Frequently Asked Questions -- General (FAQ)

  • From: (Al Crosby)
  • Organization: The National Capital Freenet
  • Date: Wed, 8 Sep 1993 02:57:26 GMT
  • I received a lot of advice from some very "patient" people about buying tires.
    Out of guilt I guess, I created this draft FAQ and I hope it helps the
    next person. Please save and send to others who ask for tyre/tire info. 
    This info will fall off the bulletin board in about a week.
    If you can search, look for "===" to skip sections.
    Make sure the cursor is on this line first.
    BUYING DIFFERENT SIZED TIRES/TYRES (I alternate the spelling)
      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)
                 /         \                       To find Height    
                (<---155--->) <-- the width here   (.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.)
    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 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
    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.
    The Traction code is a letter A=High, B=Medium, C=Low 
    These codes are also A, B, and C from high to low. People who drive in the
    hot desert should buy "A" rated.
    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
                  /]-rim-[\--------               6"
                 /         \                  /]-rim-[\------------
                (    75     ) 161 mm         (   70    )  150.5 mm 
                 \_________/________          \_______/ ___________
                 <---215--->                 <---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"
    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):
    From: (Ron Bense) 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)
    From: wdh@oversteer.Eng.Sun.COM (Dennis Henderson) 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. 
    From: (Eddie Renoux) 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).
    From: (Eric Lutkin) 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. 
    From: (Steve Romanski) 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
    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.
    From: (Big Stubbs) 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 XB (#1) + CompuServe 70641,2461 
    Ottawa, Ont   + +
 (Al Crosby)