speedometer doesn't read the same as speedzone signs

I have a 91 Astro van 4.3 litre engine as I travelled on vacation and through the city I noticed my speedometer reading higher than the speed signs. It doesn't matter what speed I travel my speedometer is 5 kph faster
than the speed zone signs read. I started looking at these speed signs since I went for my e-test and the mechanic said that my speedometer was off. Any suggestions on what could cause this.
Thanks Mike
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Mike & Chris wrote:

Tires.
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Sharon K. Cooke wrote:

he travels. If the tires were undersized, the speed would be off more kph at a higher speed.
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... which is quite hard to believe.
--

-Mike-
snipped-for-privacy@alltel.net
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Mike & Chris wrote:

It's probably normal. In some countries (Germany for one) the speedometer is required by law to be accurate within a certain tolerance, but never read faster than actual speed. (they also have laws there mandating that replacement tires must be the exact size and speed rating as originally specified, with a few exceptions that need to be approved for the particular vehicle and tire combination.) Therefore a speedometer reading a small amount low (and I would consider 5 kph to be "low") I wouldn't worry about it. If you feel the need to compensate for this, then do so, but I wouldn't call appx. 3 MPH anything to worry about.
nate
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The original tires were 215/75/15 I changed them to 215/70/15

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Mike,
The tires you now have are smaller in overall diameter than the originals. That means they will turn more revolutions per mile/km, which will make the speedometer read higher than you actual speed.
To my way of thinking I'd rather have my speedometer read higher than my actual speed - gives you a little "margin".
Regards, Bill Bowen Sacramento, CA USA

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Sounds good thanks

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Mike & Chris wrote:

Which of course means your original posting was incorrect about it being off 2.5kmh at different speeds. Drive carefully.
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No it's off 5 km in Canada and 5 miles in the States I have a digital dash. This year was the first time in all the years of emission testing that the told me that my speedometer was off and I always been bring it to the same place. That why when saw a speedzone sign I would pay more attention to what the was posted. Mike

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Mike & Chris wrote:

With all due respect, your numbers don't make a bit of sense. If your speedometer is off, it's off. You just don't know how to measure it. You say it's off 5km in Canada and that is 3.1 miles. Then you say it's off 5 miles in the states. That's a big difference. Neither one of those numbers means anything unless one also states the MPH at the time. I give up as you will never figure it out.
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