Computer knows real MPH vs Dislpay?

[Zaxcom]

I have been starting to wonder if the known 2 mph error in the speedometer is created by the computer and not a function allowable error.

Why do you ask? Well the car switches modes in the CVT at very specific speeds according to another thread. If those speeds are critical surely an error could not be allowed.

Second reason, the manual says the car will lock the doors at 12mph, but mine lock at 14mph, exactly the same 2 mph error that the speedometer displays vs my GPS.

So maybe the computer does know the exact speed but sends an offset speed to the display since most drivers are used to the error that cars have always had.

[qbee42]

Generally the display error is built in to ensure compliance with the law. Most countries allow a speedometer to read high, but not low. The displayed value is fudged high intentionally.

Tom

[krisirk]

If you think that's bad, try driving a motorcycle. Mine's high by about 10% (Suzuki Vstrom 650). It seems that most Japanese bikes are 5-10% high on the speedo, but the odometer is right on.

Kris

[DaveinOlyWA]

well, i hear all the time that the speedo is set to read a bit high, but then again, everything i own would be the same way.

there is an elementary school on the corner of the housing development where i live and they have had one of those speedometer signs up for the past month. my 2010 is dead on with that sign and is also within one mph with two different GPS systems.

now at higher speeds, i can see a varying reading that is partially caused by elevation changes (sure GPS is supposed to take elevation changes into account, but in my case, i dont think either system i use is high end...heck one is just a cellphone!!) that sometimes maybe as much as 2 mph off (phone does mph in tenths) but the difference does seem to settle maybe a single mph low.

so, i dont know. maybe...

[Mike Dimmick]

On the 2G car there's a diagnostic menu available through the MFD which has a 'Vehicle Signal Check' option. In there, the vehicle's speed is displayed in km/h. The value on the speedometer, at least on my car, is exactly 10% higher. I believe the US rules are different. Reportedly the 3G is set to 5% higher in the UK.

The car needs to know precisely where the motor shafts are in their rotation to control the currents going to the motor windings, so it knows exactly how fast those motors are rotating. It works out the road speed by multiplying that by the rolling circumference of the standard tyres: this is the speed shown in the diagnostic screen. It then adds the fudge factor to produce the displayed speedometer reading.

The actual rolling circumference of the tyre - the distance travelled for one rotation - may well be a bit different if you change to a different brand of tyre, and it changes as the tyres wear down. I'm also not sure whether the programmed value is different for cars that came with 17" wheels versus 15" - the 17" tyre actually has a smaller rolling radius than the 15".

[wvgasguy]

Originally Posted by Mike Dimmick

On the 2G car there's a diagnostic menu available through the MFD which has a 'Vehicle Signal Check' option. In there, the vehicle's speed is displayed in km/h. The value on the speedometer, at least on my car, is exactly 10% higher. I believe the US rules are different. Reportedly the 3G is set to 5% higher in the UK.

The car needs to know precisely where the motor shafts are in their rotation to control the currents going to the motor windings, so it knows exactly how fast those motors are rotating. It works out the road speed by multiplying that by the rolling circumference of the standard tyres: this is the speed shown in the diagnostic screen. It then adds the fudge factor to produce the displayed speedometer reading.

The actual rolling circumference of the tyre - the distance travelled for one rotation - may well be a bit different if you change to a different brand of tyre, and it changes as the tyres wear down. I'm also not sure whether the programmed value is different for cars that came with 17" wheels versus 15" - the 17" tyre actually has a smaller rolling radius than the 15".

Looks like it should be an easy recalibration for Toyota to do for the different tires. I assume they don't.

[qbee42]

Originally Posted by DaveinOlyWA

well, i hear all the time that the speedo is set to read a bit high, but then again, everything i own would be the same way.
<snip>

The speedo on my 2006 is pretty good. It consistently reads one mph high, based on many radar speed signs and my portable gps. I got rid of the OEM tires early on, and the replacements have a slightly different circumference, improving the accuracy of my speedo.

Tom

[greylar]

I passed by 2 different radar checks within an hour of purchasing my 2010 and both indicated that I as going 2 mph slower than my speedo said.

G

[Falcon]

I have installed the Speedo-Healer on my motorcycle I wonder if it can be installed in the Prius also.

[eglmainz]

Originally Posted by Zaxcom

I have been starting to wonder if the known 2 mph error in the speedometer is created by the computer and not a function allowable error.

Why do you ask? Well the car switches modes in the CVT at very specific speeds according to another thread. If those speeds are critical surely an error could not be allowed.

Second reason, the manual says the car will lock the doors at 12mph, but mine lock at 14mph, exactly the same 2 mph error that the speedometer displays vs my GPS.

So maybe the computer does know the exact speed but sends an offset speed to the display since most drivers are used to the error that cars have always had.

Does the estimated 2 mpg difference that is displayed also mean that for every hour I am actually driving, the miles recorded on the Odometer will be 2 miles per hour more than I am actually driving?

If I am stuck an hour each way driving to and from work, that would be 20 miles per week, or just over 1000 extra miles per year for 'error correction' added to my car's mileage (even if not driven that much in actuality?)