So you read on a forum that a fellow enthusiast plus-sized their car or truck tires recently, and they’ve been touting the performance benefits of the change ever since. Maybe he thinks his traction and handling has improved, and maybe his friends are impressed with the look of his new oversized tires. The story got your wheels turning and now you’re thinking about doing the same. Before you move forward, consider the following two questions:
- is the proposed change safe and
- is it the best decision for you and your vehicle?
Credit: Pavliga, John.
To help understand these questions, we need to direct your attention to some important technical considerations. For starters, do you have ABS (anti-lock brake system) installed on your vehicle? Most new cars come pre-equipped with ABS. You may already be aware that the ABS system on your car was designed with the OEM (Original Equipment Manufacturer) tire size in mind. ABS pulses at a certain rate based on the tire’s dimensions. Changing the OD (overall diameter) of the tire more than +/- 3% from the original specifications can begin to cause the ABS system to malfunction, thus increasing your braking distance. Large changes of +/- 15% can cause the system to fail completely – an obvious safety concern for any driver.
In addition to the overall diameter which should not change much, you must also consider the “Approved Rim Width Range” for your vehicle. Due to the flexibility of a tire’s sidewall, each tire can fit on a variety of rim widths. The approved rim width of a tire is the range from widest to narrowest of rim widths that a tire is designed to fit. For example: the Michelin Primacy MXV4 in size 205/65R15 94H. While the diameter of the rim must be 15”, the rim width merely has to be between 5.5 and 7.5 inches (you can’t tell from the size, you have to look it up in a reference). Mounted on a rim that is wider or narrower than the approved rim width, the tire runs the risk of unseating itself under loaded situations, with potentially catastrophic results. So, please make sure you look up the approved rim width range for a specific tire and verify that your wheels are within that range. The approved range is generally found in data sheets for the tires on tire manufacturer websites.
cc: soulrider.222
There is a third plus-sizing factor that will not affect most people. If you drive a dual rear wheel truck, “dually” for short, you must consider something called “Minimum Dual Spacing”. This factor refers to the distance between the centerlines of the pair of wheels. “Dual kissing” results from there not being enough space between the two wheels, causing them to rub against one another under load. This rubbing will cause the temperature of the tires to exceed their designed tolerances and can lead to tire failure. When plus-sizing the rear wheels of a dually truck, it is almost guaranteed that you will need new wheels or will have to add a spacer between the existing wheels to provide enough space.
In addition, you must consider whether the new tire will fit within the wheel well and not come into contact with the sides, top, or internal structures like struts – and maintain safe clearance through the entire range of motion of the tire/shocks/axle. If the tire hits something when the suspension is compressed, or at high speed, it can lead to damage or catastrophic failure. If the new size cannot safely operate within the wheel well, your option is to lift the suspension, like the Blazer in the picture above.
With all this in mind, you are not done yet! You must also make sure that the new tire you want to mount meets both the minimum speed rating and load index for your vehicle. While a small downgrade in speed rating is generally considered acceptable (as long as you understand the performance loss) you should never downgrade the load index/load rating of the tire. The load index/load rating indicates the amount of weight the tire is designed to carry and overloading a tire will cause failure.
Once you have confirmed that the new tire(s) meets all these safety criteria, take a minute to consider the potential impact of plus-sizing on vehicle performance. Plus-sizing tires without making adjustments to other components of the vehicle may lead to slower steering response and premature wear of brakes, suspension and transmission components. On a positive note, plus-sizing can increase your traction (both on and off road) and in some cases make your ride smoother. Not to mention the head-turning appeal of a vehicle – especially an offroad light truck or SUV – with oversized tires.
Therefore, as you consider plus-sizing your tires, we recommend that you consider ALL the pros and cons before moving forward. Hopefully we’ve convinced you that there’s much more to the decision than the appeal of better performance or the look of “bigger tires”. Always consult a tire professional if you are unsure of what effects plus-sizing may have on your vehicle. Your safety, and your vehicle, may be on the line.
Comments?
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