Speedometer not accurate?

[peterbrown77]

Wildly inaccurate speedo - and I hate it. I'm always doing the math in my head, trying to figure out at what indicated speed am I pushing the limit of what Johnny Law will tolerate. I think it's just inexcusable - my Toyota is dead nuts to the GPS as is my Explorer. Yet my most expensive car can't get the most basic instrument correct. It's total BS. Makes you wonder if the tach is wrong too.

[jb92563]

Yes, all of the speedos read Higher than actual depending on your tire diameter/size.
~3 mph faster.

It helps keep your speed down a bit and many times I have been glad it has as I cruise by an unseen CHP a little on the high side.
(Radar Detector ordered before its too late)

Your allowed 10% over actual by most police unless they are feeling ornery or are just plain bored.

They are generally not amused when you mention that due to the rotation of the earth you are already traveling ~300mph plus or minus your speed depending on direction.

If they offer to write you up for 300mph, accept, so that when they show up in court and cannot demonstrate to the Judge how they added or subtracted your cars speed vector from the 300, it will get thrown out.
Plus the judge will want a demo in your 300 mph Porsche
and you can frame the ticket and put it up in the garage as a conversation piece.

[Crono0001]

Originally Posted by jb92563

Yes, all of the speedos read Higher than actual depending on your tire diameter/size.
~3 mph faster.

It helps keep your speed down a bit and many times I have been glad it has as I cruise by an unseen CHP a little on the high side.
(Radar Detector ordered before its too late)

Your allowed 10% over actual by most police unless they are feeling ornery or are just plain bored.

They are generally not amused when you mention that due to the rotation of the earth you are already traveling ~300mph plus or minus your speed depending on direction.

If they offer to write you up for 300mph, accept, so that when they show up in court and cannot demonstrate to the Judge how they added or subtracted your cars speed vector from the 300, it will get thrown out.
Plus the judge will want a demo in your 300 mph Porsche
and you can frame the ticket and put it up in the garage as a conversation piece.

You'd lose the entire court room if you tried to go into centripetal force and angular mechanics

[Bruce Wayne]

in Europe, manufacturers must not allow a speedometer to under-read and the maximum permissable overread is by a calculation (this was discussed on a professional pilot forum i use).

by law in the UK we are allowed 110% +4

yes, i know it's europe, but the cars are designed and built to be sold across continents

for those interested in the legalities it's here: http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:1975:196:0001:0005:EN:PDF

honda got drawn up on the over-read in US, here: Odo Uh-Oh: Honda extending warranties on 6 million cars

[bar10dah]

Originally Posted by Bruce Wayne

by law in the UK we are allowed 110% +4

I imagine 110% and +4 can only really truly happen at one speed. You can have your speedo read 110% faster, or you can have it read 4 MPH faster. But only at one specific crossover point, will both be true. Am I thinking about this correctly?