I got into my first four-wheel drive truck in the nineties after my father died. He owned a 1987 Nissan Patrol pickup and I had always wanted to own a four-wheel drive vehicle. I had been a huge collector of off-road magazines over the years, but actually owning a four-wheel drive truck was still a huge learning experience for me.
One of the areas that was trial and error for me was the selection of tires based on size. The following is what I learned during the next few years about purchasing tires for an off-road truck.
When I got the vehicle, it came shod with 225 75 16s on split rims. These weren’t the stock tires on this vehicle, so I changed to the stock 7.50 16s soon after I started driving it. At 32.5 inches in height, these tires were quite a bit taller than the tires that came on the vehicle. The 225 75 16s were 29 inches tall.
The 225 75 16s that came on my truck are measured using an ISO Metric tire code. To calculate the height of the tire, you multiply the first number by the second, which is in hundredths (the aspect ratio). Multiply that by two and then divide by 25.4, e.g, (225x.75x2)/25.4. Then add that to the last number, which is the rim height. The height based on this formula is approximately 29 inches. The 7.50 16s that I purchased later are measured using a Light Truck numeric. The first three or four digits are the width of the tire in inches. The aspect ratio of these tires is one hundred percent. So using this formula, the height calculated would be about 31 inches. However, most 7.50 16s are generally closer to 32 inches or a bit over. Flotation sizes such as 31 10.5 15 are more easily measured, as the height is the first two digits. Bear in mind, though, that the size on the side of the tire can vary slightly from manufacturer to manufacturer.
The static loaded radius of a tire is the height from the ground to the center of the wheel when maximally loaded. In comparing two tire sizes, the difference in the static loaded radiuses of both tires will be the amount of ground clearance gained or lost between both tires. The radius is a bit less than the actual diameter of the tire, depending on the weight of the vehicle. The increase in height/ground clearance on my truck was 1.5 inches. The height increase was quite visibly noticeable.
Another noticeable effect of putting on taller tires was the decrease in RPM. With the shorter tires, the vehicle was quite revvy even in fifth gear. With the taller tires, the Patrol became much quieter. This is because taller tires decrease the gear ratio of your car. The stock gear ratio on my vehicle was 4.88. Moving from a 29-inch to a 32-inch tire decreased that gear ratio by 10%. Bearing in mind that the 7.50s were stock, the effective gear ratio on the 225s was 110% more, or 5.36 (a lower gear). No wonder the engine was so revvy! There are many sites online where you can calculate the effect on gearing that a change in tire size will have on RPM for your vehicle.
Effects that a change to a taller tire will have include:
If you want to maintain the performance of your truck after changing to larger tires, then a swap to lower gearing in your differential will go a long way toward keeping the RPM in the same range, providing you lower the gear ratio the same percentage that you increase the height of the tire. That may not be necessary if the increase is small and you can put up with a slight loss of power.
Larger tires will definitely give you an advantage in driving off-road, especially over more extreme terrain. An inch or so over stock height should be well within budget, so long as one considers the aforementioned points in buying off-road tires.