Having the wrong tire size, worth the risk?

Your brother in law has an old set of tires in his shed and “graciously” offers them to you… “It’s almost the same size as yours!”…
Good offer? Well... Might be not!

Run the numbers first! Let's recap!

Tires have 3 important numbers on their sides.

E.g.: 205-55-16:

• The first number is the width of the tire in MM, in this case, 205 mm or 20.5 cm.
• The second number is the ratio of height of the sidewall, in our example, 55% of 205 which is 112.75 mm.
• The third number is the size of the wheel required to mount the tire on.

Now, on the driver side of your car, you will find a sticker in the archway stating the recommended tire size for your car.

Let’s assume that your in-law as some 205-60-16 tires for you and your car needs 205-55-16. Close enough, no? Answer: Not at all!

While both tires have the same width, at 8.1 inch or 205mm, the sidewall and height is very different.

The golden rule

In the tire market, the maximum tolerance is -2% to +2%. Out of this range, there are some risks. In our example, we can see that the diameter differs from +3.2%.

Worth the risk?

Running a different tire size might cause a lot of problems.
Here are a few:

• Problematic data on the onboard computer
  A different tire size will send conflictual data to your onboard computer. This could cause the speedometer to display an incorrect speed, thus putting you at risk of being arrested for speeding.
• If the data is not the right one, your transmission, especially your AWD system might interpret the reading wrongfully, wearing it and possibly damaging itself on the long run.
• If the speed is wrong, then the fuel consumption might increase, costing you precious gas on the long run.
• Tire size VS wheels wells
  The wheels wells of your car can accommodate a certain tire size. If the tire is too big, it might rub against the body of the car, damaging it and damaging the tire.
• Wider better?
  Not always! If you use wider tires, they might rub against the suspension, damaging them. Some people will use spacers to prevent this. While this could sound like a good option, it is sometime problematic if not installed properly. Beware!
• Tires too small VS ground clearance
  If the tires are too small, you have a high risk of hitting something on the ground, damaging the underside of your car.
• Instability problem
  Your car suspension and direction are calibrated to have a specific tire size, have the wrong tires could cause instability while driving.
• Tire size VS warranty
  We have heard of some situation where a dealership refused to repair a car under warranty because the tires where not of the recommended size. Take this in consideration!
• Last but not least
  The age of the tires. If you in-law had them is his shed for a few years, the tires might be too old or have deteriorated due to the storage being inappropriate. Check the sides for cracks and check that DOT.

So, your in-laws can be well intentioned, but running the numbers on the tires that they are offering is important. Having the wrong tire size might damage your car and put you and the drivers around you at risk. Your safety is more important than saving a few dollars!

Drive safe!