I’m not an automotive expert, but if I understand how speedometers work, it should be simple.
If you have both sets of tires available, the simplest thing to do will be to measure the circumference of both. Any percentage difference will be the amount your speedo has changed.
For example, let’s say your old tires were 75 inches around (just pulling numbers out of the air) and your new tires are 70 inches. That’s a reduction of 5 inches, or about 7 percent. Your speedometer would now read about 7 percent slow, so if it said 65 miles per hour, it would really be about 60.5 mph.
I think your odometer would be off by the same percentage.