You can disagree all you want - what do you have to back it up?

i believe the roll out will be what you need to be the most accurate. to back this up, just take a tank "tread". in use, it has a super short "loaded radius". if you put it in a circle, it would be pretty tall. un-link it, and it is a set length, never changing length in any shape. i believe a tire acts the same. the tank tread will roll a set distance no matter the shape because of the "length" of the tread. it takes "x" time to cover "x" distance [roll out, or circumference] at "x" speed. we are not talking a top fuel slick growing at speed by any measure.

Speedo gear coming from CRT...thanks John

My actual measurements...try it yourself.

Still wondering why anyone thinks measuring the tire is more accurate than the manufacturer specs that are readily available?

Example, the manufacturer of my 235/70R15's says they are 28" in diameter but the rollout is 84" which equates to a diameter of 26.73".

I agree with John, my way is to use gps and install the proper speedometer gear...

John exactly illustrates my point in asking the question of math whizzes. Also completely agree with gps recommendations as an exact time saving tool.

At the risk of repeating myself, I think it's ill-advised to change a speedo pinion based on a GPS speed readout; the odometer is the best indicator of correct/incorrect pinion, not the speedometer. If the odometer is correct, then any speedo error is in the head, not the pinion.

Thank you John, for the clarification.
That's why I like Moparts.com

The above illustrated app if you forego the 'free' version and step up to the paid version(2 bucks was what I paid) you get among other niceties, includes odometer support and a history/log file. .

The cost of a small order of fries at Macdonald's.

What could be simpler and more effective?

Agreed, with the right speedo gear the ODO will automatically be corrected & most always then the magnetic gismo speedo will also be correct & if not then the speedo head in that rare case will need to be sent to a speedo specialty shop to be recalibrated (or another one at a boneyard may be cheaper).

The app Speed View is totally free and provides an odometer reading.

https://play.google.com/store/apps/details?id=com.codesector.speedview.free&hl=en

I think I'd be more worried about the quality of the tire than the speedo accuracy if the vendor can't make a tire that's not 1" off from a pretty standard spec.....

The same size tire from the same manufacturer can be different in diameter due to coming from different molds and the diameter of the same tire will vary with the width of the wheel it's mounted on.