GPS factors in new tire size without having to measure diameter and calculate to the original 7.00 X 14 tire, which the 'math' does not do.

Not exactly, what the math does not take into account, because it is an unknown, is the EFFECTIVE diameter of the old and new tire. Effective diameter is the actual diameter with the load of the vehicle on it, not the listed diameter specification.

And that is why the math is wrong. Problem solving is a [censored] when you don't know how the formula works.

FWIW, finally got around to checking my original factory reduction unit and it's .80 reduction.

I dont have GPS but I had assumed it was high tech dead on accurate, ain't so ???. What John said I would get the odo correct with the correct pinion then after that if the speedo ain't correct then it would need to come out & go to a speedometer specialty repair shop. I use 5 mile markers in a row on the interstate to check the odo and those portable lit up digital roadside MPH signs the city sets out on occaision to check the speedo

Free version gives you actual mph, paid version gives you actual miles plus a wealth of other features. I have on IPhone, likely Android version. Invaluable for syncing speedo/odometer.

Actually to my way of thinking the GPS is MORE ACCURATE than the math because you're never quite sure how many ACTUAL revolutions the tire will require per mile. You can quote rolling radius all you want, but it has to vary some and that's a dynamic process. Think of drag slicks.

We use GPS to measure movement on civil structures, and that is well below inch accuracy. It's repeatable, as well.

R.

When Lacking a GPS I would drive the car on the Interstate from mile marker to mile marker. Needs to be over 25 miles though, the more miles the better the accuracy.

I divide the miles traveled on the odometer and the actual miles travel by the mile marker signs to get a percentage. If it is 20 percent too low I get a speedometer gear with 20 percent less teeth to speed it up.

That being said, John is right about speedometer heads being off. My method works for calibrating the Odometer with the hopes the speedometer will be right afterwards.

Try this http://vexer.com/automotive-tools/speed-rpm-calculator

If the actual odometer head is off, the GPS method still factors that in and corrects it. Along with the effective tire diameter and all variables, so it seems. I don't see the GPS being inaccurate, so I'm confident the .67 ratio adapter is the answer. Thanks for all the responses!