Accuracy of Speedometer for TCH

MileHighCity · 08-30-2009, 11:31 PM

Read on another forum where the speedometer is not accurate and normally reads over actual speed. Used my Garmin today and validated that at 60mpg on the car that I was really only doing 58mpg per the GPS. That’s a 3.33% error … fairly significant. If the speedometer is wrong and used in the calculations that show the MPG then it would be wrong as well and overstated. 40mpg with a 3.33% correction would really mean your average dropped to 38.67mpg. Have others noticed this error? Can the dealer reset the speedo in the car to make it more accurate?

ken1784 · 08-31-2009, 12:00 AM

Quote:

Originally Posted by MileHighCity

Have others noticed this error?

The speedometer reading is intentionally biased to meet the UN ECE Regulation.
UNECE - United Nations Economic Commission for Europe

The MPG number is based on distance traveled(odometer/trip meter) and fuel usage, then the number is nothing related to the speedometer reading.

The odometer/trip meter reading is completely another story, and the number is relatively accurate.

Quote:

Can the dealer reset the speedo in the car to make it more accurate?

No.

Ken@Japan

fuzzy1 · 08-31-2009, 12:06 AM

MPG is calculated from the odometer, not the speedometer. In general, they don't have the same error.

From other discussions (Prius & elsewhere), it seems that the speedometer display is intentionally biased high for reasons of industry standards, liability, and legal requirements of some jurisdictions. The speed that the ECU passes on to the trip average speed display, and to ScanGauge, is not biased.

The odometer is likely not similarly biased, especially after one manufacturer suffered a class action lawsuit over warranties vs. biased odometers.

On my old analog cars, the odos and speedos clearly did not have the same error factors.

Airbalancer · 08-31-2009, 08:21 AM

Why do you think your GPS is correct

Do you know that GPS are not like the military ones that are correct to probably one mm now

qbee42 · 08-31-2009, 08:31 AM

Quote:

Originally Posted by Airbalancer

Why do you think your GPS is correct

Do you know that GPS are not like the military ones that are correct to probably one mm now

At automotive speeds commercial GPS systems are pretty good. You may see some bounding where the speed drops and jumps back up if there is a loss of signal, but the overall speed reading will be good.

Tom

MileHighCity · 08-31-2009, 10:29 PM

Thanks for the comments. Guess I incorrectly assumed that the sensor that provided the speed signal was also the one that provide the odometer reading (or course a computer does the calculations). This weekend driving over 550 miles to Kansas City so will be able to check the odometer against the GPS to see how they compare. I also assumed that the GPS was fairly accurate as it matched identically in my Diesel Dodge Pickup, Honda Accord, and RAV 4. Will probably order the scan gauge this evening, but won't have in time for the drive to Kansas City. Darn

Again thanks for the comments. ...

MileHighCity · 09-09-2009, 12:14 AM

Made the trip from Denver to Kansas City and return over the past 4 days. Not so sure of the accuracy of the GPS now, as it seemed the GPS had a wider variance at slower speeds (at least 2 mph) and at higher speeds may have only been about 1 mph off. Checking the trip meter on both and this is even stranger – the GPS would show 505 miles when the car would show 500 (each time when I filled the GPS read a little higher than the trip meter in the car)!! You would think it would be the other way. The GPS is a Garmin 200W. For the mileage I drove 77 – 78 MPH in Colorado and 73-74 in Kansas. First fill up at 329.6 (trip meter) the gauge in the car read 37.5 – computed actual was 35.91. Second fill up at 486.3 (trip meter) the gauge in the car read 37.0 – computed actual was 36.29. Third fill up 509.6 (trip meter) the gauge in the car read 36.3 – computed actual was 36.17. Haven’t filled up over the last 90 miles Limon to Denver but so far the gauge in the car reads 39.7 – so could possibly have when computed a little over 38 MPG. Pretty happy with these figures as this is a fairly large and comfortable car to drive – and to get that kind of mileage at approx 75MPH is great!

BTW – I have 40 pounds pressure in the tires. If you’d like to hear, also had time to play with the ECO – both on and off to see the difference.

acdii · 09-10-2009, 03:58 PM

Yes, the speedo's are off. I have passed many road side "your Speed Is" signs, and everyone has been off by 3 MPH. There is a 25 MPH sign on the road my work is on, and it shows 25 when I am doing 28. All my cars, except for one, show the 3 mile difference, and the one that showed the correct speed was an old Police Interceptor crown vic I had with the calibrated speedometer. The MPG computers are NEVER accurate, no matter what car you are in. I found they are always off on the high side, whether by design or just that's how they are is unknown, but I have yet encountered one that was dead on.

08-30-2009, 11:31 PM	#1
MileHighCity Junior Member Join Date: Aug 2009 Location: Aurora, CO Posts: 10 My Car: Other Hybrid Model: N/A Package: N/A Thanks: 0 Thanked 0 Times in 0 Posts Friends: 0	Accuracy of Speedometer for TCH Read on another forum where the speedometer is not accurate and normally reads over actual speed. Used my Garmin today and validated that at 60mpg on the car that I was really only doing 58mpg per the GPS. That’s a 3.33% error … fairly significant. If the speedometer is wrong and used in the calculations that show the MPG then it would be wrong as well and overstated. 40mpg with a 3.33% correction would really mean your average dropped to 38.67mpg. Have others noticed this error? Can the dealer reset the speedo in the car to make it more accurate?

08-31-2009, 12:06 AM	#3
fuzzy1 Senior Member Join Date: Feb 2009 Location: Western Washington Posts: 1,006 My Car: 2010 Prius Model: II Package: No Package Thanks: 22 Thanked 97 Times in 81 Posts Friends: 0	Re: Accuracy of Speedometer for TCH MPG is calculated from the odometer, not the speedometer. In general, they don't have the same error. From other discussions (Prius & elsewhere), it seems that the speedometer display is intentionally biased high for reasons of industry standards, liability, and legal requirements of some jurisdictions. The speed that the ECU passes on to the trip average speed display, and to ScanGauge, is not biased. The odometer is likely not similarly biased, especially after one manufacturer suffered a class action lawsuit over warranties vs. biased odometers. On my old analog cars, the odos and speedos clearly did not have the same error factors.

08-31-2009, 08:21 AM	#4
Airbalancer Senior Member Join Date: Aug 2009 Location: Cobourg, On, Canada Posts: 225 My Car: 2010 Prius Model: N/A Package: Premium w/ Solar Cells (Canada) Thanks: 3 Thanked 12 Times in 12 Posts Friends: 0	Re: Accuracy of Speedometer for TCH Why do you think your GPS is correct Do you know that GPS are not like the military ones that are correct to probably one mm now

08-31-2009, 10:29 PM	#6
MileHighCity Junior Member Join Date: Aug 2009 Location: Aurora, CO Posts: 10 My Car: Other Hybrid Model: N/A Package: N/A Thanks: 0 Thanked 0 Times in 0 Posts Friends: 0	Re: Accuracy of Speedometer for TCH Thanks for the comments. Guess I incorrectly assumed that the sensor that provided the speed signal was also the one that provide the odometer reading (or course a computer does the calculations). This weekend driving over 550 miles to Kansas City so will be able to check the odometer against the GPS to see how they compare. I also assumed that the GPS was fairly accurate as it matched identically in my Diesel Dodge Pickup, Honda Accord, and RAV 4. Will probably order the scan gauge this evening, but won't have in time for the drive to Kansas City. Darn Again thanks for the comments. ...

09-09-2009, 12:14 AM	#7
MileHighCity Junior Member Join Date: Aug 2009 Location: Aurora, CO Posts: 10 My Car: Other Hybrid Model: N/A Package: N/A Thanks: 0 Thanked 0 Times in 0 Posts Friends: 0	Re: Accuracy of Speedometer for TCH Made the trip from Denver to Kansas City and return over the past 4 days. Not so sure of the accuracy of the GPS now, as it seemed the GPS had a wider variance at slower speeds (at least 2 mph) and at higher speeds may have only been about 1 mph off. Checking the trip meter on both and this is even stranger – the GPS would show 505 miles when the car would show 500 (each time when I filled the GPS read a little higher than the trip meter in the car)!! You would think it would be the other way. The GPS is a Garmin 200W. For the mileage I drove 77 – 78 MPH in Colorado and 73-74 in Kansas. First fill up at 329.6 (trip meter) the gauge in the car read 37.5 – computed actual was 35.91. Second fill up at 486.3 (trip meter) the gauge in the car read 37.0 – computed actual was 36.29. Third fill up 509.6 (trip meter) the gauge in the car read 36.3 – computed actual was 36.17. Haven’t filled up over the last 90 miles Limon to Denver but so far the gauge in the car reads 39.7 – so could possibly have when computed a little over 38 MPG. Pretty happy with these figures as this is a fairly large and comfortable car to drive – and to get that kind of mileage at approx 75MPH is great! BTW – I have 40 pounds pressure in the tires. If you’d like to hear, also had time to play with the ECO – both on and off to see the difference.

09-10-2009, 03:58 PM	#8
acdii Senior Member Join Date: Apr 2007 Posts: 591 My Car: Model: Package: Thanks: 12 Thanked 16 Times in 11 Posts Friends: 0	Re: Accuracy of Speedometer for TCH Yes, the speedo's are off. I have passed many road side "your Speed Is" signs, and everyone has been off by 3 MPH. There is a 25 MPH sign on the road my work is on, and it shows 25 when I am doing 28. All my cars, except for one, show the 3 mile difference, and the one that showed the correct speed was an old Police Interceptor crown vic I had with the calibrated speedometer. The MPG computers are NEVER accurate, no matter what car you are in. I found they are always off on the high side, whether by design or just that's how they are is unknown, but I have yet encountered one that was dead on.

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