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I am going to get a speedometer accuracy test on my car to fight a ticket and was wondering if the tire pressure will affect the speed reading. I know larger diameter tires cause the speedometer to read slow, so will extra tire pressure affect the diameter of the tire enough to make the speedometer even read slower? Or will it be so insignificant it's not even worth thinking about?

 
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Not only will the tire inflation level effect the reading, so will the tread pattern on your car. When new, the tire has 10-11/32 " of tread, when "worn out" will have 3-4/32" of tread. 1/4" max difference which makes the tire roll .78 (3.1417*.25) of an inch more on a new tire versus a worn out tire, assuming constant tire pressure.

Air pressure can add (or subtract) from the diameter, also. Consider the racing circuit, when they tell you they are adding/subtracting a pound of pressure. They affect a change to the handling of the car by making that wheel(s) roll more/less.

Make sense?

 
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So in a nutshell, higher pressure would make the speedometer read slightly slower since the additional pressure would increase the diameter of the tire.

 
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Correct. Because the diameter is larger you cover more ground.

So if the speedometer reads 50 mph, and you one hour, at exactly that speed, you will have traversed more than 50 miles, thus your real speed would have been more than 50 mph.

 
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speaking of tire pressure, i know the cold reading should be 30psi but that's only convenient if you have an air pump at home or live like less than a mile from the gas station.

so i was wondering, what should the warm/hot tire pressure reading be?

thanks

 
Rep Power: 424

You're arguing a couple fractions of an inch. Even a difference in tire size doesn't make much of a difference unless you're way off. And the circumference of the tire doesn't change much because its not inflated. You still make X revolutions per mile if you were to start rolling the tire out like a roll of tape, regardless of the pressure in it.
Trying to compare that + the 1-2% error in the speedometer (I know mine reads too slow for a fact) and you're better off arguing a tail wind blew the car down the road.Stack up the tolerances if you want..... the actual diameter of the tire vs. spec, the true ratio of the transmission gears, the actual RPM vs. what the gauge said (this is going to give you your true speed), you can check the speedo for error, but I dont' think its going to help. They'll just say you were negligent in keeping the car maintained.
Usually it takes more than a few MPH over to get a ticket for it, so unless you can somehow account for 5 or more MPH through error, in which case the cops will come up with more reasons, I don't think I'd bother.
Get the radar calibration and stuff, see if you can pull dirt out of that.

 
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how fast were you going? generally a lot of places around i've heard that you can go to court and apply for a moving violation and just pay a bigger fine so it doesn't go on to your ins record... and generally at least here it's 14 or under.

 
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What is OEM on the D17? 185/65/15?

What size do you have on your Civic now?