Speedometer is off by 2 mph.

[Knucklehead]

Help please. Stock tires, and everything else. Tires are inflated to 35 psi. At any speed between 20 mph and 90 mph the speedometer reads 2 mph faster than I'm actually going. I have checked it with GPS, radar "your speed is" signs, and by measuring with the mileposts at constant speed. Dealership says everything is as it should be. Part of the puzzle is that the error is consistently the same at all speeds.

If the speedlimit is 65 mph, then I want to be going 65.4999 mph. I'm nervous having to always guess on half way between the tick marks.

Thanks for any suggestions
Knucklehead

[pt_tom]

Frankly *I* wouldn't be concerned about 2mph... [:I]
99% of cars are probably a mile or two (or more) off from actual speed.

But if you're determined, then it's time to have the speedometer calibrated. If you're around a large city there probably a shop near that can do it, or search Google on "speedometer calibration".

[therents]

Hi All:

I have noticed the same thing with my 2003. I have also checked it with my GPS unit. I'm wondering if the odometer is linked to this and if it is it probably will get us to 70,000 warranty quicker then it is supposed to. That might be one up for DC. [xx(]

Neil

[quicksilverdon]

Simple suggestion - open up the instrument cluster, bend the speedometer needle a tad.
99% of the time police grant at least 5 MPH over speed limit before thay bother you.

[Jacked Up PT]

i have over size tires 205-65-15 can you tell me how far off my speedometer is thank you.

[SteelBlueGT]

That is well within the tolerance for error and I would be happy if all my cars are within 2 mph at all measured speeds. Buy another tire and see if you get more or less accurate. You do know that different tires of the same size have slightly different diameters??

[Knucklehead]

Guess I'm not very tolerant. I do know that different tires have different diameters and circumferences (after deflection) I greatly prefer Michelin tires and the Pilot Sport A/S will get me 1.01 MPH closer to accuracy.
I will be putting Michelins on in about 25K miles when I wear out the Good4ayear tires. That will be sometime early next summer. Not driving the PT full time.

Jacked Up PT: With out knowing the brand and model of the tire it is difficult to give an accurate answer. For illustration: if you have Bridgestone Potenza's in that size; Assuming your speedometer was accurate before the tire change; You are going 46.1 when the speedo says 45. You are going 56.4 when the speedo says 55. And you are going 66.6 when the speedo says 65. At 75 indicated you will be doing 76. In theory, any way. If you only go in a straight line. Cornering, weight of car, and tire pressure will change the deflection of the tread and change the actual distance traveled per revolution of the tire.

OH well. Guess I'll just have to live with it until I tear apart the dash and bend the needle.
Thanks to all
Knucklehead

[PTGT_Boy]

You have to take in account of margin of error/standard deviations. I see 3 factors playing into the 2mph error.

1. Speedometer has its own margin of error.
2. GPS system has its own margin of error (although not by much).
3. You have a set new tires with differnt ratios (I'm not sure what PT model you have).

Of course, 1+2+3 factors don't = 6. There is complicated formulas to calculate those margin of errors, and I'm not a mathmatics wiz (I forgot how to do it).

Bottom line, you might be well within the normal operations of your vehicle. Just don't go fast in front of a cop [].

[Grouch]

There should be no difference from one tire to another of the same size from different brands.

http://www.dakota-truck.net/TIRECALC/tirecalc.html

[Konkeler]

Thats it, just 2mph? I can't beleive you're that hung up on it. I've driven a car without a Speedometer before, just keep up with the traffic.