H= Speed Rated Code (H means it is rated for speeds up to 240 Km/hr or KPH for short periods of time. The Michelin brochure adds that the tires must be normally loaded and properly inflated as well.)
B=50,C=60,D=65,E=70,F=80,G=90,J=100,K=110,L=120,M=130,N=140, P=150, Q=160,R=170,S=180,T=190,U=200,H=210,V=240,Z>240 km/h (Americans can multiply by .6214 to get mph and round it off) R= Type of Construction (R is for Radial) B=Belted For conventional tires, I don't think there is a letter. 13= The diameter for the rim size (in inches this time).

There is also a new code being used. eg. 205/60R15 82S 205= 205 sectional width in millimetres
60= Aspect Ratio (60% of 205 = 123 mm high sidewall) R= Type of Construction 15= Diameter of rim in inches 82= Load Index (The list of load indexes is further on) S= Speed Rating Index of 180 KPH certified by the manufacturer

There are also other coding conventions I did not figure out. Maybe someone from the UK or Germany can add info here. eg. European Metric 155HR13 (Looks the same to me) Alpha Numeric BR80-13 (The B translates to a load capacity)

Each Company also puts on it's own codes such as 4S or +4 for all season or M+S for Mud and Snow. Easy to get confused here.
===
THE DEPARTMENT OF TRANSPORT (DOT) CODE:
The last three numbers represent the week and year of manufacture. They say you should look for a "fresh" tyre since time, moisture, heat, pollution and ultra violet light can effect your tires. Also make sure your tires
have not been stored around gas or oil. eg. DOT 032 03= third week 2=1992 DOT 163 16= sixteenth week 3=1993
===
TREADWEAR:
This is based on 100. When you see Treadwear 440, this means the tyre will wear 4.4 times normal. 260 means 2.6 times normal. Basically it is just a relative measure so you can compare tire wear. Tires that have wear bars in the tread will show up when 1.6 mm or less of tread is remaining. If you see these, it is strongly recommended that you change the tire.
===
TRACTION:
The Traction code is a letter A=High, B=Medium, C=Low
===
TEMPERATURE:
These codes are also A, B, and C from high to low. People who drive in the hot desert should buy "A" rated.
===
FIGURING OUT YOUR REPLACEMENT TIRE:
Normally, your owner's manual recommends the type of tire for your car, or it is on a placard somewhere (glovebox for example). They say use only the same size and construction as originally installed and with the same or greater load capacity. (or words to that effect). But people do change their tyre size and this is where my story really begins. You need to know what the manufacturer recommends and the vehicle
capacity weight of your car. Mine says: P155SR13 P155/80R13 775lbs for a '87 Toyota Tercel 5 door

The air chamber determines the load capacity and there is a load index associated with the load capacity. The second number is the load capacity shown in "pounds" below. The first number is the "index" on the tire using the new code.

65=639 66=661 67=677 68=694 69=716 70=739 71=761
72=783 73=805 74=827 75=852 76=882 77=908 78=937
79=963 80=992 81=1019 82=1047 83=1074 84=1102 85=1135
86=1168 87=1201 88=1235 89=1279 90=1323 91=1356 92=1389
93=1433 94=1477 95=1521 96=1565 97=1609 98=1653 99=1709
100=1764 101=1819 102=1874 103=1929 104=1984

If you substitute, you must find the aspect ratio that matches or exceeds the load capacity recommended. So here is an example for P215/75R15 to P215/70R15:

6"
/]---rim-----[\-------------------
/-------------- \
......( 75 ) .................^ to v =161 mm
\_________/____________
<---215--->

6"

/]---rim-----[\-------------------
/-------------- \
......( 70 ) .................^ to v =150.5 mm
\_________/______________
<---215--->

 

1742 lbs max 1620 lbs max

(Diagram not to scale, but you get the idea)

Obviously, the air chamber is less and you can not substitute these tyres. If you change the width of the tyre, then the aspect ratiowill also have to change to provide the same amount of air space.

In my case, going from 155/80R13 to 165/75R13 or 175/70R13 does not alter the air space in the air chamber and these substitutions would be okay. I could not find a 165/75 tire so this one was out of the running. I also asked the Toyota dealer on the recommended substitutions. This was a real shock. I was shown an 1984 list of tires. Remember, I have a 1987 car. I was also told that "SR" in P155/80SR13 stood for Speed Rating, and that the 155 was the width of the tread, not the cross section. Do not believe
everything that people tell you, even this FAQ. Figure it out yourself. "Caveat Emptor"
===
CONSIDERATIONS:
Point 1: I wanted to go to a wider tire for traction in the snow, but I found out this was wrong. Narrow tires are better in snow. I still went to a wider tire anyway. 155 to 175 (20 mm is about the width of your middle
finger, and I did not see this as a major change) I also have a front wheel drive and there is a lot more weight in the front than in the back. We'll see what happens.

Point 2: Does it change the speedometer reading? Yes it does. You calculate the diameter of a P155/80R13 tyre by: {2 (top & bottom)*[.8*155] + (13"*25.4"/mm)} = 578.2 mm The circumference of the tyre: Pi*D = 1817.2 mm Changing to a P175/70R13 means a diameter of 575.2 mm and a circumference of 1807.8 mm
which is a little less and therefore errs on the slower side of the speedometer reading. If you notice this difference, you are gifted! The error in the speedometer itself would be probably be more. Who
calibrates their speedometer anyway.

Point 3: Will it fit in the wheel well? In my case the 175/70R13 was recommended by the dealer and there is still lots of room. I checked the wheel movement full left and right and I probably could have gone even
wider depending upon point 4. I was also told by one garage mechanic that he put 165/??R13's on his Tercel and everytime he hit a bump the wheel would rub on the top of his wheel well. If you do not change the height of the wheel by changing the aspect ratio, I can not see why this would happen. I could only conclude there was something else wrong. Suspension?

Point 4: Will I have to change the rims? In one posting, I was told that you can change +/- 10 mm from the width recommended without changing the rims. I disagree with this, and +/- 20 mm seems perfectly safe. Remember that my Toyota dealer even listed the 175 as a substitute. I really do not know at what increase in cross section is maximum to justify a change of rims.Point 5: This one was generated by me and only one person gave me an answer. I will now have 175 mm wide tyres on the front and 155mm wide on
the back. If not stated, you rotate radial tires back to front every 10000 km, so you do not want to loose track of where they are. If I have problems with the wider tires in the snow, then I will put the 155's on
the front again. I think this is an unexpected advantage. In the summer you can put the wider tires back on the front for better traction, and you automatically rotate your tires! :©)

If you were wondering, I finally bought 175/70R13 82S Michelin X's at $69.99 apiece + GSTax $9.80 + PSTax $11.20 (We have a lot of taxes in Canada!) With $5.00 (+.35 tax) apiece for installation and balancing, plus $1.00 apiece to environmentally dispose of the old tires, the final sum was $173.68 Cdn (93$) Notice I did not talk about tread pattern or Manufacturer at all. Once you figure out what you want, you normally shop around for the best deal. I certainly did not go looking for a particular tread pattern either. (Read the Tire Survey for recommendations) As far as the no. of plys were concerned, I took what they had for the price.

I also thought going wider would stop them from squeeling around corners. They still squeel but a little less. Maybe it's my driving technique, or maybe it is a Michelin trademark (??) :-) :-)
===
SOME ADVICE RECEIVED (And thanks for taking the time to help me):

Ron Bense says: Depending on which car you have, you can, conceivably,put on 205/50 R15s, with the proper rims and offset, but you'll chew through front suspension parts at a faster rate. These are
all things to consider when changing your tire size more than one or two sizes in the upwards direction. When you have the larger tires to begin with, you have another problem, and that's that your suspension is
generally tuned to the specific tire size recommended, so you really have to be careful. (I'm talking about those who have 60 or lower series tires, generally sports cars)

Dennis Henderson says: Be wary of simple statement such as "higher speed rating means a better handling tire". H-rated Dunlop Qualifiers HR4s handle *worse* on dry pavement than T-rated Eagle GTs on my '85 Pontiac 6000STE. H-rated BF Goodrich Comp-TA HR4s handle *similar* on dry pavement than T-rated for my'84 Z28. Yes..the Comp is M+S and the Eagle was not. Thus all H-rated tires are not created
equal and won't necessarily outhandle a lesser speed rated tire. Usually V rated push you to a lower profile tire. Tire size will most likely make the choice for you.

Eddie Renoux says: The theory behind narrow tires in the snow is that they will have a higher loading and thus the snow will more easily be spun away sideways as easily. I believe it as my CJ5 has 15" wide tires and is bad in the snow and my wife's Pathfinder has much narrower tires with the same pattern and it is pretty good. I have 185x13 on the front of my Honda and 155x13 on the back. It does not track well on dished pavement (where the tires run in a little gully) as exists between Broomfield and Sheridan on US36. I usually run 165x13 on the front with 155x13 on the back without noticing any bad results (got a great deal on the 185x13s).

Eric Lutkin says: Skinny tires work better in the snow because you have more weight per unit area of contact patch with the ground. This factor better enables the tread to push through the snow (assuming a reasonable tread for snow) and actually reach the pavement underneath. Wider tires will tend to snowplane above the pavement. If you'll be driving a lot in snow get a set of narrower rims and skinner tires with slightly higher profile.

Steve Romanski says: The difference (in width) is similiar to an ice skate and a toboggon? on snow a skate cuts through the snow to the road (thin tire) the sled rides on top (wide tire). The formula for tire diameter is : ((width)x(aspect ratio)x(sidewalls))/(mm per inch)+(wheel diameter) Therefore 235/60 14s: (235mm) x (.6) x
(2sidewalls) / (25.4 mm per inch) + (14 in wheel)is 25.1 inches in diameter

The following is my 2 cents on tire classification.

Performance tires are - wide (thicker patch better in corners) - soft (stick to road whether wet or dry but wear fast) - close tread (more road contact) Snow tires are - narrow (better to cut through snow)
- wide open tread (snow clears out easier) Long Lasting tires - harder rubber compound (not as good traction) Any trade off between tires looses you something, either wet and dry performance, snow performance, or tire life.

Big Stubbs says: If you can find versions of your car (P155/80R13) with 175/70, I think they should work on yours. It's not that drastic of a change (20mm wider). The 65 series tires are not as standard as 70 or 80, but I think 75 are pretty common. You might try another dealer. Of course, you might find that 155/80 are ok, they are cheaper, more efficient, and some say they give a little better traction on slick surfaces because the weight of your car is concentrated onto a smaller area. Bigger ones might last a little longer, and give better
traction on dry surfaces.

Al Crosby

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