^Yes it is best to use the same year but you still may have some dash lights. Probably the dccd and the r. diff temp. But back to topic. I just called tacoma speed and they said nothing about a law but manufacturers are typically allowed to have up to a 3% error which isn't much, Honda's can have as much as a 5% error. It can be high or low. It does not have to be under! So your speedo can read slower than you are going. 3% error would only be 1.5 mph off at 50mph . I am 9% off and he said it's probably a combo of the wheel and tire size and electronic component differences between the two models.
Nobody makes a speedo correction tool for these cars like hypertech and superchip so I'm stumped. My SCT tuner for my mustang have provisions to change rear end ratio and tire size to maintain an accurate speedo and my 08' civic SI can the speed adjustment to the dash and to the ecu by use of the Hondata Flashpro.
The romraider opensource software that my car is tuned with also does not have any tables for speedometer correction.