Tire revs/mile and odometer calibration
 

What do you think?

Transmission swap relevant details
With my "new" fifth generation 1993 CX's transmission and VSS on my sixth generation 1998 DX engine with stock gauges and ECU, here is my calculation of the relative effect of my 14" wheels/tires on the transmission originally designed for 13" wheels/tires.

Method
I used ridewithgps.com to carefully plot out my commute route. "Ride with GPS" factors elevation changes into distance, unlike, I think, google maps alone. I experimented with the routes early in the week, until I had them reliably mapped and could remember to drive them strictly as mapped.

Results
On my 14.6 mile commute to work that includes a stop/detour, the car's odometer showed 13.85 (94.863%) and 13.95 (95.548%) miles on two commutes this week. On the return, shorter 12 mile commute, it showed 11.28 (94%) and 11.25 (93.75%). The average is 94.54025%.

Discussion
I'm surprised by the results really, even though they're more modest than the weird and wrong 14% figure I first had (mapping error). The speedometer gears are the same in the fifth gen CX and sixth gen DX transmissions, I'm told. And the VSS, I'm told, is not different.

I know that the short distance of the commute can be a source of error, but with four runs, I think the results are a little more reliable.

I get greater confidence that tire size can produce such a large difference on its own by studying tirerack.com. Currently, tirerack is selling 175/70-13 and 185/65-14 tires that in some cases can range apart in revs per mile as much as 922 to 871, respectively. That's a difference greater than 5.5%.

I'm concluding that the D-series transmissions were designed for the 13" wheels that were stock on the fifth generation, and when Honda put 14" wheels on them in 1996 they adjusted for gearing problems with a brand of 14" tire that fixed most or all of the VSS under count through tire revolutions. Judging from tirerack they should have been able to get to within a percent or two.

Lastly, if all this is correct, my 14" "Ultra GT" crappy tires have been under counting miles by more than 5% ever since 2007 or 2008, across maybe 70,000 miles.

[EDIT 1: result on the 12 mile route (5-13-2013) is 11.25 miles on the stock ODO (93.75%).]

[EDIT 2: result on a 71.9 mile route (5-24-2013) is 68.05 miles on the stock ODO (94.645%) and on a 72.0 mile route on the same day 68.05 miles (94.514)]

[EDIT 3: Retested with a GPS on 6-18-2013. OEM odometer showed 21.7 miles when my calibrated Ultra Gauge showed 23.01 and Garmin showed 23.0. That's 94.35% accuracy OEM to GPS and a 5.65% under count shown on the OEM odometer.]

It makes sense: the more you deviate, the more slop there is to correct for. Do the best math that you can going forward, and next time you have a long trip, stop at a mile marker, zero out an odometer, fire up the GPS and go as far as you can to get the longest sample you can.

Then stay with that math until you change hardware again. ;)

Undercount of 6.7%
 

Had to go to a meeting in a nearby city and so I got a chance to do more runs of somewhat greater length today. On a 20.9 mile route plotted using ridewithgps.com I recorded 19.5 miles on the car's ODO and very close to 20.9 on my Ultra Gauge, which is calibrated at 1.055. The miles on the stock ODO were 0.933:1 against the ridewithgps route. I had a 26.6 mile return route planned, but messed up data recording so I have no data.

I know that both the 1993 and the 1998 transmissions have the same speedo gear inside and I know that the vehicle speed sensors on each spin at almost exactly the same rate at 60mph (1026 rpms in 1993 and 1025 rpms in 1998), according to the FSM for each model. I am running factory 14" wheels. All I have to account for the difference is two things: honda built the d-series transmissions with 13" wheels in mind in 1992 and then there are my non-OEM tires. The combinations seems to mean an undercount by about 5% or more. And I have had these tires on the car since probably 2007. Yikes! I'll do more calculations across longer distances when opportunity arises.

I have a garmin gps and I always check the miles driven to the gps. Then I set the scangauge.
The trick to setting the scangauge is to switch over to the metric mode because it is more precise (calculates in 1/10s).

Nice! I just borrowed a Garmin hand-held unit and did a short test: Garmin showed 6.50 miles when my calibrated Ultra Gauge showed 6.48. I'll do further testing of the calibration, across longer distances, this week, but it's exciting to see preliminary confirmation that "ridewithGPS.com" gave me pretty accurately measured routes with which to calibrate. :D

Rev's per mile.
 

Just went on ride with GPS and plotted out my current commute. 19.8 miles is closer than I expected. I was using my ODO to get tank mileage. After resetting my trip meter and driving work 19.6 and half way to 7/10ths. I am running a 15" wheel and 195/50/15 tire with 5th gen DX trans. I used the tire rack spec sheets to determind which tire rev/mile was closest to the OE for the factory DX.

If my calculations are correct then 99.24%. I'll take .76 miles on the hundred for variance. This same difference could be made from driving in the left lane versus the right lane. Feel good about my findings. I will have to check again when the tires are replaced at the end of summer( 3/4 wear at this time).

I'd spend more time finding the 'upsize' that helps to get the best mileage....but that's me.
You can buy used garmin gps all day for $50-60. And get the exact mph without trying.

I realize some are obsessed w/ the speedo being 'correct' but that is 'limited inside the box' thinking. If you can pick up 5-7% taller tire and drive down the final rpm at cruise.....why wouldnt you?

I just had this conversation with my wife about her Subaru... she'll go for strictly accurate OEM gauges all day. I tested her OEM tire/ODO combination yesterday: 99.6% accurate against the Garmin GPS.

Tested my own car further today: OEM odometer showed 21.7 when my calibrated Ultra Gauge showed 23.01 and Garmin showed 23.0 for 94.35% accuracy OEM to GPS. That's a 5.65% undercount, and I have been calibrating my fuel log at 5.5%.

I suppose my caomment were more directed at male ecomodders!!! :thumbup:

My girl friend is probably the same........ but if I up size her.....then she'd really be going slower.....(w/o her knowledge :eek: ) and that would be a good thing!!! :D

I had been wondering what people did to calibrate the odo specifically on civics. I'll have to check my numbers now. I just replaced my 175 70 R 13s Prime Wells with a set of LRR Michelin Defenders 175 65 R 14 with aluminum rims from an 04 Civic hybrid.

Old were about 917 rev/mile
New are rated 905 rev/mile

wich gives a 1.31% difference

I always just use the stock calibration on my scan gauge for its numbers however my fuel econ numbers come from the stock odometer versus gallons pumped with no correction and no scangauge data. Perhaps I should do some analysis on my new and old setups to see if any big adjustments are in the cards for me. I still have the old tires so it would be easy. A 5.5% bump would be huge even if it is only an arbitrary number at that point with no actual bearing on consumption.

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 recently put a new set of tires on my truck. The old tires were not quite down to the wear bars. The revolutions per mile changed about 1%. There is also a tire pressure effect where increased pressure reduces the revolutions per mile. I do not know if that effect is significant, but it is definitely there.

it's a steel belt.....40 or 50 psi isnt going to change the diameter

Seems to me it could, if the lower pressure is allowing a flat spot. The smaller effective diameter of the flat spot leads to higher revs/mile. Increasing pressure can push that out closer to round, making the tire act like it's bigger.

I can see low pressure with a car sitting on top of it, allowing deformation and reduced diameter. Higher pressures really shouldn't stretch the steel belts past nominal size. I'd think higher pressures would increase resistance to deformation (=LRR) without an actual increase in diameter.

This doesn't account for people like Ford specifying really low pressures in their Firestones for a cushy ride, but if reasonable people set the spec, going to max sidewall or beyond shouldn't affect size.

This makes sense, but I think Pale still has a point. If the OEM psi spec is low enough to allow deformation under the car's weight (which it does of course) then higher pressure might help the tire keep more of its max design diameter. If the car makers factor the deformation into their expectations for the odometer/speedometer, then wouldn't higher pressure produce fewer revs per mile? What am I misunderstanding?

Does the outter diameter actually change? Does the tire just touch more ( or less) of the road. A ballons circumference will change due to its phisical properties. Does the total circumference change or does the tire just become more round?

Increased pressure causes the cords, even steel cords, to get longer. The exact amount is pretty small for steel cords. I did some rough calculations that seem to show the effect is too small to measure. But it is there.

Increased pressure will make for a smaller contact spot. I don't know if this changes revolutions per mile in a car tire.

I know that tire pressure changed revolutions per mile in a bicycle tire. I could tell the tire pressure from the odometer distance for a known trip.

Q: Does outer diameter actually change?
A: Yes!

FWIW, the "loaded" tire diameter (what you'd use to calculate tire-revolutions per mile) is typically about 0.97 of the tires un-loaded diameter (my analysis of Michelin and Goodyear tire data)

But, the amount of diameter INCREASE from higher airpressure is MUCH smaller due to the constricting force of the steel belts (<0.03).

you gotta love Mr. Wiki!!!! :thumbup: :D

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.

http://www.vibratesoftware.com/image...ded_Radius.JPG

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.

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?

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.

http://ecomodder.com/forum/attachmen...1&d=1371695052

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.

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
http://ecomodder.com/imgs/chart-tire...n-pressure.jpg

Got It. Thanks for the explanation.

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


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.

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

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.

HMM? question everything that you know to be fact! LOL
Well Ive done similar and had a similar result and also gained slightly more at higher speeds. So I determined distance with combination measurements from yahoo maps and google. I dont know if they factor in cornering etc. but they did fall closer to the distance traveled by Mile marker count on a trip instate.
Afterwards I found things get real messy trying to figure multiple states but I firmly base my weekly mileage against yahoo maps rite or wrong I cant say.

One thing that I am sure most of you are aware of, but it is worth pointing out, is that tires change diameter as they wear. On a 25” OD tire, if you wear it half way (suppose 6/32 tread wear) the radius would change (6/32”)/(12.5”)=1.5%. Thus even if you were able get the absolutely perfect speedometer adjustment done initially, as your tires wear you will start to drift away.

Which can also be the source of a class action suit. It happened with a number of 02-04 Hondas, including my Odyssey. My warranty ran out of years first, so it wasn't relevant to me.

Found it!
 

I did some digging and I was successful in finding the OE tires and size.

This doesn't apply to me but pretty much all other 6th gen Civic owners.

1996 Honda Civic EX - Technical Specs - Long-Term Wrapup- Motor Trend


More digging...

Tire Sizes and Specifications Firestone FR680FR680 02

http://ecomodder.com/forum/attachmen...1&d=1371773852

So we are looking at 886 revs per mile for the OE tires for 1996 Civics if you are on 185 65 R 14s. I am on 175 70 R 13 originally and now 175 65 R 14" so 906 revs per mile is probably closer. I would assume that all 6th gen Civics use the same tires although this may not be the case. This compares nicely to the reported revs per mile on my new Michelin Defenders at 905 revs per mile. Still doesnt change much as far as odometer accuracy but now we know.

Nice job with the OEM tire identification. The EX might have had a sportier tire, than the DX or the HX. But thanks for the citation.

Doing more calibration tests. today so far, using a good android app "GPS Odometer" I have measured a 4.14% stock odometer undercount on the stock odometer. yesterday, I measured a 4.88 undercount. The app claims to be accurate to within a % and these numbers compare well the 5.5% average undercount I got using ridewithgps.com and a handheld Garvin gps unit. I'll keep testing. But when I change over to the175/70-R13 tires that just arrived for my VX wheels, it will be fun to see of the calibration changes, because if 906 RPM was the stock rate, then these tires should change my calibration from correcting an undercount to correcting an over count on the stock odo.

EDIT: 35.93 / 34.31 = a 4.72% under count on the odo, as measured by GPS Odometer (Android tablet app).

Having change the wheel tire combination from 185/65-R14 to 175/70-R13, I neededto re-calculate my odometer calibration. With GPS, on a 112 mile ride today, I calculated a 2.9% undercount on the OEM odometer. 112.54 miles (GPS) ÷ 109.3 (OEM odometer) = 1.02964. I'm gonna be slihtly conservative again and go with 2.9%.

I do not understand why this undercount persists. I expected these tires/wheels to yield an even count or and overcount. But there it is... that's what the instruments show.

...a useful equation to know: http://ecomodder.com/forum/showthrea...tml#post270069

Yes, quite cool. I also foundthis post by you a couple items downin the same thread. It's also very interesting:

The GPS reported my average speed about 51 mph across the 112 mile trip today. But I wonder if this table applies to steel belted tires.

james

Those were Goodyear Polyglass tires on my AAR 'Cuda, not steelbelted radials.

Ran another check of the odometer calibration using a work-related road trip today: odometer read 121.1 miles when the android GPS app showed 124.43, which is about a 2.9% undercount on the stock odometer (again).