Tire Size questions

So, I need new tires for Saph. She’s a solid truck but needs new shoes. The current tires are what came with it and they’re size 285/70/R17. I’m curious what would happen if I went a little smaller or bigger. I don’t have quite enough to buy a whole new set. But, my dad has a set that are 255/75/R17. What sort of issues would I run into if I put those on? I know they’re smaller and the odometer would be off just a little. But, how much is what I’m trying to find out. Any help or recommendations would be great. Thank you!

 

255/75 is about 2% smaller in circumference than 285/70, so at 65 MPH, it'd be showing just under 64 (63.7). the speedo is already 3-5% off (higher) from the factory, so you'll be closer to your actual mileage/speed.