I have a set of TOYO Versado Noir 195/65-R15 tires that came on my 08 LS, the BC. I have chedked the speedo against 3 different GPSs (Garmin, Bad Elf, IPAD) and a chronometer (Seiko 7A38 ) and find my speedo matches all of them very well. In fact, it is close enough that I trust my speedo at all street leagle speeds.
Now every tire mfg's tire diameter are NOT the same for any given tire size. In other words, a Goodyear 195/65-R15 is NOT guarenteed to be the same outside diameter as a Firestone, BFGoodrige, or any other. While they all will match a given wheel size( 13,14,15,...).
This is why you want to run match sets (4) or at least match pairs on an axel.
PS there is a tire calibration setting that can be adjusted in the body control module (BCM).