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Your tires would have be WAY off in size to give you that much error. If you are reading 100 KMH and actually doing 85 KMH it goes something like this. Your car probably came new with 205/70-15" tires or something close. They turn 792 revolutions per mile and your car is calibrated for that. Just check tire-size sticker on the door pillar and compare to what you are actually using. To get the kind of error you are reporting JUST from the wrong tires - you'd have tires spinning 931 turns per mile instead of 792. That would be pretty hard to do and I think you'd notice if your tires are that small? A 185/60 - 14" tire almost gets you off by that much at 914 turns per mile. Unless you took some 13" tires off a Subaru and bolted them onto your Buick - I think your problem is elsewhere. Without knowing the vehicle repair history - I'd suspect the wrong trans. drive gears, vehicle speed sensor, axle gears, etc. If you owned it when it was OK, and it all-of-a-sudden got wacky - then you've got a better handle on this than we outsiders do. One example is my Dodge W250 diesel truck. It had always been off since I got it used and read 65 MPH on the speedo when actually going 56 MPH. Odometer was off the same. Speedo is electric. I finally discovered the wrong speedo-drive gearbox. It was probably changed long before I owned the truck. I bought a new gearbox with the correct ratio for $50 and now it's fine. For $65, I could of used an electronic corrector instead.
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