Actual diameter of the Toyo 285/70r17 calculates at 32.7". 275/65r20 diameter calculates to 34.1. That's a gain of 4%.
A 4% gain in tire diameter means that when your speedometer reads 100mph, you're actually doing 104. At 50mph on the meter, you're running 52.
If you're 5mph off at regular driving speeds, there is an error already in your speedometer (or somewhere) because the tire size change alone doesn't account for that. A 5mph difference at 40mph, for example, would be 12.5% difference. To get that, you'd have to be running a 36.8" (37") tire.
It's pretty normal for speedometers to be inaccurate, though. I once had reason to look up the accuracy of the stock speedometer on my Jeep. It was rated by the maker at -5/+8mph nominal error at 60mph. The auto industry generally accepts 10% as a perfectly acceptable.
Of course, you can't have any such thing as a perfectly accurate speedometer without using doppler radar. Even if you get it accurate, things change as you drive. Tire temp shifts a bit, causing outer diameter changes. This is based on your driving speed, road temp, air temp, and aggressiveness of your driving. Tread wear causes it to shift more slowly, but much farther over the long haul. A 1/4" of tread wear equates to about 1.5% error. And you certainly can't use GPS devices for calculation - consumer GPS is only accurate to about 65 feet. Plus GPS devices don't calculate distance accurately when there are elevation changes.