285s are about an inch taller and will cover more distance per revolution of the tire. The Speedo & odometer will be about 3.8 % slower or behind.
See this "tire size comparison" site. Provides visual & numerical comparisons.
https://tiresize.com/comparison/
Calculate speed on the fly?
So basically, just multiply your indicated speed by 4% (round up 3.8 for easier math) to get a number to add to your speed.
50 mph x .04 = 2.0 ..... 50 + 2 = 52 mph
Been doing this for decades with my lifted Jeeps (an easy 10% calculation)
Same for your odometer. 250 miles is actually close to 260.
250 x .04 = 10.
250 + 10 = 260.
Eventually, you'll do it in your head without thinking too much about it.
60 mph? 6x4=24. Add 2.4 = 62.4 mph.
70 mph? 7x4=28. Add 2.8 = 72.8
80 mph? 8x4=32. 83.2
Screenshot from the site I listed above:
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Most all tire diameter & width increase will cost mileage. While it may lower rpms at a given speed, it requires more fuel to keep the extra weight rolling at the same speed.
..... The killer is the added wind resistance of the extra width (tire surface area) as well as increased area for air to go under the vehicle (very un-aero dynamic under there) causing more resistance.... requiring more throttle/fuel.
There's multiple force vectors at work in the physics of increased tire size.