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

Yes the speedo was out 10% at 60 MPH. The UK law although part of the EU allows a much higher figure than this I believe it can be 10% + 4 MPH fast but no under reading, but I will check and post tomorrow.

John (Britprius)

 

That's 'cause Toyota likes to BS.

As mentioned by other responders, that's due to legislation, basically the speedometer is mandated to err slightly on the side that would get you to go a bit slower. But there's no direct connection between speedometer and odometer readings, ie: the speedo having an error doesn't mean there's a corresponding error in the odo.

Assuming you mean the speedo error, I highly doubt it. Would give you really good odds it's not doable. And to what end? If you really have to adhere to precisely to the speed limit, just go 52 in a 50 zone and bob's-your-uncle, LOL.

 

Wow! At 60 mph, it could indicate as high as 70 mph.

 

You're right. I was trying to keep things simple and used the term engine to mean the whole of the arrangement that turns energy into motion. It seemed reasonable in the context of the discussion, but I should have known better.
The pick-up for the speedometer is normally on a rotating component on the output side of the engine; that rotating component being downstream of any variable gearing arrangement so that it has a rotational speed that bears a fixed relationship to the rotational speed of a road wheel axle. The essence of my argument was that though the ratio of the rotational speeds of the pick up component and the wheel axle are constant, this does not guarantee a constant relationship between the rotational speed of the pick up component and the working peripheral diameter of the tyre.

 

Not really. Try adding the functionality for auto/on/off headlights for example. The sensor is on the dash. It already auto dims the dashboard but to "create an offset" for headlights is $2K in computer mods for Toyota to do it.

If, as people say, the miles the car travels and the computation on the mpg have nothing to do with the car's miles per hour, it would require separate systems, separate programming and some added expense to separate those functions.

As for the 205 tires causing the Prius to report accurate speeds, just noting what the police radar's all report. If someone wants to "argue" with the police radar, go right ahead.

 

UK regulations for speedo's:-
The Motor Vehicles (Approval) Regulations 2001[12] permits single vehicles to be approved. As with the UNECE regulation and the EC Directives, the speedometer must never show an indicated speed less than the actual speed. However it differs slightly from them in specifying that for all actual speeds between 25 mph and 70 mph (or the vehicles' maximum speed if it is lower than this), the indicated speed must not exceed 110% of the actual speed, plus 6.25 mph.
For example, if the vehicle is actually travelling at 50 mph, the speedometer must not show more than 61.25 mph or less than 50 mph.
or at 70 mph the speedometer could show 83.25 mph!!!

Australian regulations follow the UNECE regulations for mass produced vehicles from 1 July 2006:-

Reading must not be under but can be 110% + 6km/h over.

John (Britprius)

 

No one is saying that a larger diameter tire will not cause the speedometer to read slower and thus become more accurate. However, we are saying that if you go with a larger diameter tire, your odometer will read slower as well and become less accurate. So using your method of measurement, a police radar, I would agree that the speedometer change will occur but you are failing to make additional measurements to test odometer reading changes and ECU speed readings vs. dash displayed speed readings.

As for the costs to create an offset, I don't see where you assume it has to be expensive to do. All that is being changed is the dash displayed speed. The ECU is always reading without the offset. Have you ever looked inside the programming of a vehicle's ECU? There is a lot of complexity and so many parameters that can be adjusted. Back when I would program Corvettes and Camaros for performance there were many parameters like tire size offsets to timing retard decay times that could be changed.

 

Thanks John. That's amazing that they allow such a wide range.

 

Updated my thread to include Australia.

John (Britprius)

 

I should have mentioned this in my post but a Garmin or other portable GPS doesn't necessarily show an accurate speed or match the speeds shown by other portable GPS devices. I once conducted "the War of the GPS Devices" while my wife was with me - much to her displeasure. Other than the constant barrage of notifications from all the devices, the most interesting aspect were the conflicting routes. The portables generally showed speeds within one or two MPH of each other.

 

Don't really care more than failing to research. Take your word for it that Prius has created an odd system where miles per hour is not connected to miles driven or miles per gallon.

Don't know how much more expensive it is to provide separate sensors, programming etc. to separate speed from odometer from mpg but it certainly costs something extra. The "expensive" was an actual example of how adding auto/on/off lights was about $2,000 from Toyota because Toyota did not have a way to tie in existing daylight sensing to the controlling the lights and had to add modules to the computer system. Likely much less to do it from the beginning.

 

The sensors are the same. The odometer and the speedometer being different is a matter of calibration. Your speed sensors don't output MPH or miles. It outputs a voltage. The odometer and the speedometer convert that voltage to a number that is usable for the mileage and speed.

 

Get a Scangauge, it will tell you the true speed which the ECU is used to calculate the odometer.

 

If the sensors were the same then a change in output (from the change in wheel size) which does affect the speedometer would affect the odometer and the mpg.

But it does not, according to the comments above, so it must be using a different input, another sensor.

 

Wheel size does not matter, only the rolling circumference of the tire.

 

I believe that a tire size (circumference) change will in fact change all of those. The ECU has to assume a rolling circumference because it can't measure actual moving vehicle speed or distance. It makes no sense to use separate sensors when you can use software to display/record what you want from a single sensor.

 

Speed and distance aren't determined by sensor voltage output. The sensor put out a certain number of pulses per revolution, the computer counts the pulses and determines speed from the pulse rate and distance from the pulse count.

 

Who said it didn't?

I stated quite simply:
1. A significant change in tire diameter will affect mpg, odometer and speedometer.
2. The odometer is very accurate with OE sized tires.
3. The displayed mph is off by 1-1.5mph too fast with OE sized tires.
4. The ECU reads the correct speed with OE sized tires.

 

I have seen threads on Prius Chat that state that the Prius mpg calcs are also reading high as is the speedometer. If that is the case then the speedometer correction gained by 205 tires should also provide the same correction to mpg.

 

I think this horse has been dead for about a week.....