Is there any reason why I should not change my tire size from the manufactor’s suggested size of 265/70/17 to 285/70/17?
Possible lack of clearance and a mis-calibrated speedometer.
I may have forgot to mention, the tires are going on a 2003 Dogde Ram 2500 4x4 diesel.
Your performance in snow will be reduced a bit, if that matters to you.
The 285’s are a bit wider and perhaps a bit more circumference too. Likely not enough to make a big difference; IF the wheels can handle the wider tires.
I’m not sure what you hope to gain? There is virtually nothing to be gained and there could be some issues with the larger tires.
Most likely the main change you’ll notice is your speedometer reading will be a bit different. Perhaps more accurate, perhaps more off your actual speed.
The 285 tires weigh more and the increased width could cause a small decrease in fuel economy. I don’t see any real gain here either.