OK, thought you meant you went to 35 aspect ratio tires. Anyways, the difference between 275/55-20 (31.9" tall) and a 35" tall tire is almost 10%. So, if your speedo was dead on with the original tires, at an indicated 70 mph with the 35" tires, you would be going about 77 mph. Now, about being only 2 mph off, all things being equal and linear, you would be only 2 mph off at 20 mph. So, not sure how you calculated anything, but that is the cold hard math of the situation. Math is really quite simple; if you need help, let me know.