Automotive Mileage

This has more to do with actual tire sizes, which vary manufacture to manufacture slightly.

However, if there is an error, manufactures tend to err on the side of caution and the speedometer will read slightly higher than actual true speed.

As AH said, they're all fast.

The manufacturers don't want to get sued every time someone gets a speeding ticket.

They always err on the side of reading too fast.

If you are not using the original exact OEM tires 5% error is easy to get from just a bit taller tire, or even more air pressure than what is on the label on the door jamb that the manufacturer suggests you use for that model.

If you pump up the tires to their maximum, the sidewalls will stand up and be way too stiff if they are radials, and increase the rolling height of the tire as well.

Don't use the maximum inflation number on the tire unless your gross loaded vehicle weight divided by 4 equals the maximum allowed load printed on the tire.

Use the car manufacturer's suggested tire inflation pressures.

http://www.ajdesigner.com/fl_tire/tire.php

Most modern speedometers are more accurate than that. The SAE limit is 4%.

Honda, a while ago, was accused of selling cars with odometers and speedometers that read high. The actual range was from 1% low to 3.75% high -- within the SAE standard. The lawyer argued that this shortened the warranty period unfairly.

Manufacturers worry about speedometers reading low, because there is a safety issue. The difference in stopping distance between 55 mph and 50 mph is many feet. So if someone could show that their car speedo read 5 mph low, they could claim that, had they been driving at a real 50 mph (instead of 55), they would have stopped 10 feet earlier, avoiding the pedestrian, who's family is now suing for $2 million bucks, because the evil auto company was negligent in making the driver think that he was going a safe speed when he was really travelling too fast for conditions.

Much safer to have the speedo read high, the corporate legal beagles would argue.

The fuel economy ratings are based upon very accurate speedometers on the test dynos. I doubt that any manufacturer would care about what individual owners actually appear to get. Getting 31 mpg on your Honda instead of 30 is unlikely to be the thing that makes you buy another Honda.

The same lawyer that sued Honda went on to sue Nissan, apparently. The only person benefiting is the lawyer.

The old rule of thumb was that most speedometers read a little fast, and thus protected owners from speeding fines. Speedo inaccuracy is one reason that 6 mph is the usual overspeed allowance.