Tire revs/mile and odometer calibration

[wolydotmatrix]

Are talking diameter or circumference, the total height of the tire vs the distance around the tire. Just trying to understand the logic. Even with a 32 psi tire, does not the same amout of tread touch the road inreference to its circumference as a tire with 50psi.

[gone-ot]

Originally Posted by wolydotmatrix

Are talking diameter or circumference, the total height of the tire vs the distance around the tire. Just trying to understand the logic. Even with a 32 psi tire, does not the same amout of tread touch the road inreference to its circumference as a tire with 50psi.

Short answer: No.



Notice that onehalf of 29.3" is not 13.1" -- the difference is the ~0.97 radius reduction that occurs when the tire is "loaded", ie: the bottom of the tire spreads out ("squish") slightly, which shortens the rolling (loaded) radius.

This happens most on passenger cars and least with HD truck tires.

[wolydotmatrix]

Not to continue off topic, but I guess what im trying to figure out (in relation revolutions per mile) is that if a tape measure was wraped around a loaded tire @32psi and then increased to 50psi would the tape read a longer or shorter distance. How much pressure affects how many revs per mile?

[hawk2100n]

Originally Posted by California98Civic

The revs/mile of tirerack are just corporate reporting. Worth testing for yourself in the real world conditions of your car and specific set up. The GPS I'm playing with has confirmed, almost, what ridewithgps.com showed me... so if you don't have a GPS, plot out your daily commute carefully on ridewithgps.com and compare what your OEM odo reports. I'd like to know, because I still find my calibration correction astonishing and hard to believe. I'm waiting for an error to appear.

I used all of the instrumentation I could to estimate my odometer accuracy and try and get good data. Some interesting things occurred. None of the readings agreed. I used two different GPS sources, the scangauge and the stock trip odometer.

I am running 175-65 R 14s which are only slightly larger than stock.



ODO 64.0 miles
Scangauge 63.7 miles
iPhone 4S with GPS app 63.5
Garmin GPS watch 63.31

The watch and iPhone were both started while the car was parked and before it was running. I waited until I was sure that both had a good fix before they were started. I reset the trip meter twice to make sure it was fully zeroed and the scangauge had been off a sufficient amount of time to start this as a new trip. I didn't do any engine off coast so I wouldn't loose any time on the odometer with the car off but moving.

If we average the GPS readings to be 63.4 miles, that means that my Odometer is reading about 0.946% high and my reported fuel economy would be inflated as much. However, the engineer that I am can't help but question the GPS results. I don't think that the sampling rate is very high for the watch or phone. I suspect that the watch samples at about 1 Hz as that's how often it updates the screen and moving at highway speeds the distance increments every time. Since its battery powered and very small it would be wasteful to sample more often from a power perspective. It could sample even less frequently and estimate the distance between samples for the screen updares which would be a valid approach in the intended low speed (running) application. However in the car this makes problems as you start to get jaggies as you go around turns. The distance is a direct point to point calculation and every time it cuts the corner you loose some. Long story short, if I was sampling at 10Hz I would be confident but as it stands now I'm not so sure. Plus the amount of difference between the otherwise identical GPS recordings makes me believe that the results are inconclusive but perhaps my odometer is still skewed towards the high side. What is interesting is that the scangauge and odometer don't agree. They are seeing the same data in theory so they should be very close. I don't know if that could be a floating point rounding error in the scangauge or mechanical variability in the odometer.

Honda odometers could have a variability of +\- 2% which would be realistic. Not good if you are on the high side but that's just how it is. This would also be indicative of running tires smaller than stock, which I am not. They are slightly larger than stock in fact. I would love to see more civic drivers get some data on this with your tire size.

[PaleMelanesian]

Originally Posted by wolydotmatrix

Not to continue off topic, but I guess what im trying to figure out (in relation revolutions per mile) is that if a tape measure was wraped around a loaded tire @32psi and then increased to 50psi would the tape read a longer or shorter distance. How much pressure affects how many revs per mile?

There's a section of the tread, where it touches the road, that is flat. That flat section is shorter than the free tire's curved section. So yes, it is shorter circumference.

What matters more is the reduced radius from hub to flat spot. That's the effective radius of the wheel. That flat spot moves around the tire as it turns, making it behave like a smaller tire. Imagine a hard wood or steel wheel in that smaller diameter that doesn't deflect at all. That's the size you use for calculating revs/mile.

I went looking for more answers, and found this one right back here:
http://ecomodder.com/forum/showthrea...ius-12039.html

[wolydotmatrix]

Got It. Thanks for the explanation.

[California98Civic]

Originally Posted by hawk2100n

Honda odometers could have a variability of +\- 2% which would be realistic. Not good if you are on the high side but that's just how it is. This would also be indicative of running tires smaller than stock, which I am not. They are slightly larger than stock in fact. I would love to see more civic drivers get some data on this with your tire size.

These are really interesting results. And thanks very much for them. I really appreciate the commentary on the way the GPS works. And I'd also love to have more d-series Civics test this stuff. Your last comments also raise some things I'm still curious about. (1) Do we know what the stock tire was and what its rated revs/mile was? I at least don't, and if I read the tirerack.com listings I see significantly varying revs/mile ratings for different tires in the same size category. And (2) in my experience the speedometer error tends to be higher than actual while the ODO is lower than actual. You cannot get the ODO and the speedometer to both read accurately at the same time. At least I can't. That suggests Honda designed the system to report "erroneously" for some reason, no?

Vexing.

[hawk2100n]

Originally Posted by California98Civic

These are really interesting results. And thanks very much for them. I really appreciate the commentary on the way the GPS works. And I'd also love to have more d-series Civics test this stuff. Your last comments also raise some things I'm still curious about. (1) Do we know what the stock tire was and what its rated revs/mile was? I at least don't, and if I read the tirerack.com listings I see significantly varying revs/mile ratings for different tires in the same size category. And (2) in my experience the speedometer error tends to be higher than actual while the ODO is lower than actual. You cannot get the ODO and the speedometer to both read accurately at the same time. At least I can't. That suggests Honda designed the system to report "erroneously" for some reason, no?

Vexing.

I'll see if I can do some digging to come up with data on the stock tires. One thing I know is that the 1996 DX came with 13" steelies while the 97+ were all on 14" wheels. I'll look into some parts references to see if there is any difference between VSS data for these years.

For your 2nd question, it reminds me of a good article from Car and Driver on odometers and speedometers. Pulled this one from way out of the mental archives, 2002...

Speedometer Scandal! - Feature - Car and Driver

In the U.S., manufacturers voluntarily follow the standard set by the Society of Automotive Engineers, J1226, which is pretty lax. To begin with, manufacturers are afforded the latitude to aim for within plus-or-minus two percent of absolute accuracy or to introduce bias to read high on a sliding scale of from minus-one to plus-three percent at low speeds to zero to plus-four percent above 55 mph... ...odometer accuracy is more tightly controlled to plus-or-minus four percent of actual mileage.

It goes on to give details where the error can be even worse in many situations. All supporting that the error we are seeing is real.

One thing I know is that high speedometers help reduce speeding tickets on a macro scale. Make people think they are going faster than they are and all of sudden they are now only going 63 instead of 65 in that 55 zone and the cop lets them go.

[user removed]

Basically an optomistic speedometer protects the manufacturer from a class action suit where many drivers would argue that an underregistering speedometer was the cause of a traffic citation.

An over registering odometer means fewer miles before the manufacturer can deny a warranty claim.

regards
Mech

[California98Civic]

Originally Posted by hawk2100n

I'll see if I can do some digging to come up with data on the stock tires. One thing I know is that the 1996 DX came with 13" steelies while the 97+ were all on 14" wheels. I'll look into some parts references to see if there is any difference between VSS data for these years.

Thanks for the Car and Driver cite. I'll save you the VSS research. The 1992-1995 Honda FSM says the VSS turns 1025rpms when the speedo reports 60mph and the 1996-1998 FSM says 1026rpms for 60mph. And the speedo gears across almost all of the trannies are 90.7mm diameter.