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.