1” diameter worth worrying about odometer or speedometer?

[mzeblue]

https://www.caranddriver.com/features/a15136801/speedometer-scandal/

This is for speedometers

In the U.S., manufacturers voluntarily follow the standard set by the Society of Automotive Engineers, J1226, which is pretty lax. To begin with, manufacturers are afforded the latitude to aim for within plus-or-minus two percent of absolute accuracy or to introduce bias to read high on a sliding scale of from minus-one to plus-three percent at low speeds to zero to plus-four percent above 55 mph. And those percentages are not of actual speed but rather a percentage of the total speed range indicated on the dial. So the four-percent allowable range on an 85-mph speedometer is 3.4 mph, and the acceptable range on a 150-mph speedometer is 6.0 mph

5% over 100k miles is 5000 miles, which moves the value factors for re-sale above the 100k if say you are selling it as “below 100k mi”

The error accumulated over 100k miles of use isn’t the sort of discrepancy that gets you sideways under the law when signing a statement as to the accuracy of the odometer. Surely the statute from which the need to sign an odometer statement is required is after a more nefarious manipulation of odometer readings. Good for you though and anyone downstream buying a used car from you. But, respectfully, it feels a bit like unnecessary hand wringing over this issue.

The calculations, by the way, get more complicated (and less linear) if you try to factor in the error caused by the reduction in diameter over time which offsets at least some of the error from larger tires.

Further, speedometer tolerances can go either way, plus OR minus from the design mean. But, you’ll never know which way without measuring it. So, it’s possible that the tolerance stack can work in one’s favor or against, though which direction and to what degree can’t be known without taking above-ordinary measures to ascertain - this, without doubt, statute would not anticipate nor require.

The above, however, being said in the context of the OP’s stated 0.5” change in tire radius....a larger change really should require an adjustment to the speedometer.

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[Murgatroid]

I’m considering upgrading stock overland tire to some KO2’s that are 33’s. Keeping the same wheel. So it’s only around 1” difference. Should I be worrying about gearing, speed, or odometer issues with only a 1” difference? Feel like I’m overthinking this lol.

You might not think one inch is not much, but go to a roll of double ply toilet paper and separate the layers and un-roll one layer one time around and look how many inches of difference where the perforation are now between the two layers and the only difference as one ply of thickness.

 

[Renegade]

You might not think one inch is not much, but go to a roll of double ply toilet paper and separate the layers and un-roll one layer one time around and look how many inches of difference where the perforation are now between the two layers and the only difference as one ply of thickness.

So your saying we have to measure the speed differential in toilet paper plies? Got it- thanks.

 

[Jeepnoob34]

You might not think one inch is not much, but go to a roll of double ply toilet paper and separate the layers and un-roll one layer one time around and look how many inches of difference where the perforation are now between the two layers and the only difference as one ply of thickness.

I’m having trouble visualizing your analogy, but it could be because I use cottonelle. It’s like wiping with a svelte Tempur-pedic mattress every time.

 

[Incommando]

My speedometer is more accurate now when measured against roadside radar trailers than it was before I put the bigger tires on

 

[Renegade]

I’m having trouble visualizing your analogy, but it could be because I use cottonelle. It’s like wiping with a svelte Tempur-pedic mattress every time.

Toilet paper might be the new plywood...

 

[Jeepnoob34]

Toilet paper might be the new plywood...

That would be great with the narrow wheelbase on these JT’s. Lol

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