285 - 265 is an aspect ratio. As asked, what size are these tires. The complete size.
The 285 is width of the tires tread in mm, while the second number is 75. This is 75% tall as they are wide. This 75 changes the overall circumference that effects the number to rotations per mile, so less.
The transmission will shift differently due to TPS, Timing, RPM, VSS and knock sensor feedback... this equals "Calculated Engine Load". The VCM/PCM has algorithms to change shift pattern for best operation for engine load and vehicle performance.
While going larger in the 75 aspect ratio, increasing tire circumference WILL change the speedo reading and is correct at the DRAC if possible, but a 285 is wider that effects rolling resistance. Basically, more rubber touching the road, increasing load, having to push air in front of the tire...just little stuff that adds up.
This is why "Hybrids" have low rolling resistant tires, either more expensive or not, but have a different sidewalls stiffness, harder rubber tread compound, all make changes.
It is like placing the vehicle on the flat, in neutral and rope with tension gauge on front to pull it, then measure once rolling. It would change with tire selection. Proper inflation is based upon the placard on the door, not the "Max PSI" seen on tire. This is max setting for inflation after installing for a tech to pay attention to, not the same for everyday tire air pressure testing. This change between 4 ply, 6 ply or the more common truck tire 8 ply that may say 60 PSI on the sidewall. This should work great and last longer adjusted to 40 PSI.
Call a tire shop to ask a simple maintenance question. But find the roads mile markers, test with other vehicles if needed to confirm a single mile, the start, run the mile and see what the odometer says. Adjust either in vehicle or in mind will keep you from getting tickets.
