Garmin doesn't lie Prius v does.....

Discussion in 'Prius v Main Forum' started by Northernguy, Apr 10, 2013.

  1. Northernguy

    Northernguy Junior Member

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    I'm trying to figure out why I am always getting almost three miles per gallon less when I compute the mileage myself to what the Prius v's trip computer is telling me. So the other day I took my garmin out on the highway, and discovered that it was telling me that I was going 65 mph when my Prius was reading 67 mph. I'm using original equipment wheels and tires so I assume the speedometer is just off. Anyone know if this can be corrected at the Toyota dealership?
     
  2. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    If one in a hundred speedometers are reading a lower speed than the driver is going, Toyota will be sued by drivers who got speeding tickets.

    If all one hundred read high, no one is getting tickets and giving Toyota bad press.

    Welcome to the litigious USA.

    I suggest Fuelly.
     
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  3. DAVEJAN

    DAVEJAN Junior Member

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    My understanding is that the law requires the speedo to be accurate at 30mph and never underread
     
  4. Sagitar

    Sagitar Junior Member

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    European legislation requires that a speedometer shall never show an indicated speed that is lower than actual speed of the vehicle. I would be surprised if the U.S. regulations were different in principle. The following is a direct quote from the European regulations.

    5.3. The speed indicated shall not be less than the true speed of the vehicle. At the test speeds specified in paragraph 5.2.5 above, there shall be the following relationship between the speed displayed (V1) and the true speed (V2).
    0 ≤ (V1 – V2) ≤ 0.1 V2 + 4 km/h

    So the tolerance can be quite high at lower speeds but tends towards a maximum of just over 10% at higher speeds. The difference set out by the OP would be well towards the centre of the allowed tolerance range and I doubt whether any manufacturer would consider it to be in need of correction,

    For a typical speedo to be absolutely accurate over a reasonably period is impossible because it depends upon the relationship between the rotational speed of the engine and the working peripheral diameter of the tyre. That relationship varies for a number of reasons such as - physical size of tyre fitted; wear; inflation pressure; ambient temperature; speed; axle loading etc.
     
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  5. The Electric Me

    The Electric Me Go Speed Go!

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    Who said anything about suing anybody?
    I think the OP just wants an accurate speedometer.

    I'd just call and ask the dealership what might be able to be done.

    2 mph, is not a big deal and you're probably going to get the "it falls within acceptable spec's" answer, but you can always ask.

    I'd be more concerned about whether your odometer is accurate. Is it reading you've gone further than you've actually gone?

    If you're reading that you've driven more miles than you've actually driven, that can affect length of warranty coverage, and even eventual resale value.

    I've used my Garmin to compare against my Speedo (Honda Fit) and for whatever reason it seems dead accurate. I'm surprised how accurate the Garmin seems to be. However, I don't think a GPS navigation unit is necessarily infallible.

    Call the dealership, and let us know what they say. I'd be curious.
     
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  6. mmmodem

    mmmodem Senior Taste Tester

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    Also consider your new tires with 11/32nds of tread when new could be worn to the recommended minimum of 2/32nds at replacement. That's a diameter difference of 0.56 inches. Which equates to a 98% smaller wheel which means in 50k miles or so , your speedometer will read 65.5 mph when going 65 mph. I doubt you'll notice the difference.
     
  7. schmuly

    schmuly Member

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    I used our Garmin to check those roadside speedo checks and it was out 2/10's of a mile at the end of the 5 miles. Now I don't know witch one it correct??? I might try two Garmins side by side to see if they read the same!
     
  8. GasperG

    GasperG Senior Member

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    Europe version of Prius and Prius + is even more off, doing 130 km/h you are actually going 123 km/h (90 km/h = 85 km/h ).
    The speed reading is not to be mistaken for miles traveled, the odo/trip on my Prius is completely accurate with GPS (less than 0.5 % difference) on 15" tires.
     
  9. BTH

    BTH Junior Member

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    I bought a 2012 Prius v model 3 last February, I have noticed that milage reported by the car is about 2 miles per gallon more that when I compute the milage by the tank full of gas. However, I would trust the garmin's speed over the car's because of way GPS computes speed. In the underlaying system, speeds are computed with position velocity vectors which don't depend on the car dimensional mechanics.


    iPad ? HD
     
  10. iClaudius

    iClaudius Active Member

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    You can correct it when you get tires. Get 205 vs. 195's and your speed and mileage will be accurate.
     
  11. F8L

    F8L Protecting Habitat & AG Lands

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    But then your odometer will be off. ;)
     
  12. F8L

    F8L Protecting Habitat & AG Lands

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    I've tested the GenII and two GenIIIs and in each case the speedometer was off by approx. 1.5-2mph. The odometers are very accurate such that it is difficult to tell where the very small error stems from. Was it the GPS or the car? That's how accurate it is.

    I've also played with different sized tires and watched the speedo error go away but then the odometer is off. Toyota just programs the speedometer display to read high. The car's computer knows your real speed. You can observe this by monitoring speed through an OBDII device.
     
  13. Offline

    Offline Active Member

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    F8L is providing accurate information that matches my experiences.

    Take your Prius on a highway with mile markers and check the accuracy of your odometer .... maybe on a trip where you are driving a significant distance on an Interstate highway. I've done this in my Lexus and found that my odometer was perfectly accurate even though the speedometer read a couple of mph slow at highways speeds. My average mph display in the instrument cluster trip computer also read the true speed instead of the speed shown by the speedometer. The legally required "buffer" dialed into speedometers is on purpose and does not have anything to do what is shown by odometers. Of course you can screw it all up by using incorrect tire sizes.
     
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  14. iClaudius

    iClaudius Active Member

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    Nope. If your speed is off, as it is with 195's, then your odometer is also off which is why the mileage calcs are off with 195's along with the speed.

    205 tires correct the problem. Your Prius speed will be accurate. Drive past any police radar/speed sign to check. Your mileage won't take that much of a hit. But your speed and mileage will be accurate.
     
  15. F8L

    F8L Protecting Habitat & AG Lands

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    Your conclusion is not in agreement with every other PC member who has measured their odometer vs. GPS and mile markers. It also disagrees with measurements taken at the ECU via Torque and Scangauge II.

    I suggest doing some actual testing with one or more of the above mentioned devices. In the case of GPS, ensure you have a solid signal and accuracy is at 15' or less. :)
     
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  16. xs650

    xs650 Senior Member

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    It has nothing to do with rotational speed of the engine. If you don't believe that put you car in neutral at highway speed and let the engine drop to idle speed. Notice that the speedometer reading only changes as the car speed changes, not as the engine speed changes.
     
  17. iClaudius

    iClaudius Active Member

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    I suppose Toyota could build a car where the speedometer, odometer and trip computer were independent of each other. Seems like it would be costly to duplicate functions that way from design to build to maintenance.

    As for "testing" it, I've only tested the 205 tires and the fact that the police radar and the Prius speedometer now agree. If you say that it has not had similar effect on mileage and odometer, I'll take your word for it.
     
  18. Britprius

    Britprius Senior Member

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    I have found in my UK gen2 Prius the speedo was out 10% "reading high" at 30 mph, so was the MFD fuel consumption display telling me I had done 2 to 3 more MPG than calculated.

    After fitting 205/55/16 tyres against the OEM 195/55/16's the speedo is now more accurate "still a little fast" but the MPG readout is now always very close to calculated after adjusting my oddo reading by + 2.8% "the increase in rolling circumference of the tyres.

    The distance travelled on the oddo under reads by 2.8% according to our motorway distance markers. So in short I agree with F8L.

    John (Britprius)
     
  19. skwcrj

    skwcrj Member

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    It's very easy when it is just a computer. You can create an offset on the speedometer.

    Assuming OEM tires...

    I have a Garmin GPS that shows speed to the tenth of a mph. Assuming that you are right and the speedo is off by a % with , the difference in the indicated vs actual (delta) would increase at higher speeds. It is actually off, by a fixed amount (about +1 to 1.4 mph) for all speeds. The speedo has an "offset" not a percent error. I've driven 500 miles with two GPS's on and the difference in total distance was less than 0.2 miles. Mostly due to GPS errors while driving around curvy mountain roads.

    If you change the tire diameter, then you introduce a % difference to the speedo and the odometer.

    I'm pretty sure that we have had this argument before....
     
  20. skwcrj

    skwcrj Member

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    Was it 10% off at 60 mph? ie. indicating 66 mph when the gps says 60 mph.
     
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