Speedo/odo calibrated for OE tire size?

Each tire size has a revolutions/mile rating. You will find that each of these different wheel diameters is compensated by the tire's aspect ratio so that each of them have approximately the same revs/mile.

JeffD

 

There is no practical way to recalibrate the speedometer or odometer unless you are capable of dissecting the factory firmware that generates the ODO and Speedo readings, correctly interpreting how it works, and modifying it.

The speedometer and odometer are calibrated differently. The odometer reads quite accurately and the speedometer is optimisitic.

 

Those are the advantages and disadvantages, although I would also say the wheels/tires you have now are 'free' while wheels/tires you don't own yet cost money.

 

A larger diameter wheel with a constant diameter wheel/tyre combo means the larger wheel's tyre will have shorter sidewalls. This means a rougher ride (usually) but less give when side loaded (cornering) hence "better" handling.

A larger diameter wheel also usually means a heavier wheel, which also means a rougher ride and potentially lower fuel economy. The wheel/tyre combo will be heavier, as the wheel is heavier and usually the tyre is also heavier (sidewalls have to be thicker to work properly).

 

Interestingly, the OE 215/45/17 tire is smaller in diameter than the OE 195/65/15 tire. So using that tire size on a car originally equipped with 15s may cause an even larger speedo error. The error is programmed to be about 1-1.5mph optimistic from the factory.

 

So, are folks here thinking there is no adjustment to the odometer to compensate for models that come stock with 215/45R17? I'm in that boat, have been wondering about it. FWIW, the 17" OEM does about 1.5% more revolutions for any given distance.

 

Actually that I am not sure of. Toyota can obviously change the calibration during manufacturing/programming. Do they do it? Who knows.

I've yet to drive around in a Model Five and monitor the speedo error like I have done in my other three Prii. Reports here on PC seem to indicate the Five has the same speedo error which makes me think the Five is calibrated for the 17s. I just don't know.

I do know that the 215/45/17 (24.7") specs at 844 revs/mile on average and the 195/65/15 (25") is 829 revs/mile on average.

 

I'll just have to team up with someone with 15", and go for a measured drive.

 

844 and 829 are close enough to be within normal variances. Did you get those numbers for the make of tyre on the car? Different tyre manufacturers tyres differ in diameter, even for the same size. In fact -some- manufacturers have more than one tyre in a given size and model of tyre and the diameter is slightly different.

I think it's safe to say the Prius has -one- "calibration" used for all wheel/tyre combinations used. It may change between the "standard" hatchback, the "V", and of course the "C".

 

The revs/mile numbers come from the manufacturers. I quoted the revs/mile from the 215/45/17 Michelin Primacy MXM4 and the Yokohama AVID S33D because I know them by heart.

I disagree to an extent with your opinion that 844 and 829 are close enough. I have observed the difference between various tire models in these two sizes and there is an obvious correlation between the smaller sized tire and an increase in speedo error. Now, comparing a worn 195/65/15 to a brand new 215/45/17 may reduce this error because the tires are closer in overall diameter but we are comparing new to new. Different model tires within a specific size do vary in revs/mile but they are generally 5 revs/mile of each other. You can check out specs fro different tires at TireRack.com. Choose a tire model and click on the specs tab.

Personally I do not feel it is really a big deal. The differences in size are not going to hurt the car but one should verify their speedo accuracy if they want to avoid tickets, annoying other drivers and be truly honest about their fuel economy. Running a small tire artificially inflates displayed mpg as does driving 1-2mph slower because the speedo error increased. This is another reason I chose the 205/50/17 over a 215/45/17. The speedo error decreased, fuel economy is increased, ground clearance increased, wheel well gap decreased. The only con was the odometer now reads a tad slower. Criminal I know.

 

The difference between 844 and 829 is 1.8%. You are very optimistic if you think the metering systems in the Prius are anywhere near that accuracy! Just the wear on the tyres will exceed that %. MOST law enforcement organizations will give you at least 10% leeway on speed. If they don't they risk having the citation thrown out of court. Radar guns are specified at +or - 10% -after- calibration for example! I have seen many tickets dismissed around here because of this alone. "Pacing" is even less accurate.

 

I've observed the difference. Have you?

I don't think anyone cares about legality of this difference. I was just stating my observations and how the difference in readings can affect mpg due to:

1. Traveling faster or slower than you think you are
2. The odometer is reading faster or slower than actual

 

The Prii over here are 10% optimistic....in speed and also in FE.
When you hook up a OBD2 device you can see the real speed, but Toyota made the decision to add 10% and show that speed on the display.
So it's a software thing.
I didn't like that and called Toyota EU, and they told me that they have to add a safety margin.
I told them that in the EU the max safety margin that is allowed is 10%, and I don't want it to be that high.
When I am cruising on the highway at 100 km/h on the display, then truck are at my rear bumper and Prius drivers get a bad name.
They could understand my concerns, but there was nothing they can do....... sure

 

And the thing that bugs me is my car has a GPS unit right there.
Accurate speed data.
Why couldn't the speedo and odo be self calibrating.
Every time you change from old to new tires the error is more noticeable.
Maybe in the future we could have Accurate Speed and Odo, is that too much to ask in this day and age?

 

^ Odometer is legislated to be accurate. Speedometer is legislated to be slightly high.

If Odo is significantly off there are lawsuits, and I think they typically are accurate.

I've got no problem with a speedometer reading a bit high. I know roughly what the factor is, don't really care. The logic is to encourage people to go a bit slower.

 

What bothers me is that the ECU "knows" the right speed, but at Toyota someone decided to fool the EU people.

North American Prii are 3-4% off (that's what I understood)....that's OK with me, but don't tell me it's a safety margin, when they are just fooling you making you think you are driving very fuel efficient, while that's not the case.....or less FE as they make you think.
I want the info on the info display to be accurate.
When the ECU reads that I'm driving 100 km/h, then software multiplies it by 1.1 and shows 110 km/h on the display.
Then I have to multiply it by 0.909 to know what speed I'm actually doing.
That's a bit of a hassle that's not necessary at all: the ECU already knows the right speed.
I'd rather keep my focus on the road and my fellow motorists.
I don't like it when other people decide what's good for me.

 

Mendel, all,
'Legislated' odo accuracy can not account for the differences between new and old tires, or a swap in tire sizes.
When a car has a built in GPS system self-calibrating accuracy is possible.
Until that is 'Legislated', we will have to continue to use our Garmins to come up with a correction factor.

And the inaccuracies of the MPG data take the fun out of having that display.

 

What about areas where GPS signal is poor or lost?

With the error built in it keeps Toyota out of the courtroom so to speak. Tire diameter is somewhat variable within a specific size. It's sort of like having the low fuel light come on and DTE show zero miles when we all know there is at least another gallon in the tank. Tire size changes are not going to affect Toyota because this is considered a modification and clearly deviates from the manufacturer recommendations.