[California98Civic]

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.]