When it comes to selecting replacement tires for a van, pickup truck or SUV, the most important consideration is to maintain sufficient load carrying capacity.
Three tire-sizing systems are used on the Original Equipment (O.E.) tires fitted to today's light trucks, Euro-metric, P-metric ("passenger" metric) and LT-metric ("light truck" metric). Many 1/4-ton and 1/2-ton vehicles use Euro-metric and P-metric sized tires, while 3/4-ton and 1-ton vehicles use LT-metric sized tires.
Euro-metric and P-metric tire sizes were originally designed for cars and station wagons, however they have also been used for light truck applications because most vans, pickup trucks and SUVs are used to carry passengers, not cargo. Additionally, most of the new light trucks being produced today are equipped with Euro- or P-metric sized tires because they offer lighter weight, lower rolling resistance and less aggressive tread designs (which makes them better riding, more fuel efficient and less noisy) than typical heavy-duty tires.
However there is an idiosyncrasy governing the use of Euro-metric and P-metric sized tires on vans, pickup trucks and SUVs because these vehicles have a higher center of gravity and greater probability of being overloaded than passenger cars. In order to accommodate this, vehicle engineers are required to specify Euro- or P-metric sized tires rated to carry 10% more weight than would be required if they were used on a passenger car. This is the equivalent of taking the tire's load capacity branded on its sidewall and multiplying it by 91%. For example, a Euro- or P-metric tire designated to carry 2,000 pounds on a car is restricted to carrying 1,820 pounds when used on a van, pickup truck or SUV. This size selection practice provides the vehicle manufacturer with the appropriate tire load capacity.
On the other hand, LT-metric sized tires were specifically developed for use on light trucks and provide the full tire load capacity branded on their sidewall. They are the little brothers of the heavy-duty tires fitted to 18-wheelers and busses, and use the same basic engineering guidelines regarding load capacity. Because of this, LT-metric sized tires are built very strong, and use higher inflation pressures to carry a given load in order to provide the desired safety margin. You really don't want the tires of the 18-wheeler along side you on the Interstate running at the limit of their endurance, do you?
While there isn't a problem determining an appropriate alternate size when replacing Euro- or P-metric tires with other Euro- or P-metric sizes, the differences in load capacity and required inflation pressure prohibits mixing the LT-metric tires with Euro- or P-metric tires, as well as often prevents replacing sets of Euro- or P-metric tires with dimensionally equivalent LT-metric tires, and visa versa.
Selecting the correct tire size will help ensure you have the performance and long term durability you desire.