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Speedometer accuracy

My 2005 Camry LE reads 5 MPH higher than my portable GPS calculated speed. A check of another Toyota vehicle from the same dealer read higher also. My 2007 Honda agreed exactly with the GPS. This disparity affects warranties, maintenence, and insurance. Questions:

1. Are there legal requirements on speedometer accuracy? ( Toyota USA sez a 10% variance is accepatable)

2. Can these speedometers be adjusted to within 1 % accuracy?

You can’t rely on that method. A stopwatch and highway mile markers are much better. 60 MPH covers a mile in 60 seconds. If you take 65 seconds to do a mile, you’re doing closer to 55. There aren’t any legal requirements. They aren’t adjustable.

You generally can’t trust the instant speed reading on a GPS because they’re only accurate to about 14 feet or so tops, so just the random variations in readings can vary enough to significantly skew an instant speed reading. A more accurate way to check it is by driving a long road trip and comparing the GPS odometer reading to the car’s odometer reading, but don’t leave the GPS on and sitting still for too long because it will record those up to 24-ft variations as motion and add it onto the GPS odometer.

Of course, the stop watch and mile marker method is only as accurate as the mile markers and you’d be surprised at how often they’re off! Some states have “speedometer check stations” every so often where they have 5 miles or so where the mile posts are certified to be correctly spaced. The only real way to know how accurate your speedometer is for sure is to correlate it with several different methods.

Just FYI, the 5mph high is pretty typical (at highway speeds-- the skew will be a percentage not a set number of MPH’s). They usually err on the side of being too high supposedly for safety reasons and shame on you for thinking they might do that to make you think you’re getting better gas mileage or to run the warranty out sooner.

I can’t speak for Toyota but most of the factory manuals I’ve been through on other makes are nowhere near 10% off. That sounds like brush-off talk to me.
Whale of a method to get you through the warranty period quicker!

Most of the ones I’ve seen allow a tolerance of about 1 MPH +/- at 30 MPH on up to about 2 MPH +/- at 60 MPH so you’ve got about a 3% swing each way at most.

Wonder if a local cop or state trooper would be willing to use their radar gun on your car for comparison purposes?

Radar Fun!

I have had more than $80 worth of fun with my Bushnell Radar Gun. You can get the Velocity or Speedster II model for $80 to $85 and free shipping at Optics Planet, online. They are virtually the same unit. They list them as accurate within one MPH. You just need to be in a fairly straight line with what you are radaring, not at a side angle.

I bought mine to make a case against a local maniac speeder commuting by my house every day at excessive speed where people walk and ride bikes. We have checked all of our cars with it, also.

Anyone watching you will want to try it! People fight over it. It’s fun. You can use it inside your car, too, if you point at a sign or someting next to the road.

I am not affiliated with Bushnell or Optics Planet in any way, just a customer.
Here’s a link if interesred:

in the uk speedos can read upto 10% faster,but must not read slower!!!

Conspiracy theory ; how to get people to slow down ?..have the auto industry set speedometers to lie !