Prescription Tires

Buying a car with odd tires sizes or expensive performance tires can be a costly mistake when it comes time to re-shod your vehicle.

As I noted in my Prescription Motor Oil posting, buying a car which requires special motor oil that costs $10 a quart gets to be real old, real fast.  When the number of places you can take your car to get the oil changed is limited, then you have to pay through the nose for something as simple as an oil change.

My next car will burn regular gas and use regular oil, and if they don't carry the oil filters at Wal-Mart, well, I ain't buying the car, period.  While having esoteric cars is fun and all, they can easily double, triple, or quadruple the cost of car ownership.

And nowhere is this more true than with tires.  In the old days, tires came in three sizes - 13", 14" and 15", and most gas stations stocked the right tire for your car - there were maybe a dozen sizes in all.

Today, tire sizes are all over the map, and not only that, they are changing with time.  What was a popular tire size 5 years ago is hard to find today.

As this report notes, the most popular OE (Original Equipment) sizes today are far different from what are the most popular replacement sizes being sold (which represents the older cars on the road).  This is a nightmare for tire dealers, as they have to stock an even wider array of tires in multiple sizes.

As the report shows, the trend is toward larger sizes, with 16" and 17" inch sizes being the norm today, and older sizes such as 13" and 14" being nearly non-existent, while 15" is fading fast.

In addition to rim diameter, there are other aspects of tire size - the width and aspect ratio.  In tire size parlance, as size such as P225/60 R16 has three numbers - 225 representing the width, 60 the aspect ratio (width to height) and 16 being the rim diameter in inches.  Some sizes are very popular, others, usually performance sizes, quite rare.  And it is possible to alter aspect ratio and width somewhat on some cars, although changing sizes beyond what manufacturers spec can be problematic.

In the olden days, you might find your car shod with P235/75 R15 tires, which were fairly "tall" tires (aspect ratio of 75%).  But today, "series 60" and below are more common - tires that in the 1960's would have been considered "ultra low profile" tires.   So the general trend is toward larger diameters and lower profile tires across the car lines.  The bulbous balloon tires of days gone by are fading fast.  HR78R15's are nowhere to be found these days.

But beyond the switch in rim sizes, there are other considerations that are more relevant.  In particular, there is the phenomenon of the "unintended performance tire customer".  Today, most cars are sold with alloy wheels and lower profile tires.  Many people buy cars with these sort of rim/tire combinations without thinking about them too much, other than maybe the rims "look pretty".

But when it comes time to re-shod the car, they are shocked to find out that these tires cost $250 apiece and more, particularly if they are an esoteric size that is hard to find.

For a performance-oriented driver, perhaps this excess cost might be justified - although as someone with two "performance" cars, I am not sure that even this is the case.  In our modern world, there are few places where you can safely drive a high-performance car to its limits, and below those limits, there is little need for extreme rubber on your wheels.  Moreover, the sacrifice in ride quality and tire life (in particular) is questionable, if the only benefit is that once in a great while, you can take an off-ramp at 1.0 g.

Thus, for example, I am looking for replacement tires for my X5.  These are Michelin MXV-4 "Energy" tires, which are hardly an esoteric high-performance model.  They are, however, low profile P255/55 R18 sized, which is an oddball size to find.  These were the "upgraded" tires which came as part of the "Sport Package" as opposed to the more plebian (and easier to find) 17" stock rubber.   Most tire stores stock 15", 16" and 17" sizes.  Few stock many 18" tires, and fewer still the low-profile P255/55 series.

The Michelins are good-wearing tires, with over 60,000 miles on them so far (which is their warranted service life) but to replace them will cost $250 apiece plus mounting and balancing, which is a lot of money ($1000+) for a car worth maybe $10,000 on a good day.  Most likely, they will be a special-order item, perhaps from the tire rack.

The M Roadster definitely has prescription tires P225/45 R17 in the front and P245/40 R17 in the rear ( a so-called "staggered" arrangement which makes rotating tires out of the question).  While these are 17" sized tires, which is a popular rim diameter, they are a fairly rare performance aspect ratio and width.  These tires ride very harshly and barely cover the rims, which tend to rash as a result.  Like many folks, I decided to go up one size and change the stock 225/45/17 on the front to 235/45/17 and the stock 245/40/17 on the back to 255/40/17.

The original tires lasted less than 30,000 miles, which is not unusual for a high-performance tire with a soft compound.  And therein lies another problem with high performance tires - they don't have much of a treadlife.  And ironically, many of these "high performance" tires are dodgy in the rain, and of course useless in snow or other hazardous conditions.  

I decided to go with Bridgestone Potenzas, which were more of a passenger car tire than a racing tire, and they have a very good service life.  I got a deal on these at a silent auction for about $200 the set (!!).  Otherwise, they would have cost over $1200 easily, if I went with a "performance tire" replacement.

So what do we learn from this?  When shopping for a car, new or used, consider tire sizes carefully.  If you want reliable, basic transportation, it doesn't pay to have a car that requires prescription motor oil, premium gas, or special odd-ball tires.

Performance or odd-sized tires often have the following problems:

1.  Short tread life (30,000 miles or less) which means replacing tires more often and paying more often for mounting and balancing.


2. Odd sizes, which means special ordering, which means little or no discounts, high tire prices, and little likelihood that such tires will ever be "on sale" at your local store.


3.  Poor performance in the real world - racing tires often have poor rain and snow performance and ride harshly and allow for rim damage.


4.  Leaving you stuck:  If you have a car with esoteric tire sizes and have a blowout on a Sunday night, chances are, you'll have to get a motel room and your car towed in, as it may take days to order the special replacement tire and have it mounted.  Many high-performance cars have no spares.

Having a high performance car is fun and all, but when you get older and realize that the amount of money you will earn is a finite amount - and that you'd rather be doing things than owning things, the idea of having fancy tires which are of little real advantage in the real world, well, it gets old real fast.

If you just want basic transportation and the optimal cost/benefit ratio, buy a car with fairly standard sized tires and wheels - tires and wheels that are readily available and that can be replaced cheaply when the time comes.

Because, when it comes down to it, what you do with your car is more important than what options it has.  And if you can't afford to go on vacation this year because you had to spend $1000 on tires that last only 30,000 miles, well, you have to ask yourself if it was worth it.

And of course, the idea that every car needs to have alloy rims is sort of silly as well, but I will address that in another posting.  Steel wheels might not be sexy, but they are cheap and durable.