Tyres: All that you have to know about upsizing

One of the most commonly asked questions on CarToq is about “upsizing.”  We thought it would be a good idea to compile some information to give you some basic facts about the whole process of upsizing and make this process easier for you.

In simple words, choosing a wider tyre and/or a larger wheel than stock is known as upsizing. However, the process itself is not that simple and should be done the right way keeping safety in mind.

Upsizing can be done in two ways:

It is fairly important to upsize the tyre in a proportionate manner. This means if the width of the tyre is increased the sidewalls of the tyre should be proportionately decreased too. This will ensure that the Overall Diameter of the tyre is not affected.

Remember!

The Overall Diameter should not change much — not beyond 3 percent of the original.

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But, what is Overall Diameter?

In simple words Overall Diameter (OD) is the outer diameter of the tyre. This may not seem significant but altering OD may significantly change things like the odometer/speedometer reading, gearing, suspension travel space, mileage and handling of the car; more often in a negative manner.

By using a tyre with a shorter sidewall, you gain quickness in steering response and better lateral stability. The visual appeal is obvious. Most wheels look better than the sidewall of the tyre. So the more wheel and less sidewall there is, the better it looks. However, lesser sidewalls mean lesser impact absorption as there is lesser rubber to flex while travelling through rough surfaces making for a harsher ride.

To get a better idea of whether you should upsize to a wider tyre or not, take a look at the following chart:-

[table id=1046 /]

What you need to know about tyre sizes.

You can locate the tyre size on the sidewall of your car’s tyre and you will come across text (usually in large fonts for easy identification) saying — 185/65 R15 or something similar.

185 — Refers to the width of the tread on the tyre i.e the portion which comes in contact with the road. This is measured in millimeters; in this case the tyre is 185mm wide.

65 — Refers to the height of the sidewall and is measured in percentages. In this example, the sidewall is 65 percent of the tread width. This is also known as aspect ratio.

R — Refers to radial construction. Almost all passenger cars and SUVs now come equipped with radial tyres.

15 — Refers to the size of the rim on which this tyre is designed to fit, in other words the diameter of the wheel. This is measured in inches.

To help you pick the right upsize, here is a simplified table which has common tyre sizes supplied by car manufacturers along with possible upsizes with ODs within a few mm or within 3 percent of each other and would mostly be acceptable, depending on the wheel rim size you would want to change to.

[table id=1045 /]

For some sizes, required aspect ratios may not be available, in which case it is best to choose the next higher aspect ratio size. 

Keeping it simple!

For the sake of simplicity and avoiding complex calculations, we shall give you some thumb rules to follow.

Upsizing with original wheels/alloys

If one needs to upsize to a wider tyre but retain the original rim/alloy then it is always safe to increase tyre width up to 20mm. So, simply add 20 to the tread width. Thus for a 185mm tyre width, you could choose a tyre with a width of 205mm.

Similarly, the sidewall height should be lowered by 5 percent for every 10mm increase in the width of the tyre. Here, since we are increasing the width of the tyre by 20mm, the side wall or aspect ratio should be decreased by 10 percent. Which means it should be 55 percent instead of the stock 65 percent.

What remained constant here was the rim size/diameter of 15 inches. Thus the max upsize for a 185/65 R15 tyre should be 205/55 R15.

The stock OD was 24.47 inches with a 185/65 R15 tyre and with the upsized 205/55 R15 tyre, the OD changes to 23.88 inches, which is a 2.4 percent drop and within 3 percent of the original OD, making this a safe upsize. 

Upsize on larger wheels/alloys

Now, if one also wants to upsize the wheels/alloys along with a wider tyre then the same thumb rules apply.

So if 185/65 R15 had to be upsized with a bigger alloy of 16 inches then one could go in for 205/55 R16 and the OD would be 24.87 inches which is just 0.40 inches or 1.6 percent off the stock OD of 24.47 inches, thus making this a safe upsize as it is within 3 percent. 

Also see: Tyre buying guide: What type and brand to choose!

Do it yourself!

In case, you like crunching numbers and are technically inclined, here is how you can calculate OD and decide for a suitable upsize yourself.

OD can be calculated using the following formula:-

Example Size: 185/65 R15

185(tyre width) x 0.65(aspect ratio, remember this is a percentage, hence the decimal) / 25.4(for mm to inches conversion) x 2(number of sidewalls) + 15 (rim diameter) = 24.47 inches = Stock Overall Diameter (OD)

Hence, whatever the upsize may be, one should ensure that the OD should be either equal to stock or be within 3 percent of the stock OD.

We previously recommended an upsize to 205/55 R15 from 185/65 R15 tyre or 205/55 R16 on larger wheel/alloy.

To see if the OD remained the same or not, we shall apply the above formula and check.

205(tyre width) x 0.55(aspect ratio) / 25.4(for mm to inches conversion) x 2(number of sidewalls) + 15 (rim diameter) = 23.88 = Overall Diameter (OD), which is 2.4 percent off the original OD of 24.47 inches.

Thus, this upsize is safe to do without changing the characteristics of the car’s performance negatively.

Similarly, for the bigger wheel 205(tyre width) x 0.55(aspect ratio) / 25.4(for mm to inches conversion) x 2(number of sidewalls) + 16 (rim diameter) = 24.87 = Overall Diameter (OD), which is 1.6 percent off theoriginal OD of 24.47 inches.

Again, this would be a safe upsize.

We shall give you more information in our next article on Effects of Upsizing.