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post #1 of 15 (permalink) Old 04-08-2007, 10:16 PM Thread Starter
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Default Speedometer error?

Both my handheld GPS and the DOT's robotic radar confirm that my speedometer reads some 2mph high - that is at an indicated 50 I'm actually traveling 48.

Is there some easy way to correct this error, or do I correct it the old fashoned way by calculating a new tire size?

Looks like 225/60R15 would come really close to correcting it, and is a readily available tire size. Does anyone have any experience with this size on their Prius, or with speedometer calibration?

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post #2 of 15 (permalink) Old 04-08-2007, 11:57 PM
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Hmm.. that's odd. The Prius is known for its dead-on accuracy (although yeah there have been some reports such as yours about overreads).

My GPS shows the exact same speed as my Prius. If any calibration is needed, chances are the dealer's the only route, unless you change your tyre size of course

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post #3 of 15 (permalink) Old 04-09-2007, 10:56 AM
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How do you sync the update time of the Prius and the GPS device? If not, the sample being displayed by the car and your GPS could be a second or two apart. Of course, if you're confident that you are maintaining a constant speed (how do you do that?), the sync doesn't matter.

Manual seat-of-the-pants calibrations are never accurate.

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post #4 of 15 (permalink) Old 04-09-2007, 11:04 AM
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My 04 speedometer is almost dead on with my Garmin GPS. In fact I was surprised as to how accurate it was.
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post #5 of 15 (permalink) Old 04-09-2007, 12:30 PM Thread Starter
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Cruise control on flat land - my Garmin unit shows I'm holding a steady speed, as does CALDOT's robotic radar, and they both agree about my speed, only the display in the Prius doesn't agree.

I misread the tire size on the car however, I see it is actually 185/65R15 and not the 195/65R15 I used for the size comparison - so I guess the size to correct my error is actually 215/65R15, not 225/60R15.

Actually I was hopeing somone would enlighten me to some Vulcan death pinch combo on the MFD that would re-calibrate me - I guess not?

- '05 Prius, BC/Driftwood pearl - new Leather! Coastal's EV, auto door lock, and Blue unlock kit. Sigma inside the glass Sirius antenna, rear bumper protector.
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post #6 of 15 (permalink) Old 04-09-2007, 03:43 PM
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From time to time, Radar Trailers are parked along Dallas city streets to remind you of your actual speed. They always read EXACTLY the same as my Prius speedometer.

I have also 'stopwatched' mileposts on the Texas freeways occasionally with the same result = exactly the same. That's close enough for me.

Have you changed your tire and/or wheel size? A smaller diameter of the tire/wheel assembly would make the speedometer read a bit high.

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post #7 of 15 (permalink) Old 04-09-2007, 04:53 PM Thread Starter
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No tire wheel change, at 7500 miles it still wears the OEM rubber.

- '05 Prius, BC/Driftwood pearl - new Leather! Coastal's EV, auto door lock, and Blue unlock kit. Sigma inside the glass Sirius antenna, rear bumper protector.
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post #8 of 15 (permalink) Old 04-09-2007, 10:45 PM
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Quote:
Originally Posted by Rambler
Cruise control on flat land - my Garmin unit shows I'm holding a steady speed, as does CALDOT's robotic radar, and they both agree about my speed, only the display in the Prius doesn't agree.

I misread the tire size on the car however, I see it is actually 185/65R15 and not the 195/65R15 I used for the size comparison - so I guess the size to correct my error is actually 215/65R15, not 225/60R15.

Actually I was hopeing somone would enlighten me to some Vulcan death pinch combo on the MFD that would re-calibrate me - I guess not?
The NAV can and does recalibrate itself by comparing measured miles via the odometer to miles calculated based on GPS coordinates. In the NAV menu, you can speed up the process by having the NAV do a recalibrate due to tire change.

But the speedo/odo cannot be altered by the owner and I don't think even the dealer can make the adjustment. When the fuel tank problem required the combo meter (speedo et al) also be replaced, Toyota had to be informed of the car's mileage so that the replacement combo meter could be set to the correct odo reading.
The combo meter gets its information from the brake system wheel sensors. It might be possible there's a problem with them. I would think a problem with the wheel sensors that could cause such a deviation in the speed/mileage reading would trigger a DTC in the brake system, but it could be subtle enough that the system doesn't think it is really a problem.
For example, a tooth could be missing in the sensor rotor so that a pulse is missing during a wheel turn. The sensors would need to be scoped to determine if there is an irregular pulse train.
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post #9 of 15 (permalink) Old 06-13-2007, 04:51 PM
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Speed sensor errors?

Almost inconceivable... If they're reading close to the speed, they pretty much have to be working correctly. It's unlikely that you'd get light intermittent readings regularly enough to register a 2MPH decrease in speed without some other symptom like twitchy readouts somewhere.

Not impossible, but it's really a stretch.

Much more likely the tire diameter (or setting?!) is slightly off. A 2 MPH error at 50 MPH is 1/25, and at a radius of about 1 foot a change of 1/2" would throw you off that much (circumference is proportional... = 2*PI*R). That's significant but not inconceivable.

Are you sure there's no calibration for the vehicle, maybe accessible through the OBD2?
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post #10 of 15 (permalink) Old 06-13-2007, 06:19 PM
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The display speed is intentionally biased from actual on newer models.
Please refer to following thread.
http://priuschat.com/index.php?showtopic=30709&st=5

Ken@Japan
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