Speedometer accuracy change due to tire wear...

[dpAtlanta]

I am looking at what tires I want when I need replacements from the OEM Goodyears. If I keep the same diameter tire I don't have to worry about the speedo being off. I have the equipment to adjust due to size, so that would not be an issue.
Then I got to thinking............... what is the speedo difference from brand new tires, to tires that are just hitting the wear bars? I never get them down that low, but what would the difference be?

It's a simple calculation actually.
Take for example, the OEM Goodyears... they are 34.4 inches in diameter.
The tread depth is 1/2 inches to the bottom of the tread, not to the wear bars.
The wear bars are 1/16 on an inch.
That means it's 7/16 inch from the top of the tread to the wear bar.
Since the diameter takes into consideration the tread on the top as well as the tread on the bottom, the diameter once the wear bars are hit will be 34.4" - (7/16" x 2) = 33.5".
It's a simple ratio of new vs. worn.

When my Bronco was new and the tires were 34.4" dia., when I was traveling on the interstate at 70 mph, the speedo should be reading 70 mph.
But when they are down to the wear bars, how fast am I going when the speedo is reading 70 mph?

(34.4" - 33.5"/34.4") = 0.026 ........... this is the ratio of new vs. worn.

Apply that ratio to the speed:
70(1-0.026) = 68 mph
You are cruising at what you thought was 70, and you are actually only going 68 mph.

The moral of the useless story is that you are less likely to get a speeding ticket with tires on the wear bars.


"But officer, I am being extra safe by not speeding due to my smaller diameter tires."
"Your tires on on the wear bars Son... they are legally bald; here is your ticket."

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[HighVelocity]

This is why Archimedes never got a speeding ticket.

 

[RagnarKon]

Fun fact: Stock setting actually does have a bit of a buffer to account for varying tires sizes and whatnot. So in your brand new Bronco with your brand new tires, you are most likely going 1-3 MPH slower than what the speedo says, and by the time your tires wear out it's probably closer to 3-5 MPH.

 

[Beggsie]

I am looking at what tires I want when I need replacements from the OEM Goodyears. If I keep the same diameter tire I don't have to worry about the speedo being off. I have the equipment to adjust due to size, so that would not be an issue.
Then I got to thinking............... what is the speedo difference from brand new tires, to tires that are just hitting the wear bars? I never get them down that low, but what would the difference be?

It's a simple calculation actually.
Take for example, the OEM Goodyears... they are 34.4 inches in diameter.
The tread depth is 1/2 inches to the bottom of the tread, not to the wear bars.
The wear bars are 1/16 on an inch.
That means it's 7/16 inch from the top of the tread to the wear bar.
Since the diameter takes into consideration the tread on the top as well as the tread on the bottom, the diameter once the wear bars are hit will be 34.4" - (7/16" x 2) = 33.5".
It's a simple ratio of new vs. worn.

When my Bronco was new and the tires were 34.4" dia., when I was traveling on the interstate at 70 mph, the speedo should be reading 70 mph.
But when they are down to the wear bars, how fast am I going when the speedo is reading 70 mph?

(34.4" - 33.5"/34.4") = 0.026 ........... this is the ratio of new vs. worn.

Apply that ratio to the speed:
70(1-0.026) = 68 mph
You are cruising at what you thought was 70, and you are actually only going 68 mph.

The moral of the useless story is that you are less likely to get a speeding ticket with tires on the wear bars.


"But officer, I am being extra safe by not speeding due to my smaller diameter tires."
"Your tires on on the wear bars Son... they are legally bald; here is your ticket."

Wow, someone needs to get a hobby!

 

[helifino16]

This kind of thinking will make you crazy.

It's easy, just say, "I'm sorry officer, my tread depth is a little low, so my Speedometer is off by several fractions of a MPH". This will assure that you will be told to "Have a great day and slow it down" before he walks away and says "Geez what a nut".

 

[Karsa]

these are the same kind of crazy rabbit holes I go down, glad I’m not only one!

 

[Braptor23_CO]

I am looking at what tires I want when I need replacements from the OEM Goodyears. If I keep the same diameter tire I don't have to worry about the speedo being off. I have the equipment to adjust due to size, so that would not be an issue.
Then I got to thinking............... what is the speedo difference from brand new tires, to tires that are just hitting the wear bars? I never get them down that low, but what would the difference be?

It's a simple calculation actually.
Take for example, the OEM Goodyears... they are 34.4 inches in diameter.
The tread depth is 1/2 inches to the bottom of the tread, not to the wear bars.
The wear bars are 1/16 on an inch.
That means it's 7/16 inch from the top of the tread to the wear bar.
Since the diameter takes into consideration the tread on the top as well as the tread on the bottom, the diameter once the wear bars are hit will be 34.4" - (7/16" x 2) = 33.5".
It's a simple ratio of new vs. worn.

When my Bronco was new and the tires were 34.4" dia., when I was traveling on the interstate at 70 mph, the speedo should be reading 70 mph.
But when they are down to the wear bars, how fast am I going when the speedo is reading 70 mph?

(34.4" - 33.5"/34.4") = 0.026 ........... this is the ratio of new vs. worn.

Apply that ratio to the speed:
70(1-0.026) = 68 mph
You are cruising at what you thought was 70, and you are actually only going 68 mph.

The moral of the useless story is that you are less likely to get a speeding ticket with tires on the wear bars.


"But officer, I am being extra safe by not speeding due to my smaller diameter tires."
"Your tires on on the wear bars Son... they are legally bald; here is your ticket."

Hmmm, is this why when Braptor was new I got 14-15 mpg now with tires almost to the wear marks I get 17-18?

 

[CarGuy]

New with OEM Goodyears was 2 mph off - speedometer reading 50 mph and was actually going 48 mph.

 

[swamp2]

Although I did not check the details of the math in the OP, I'm sure it is correct. That said that is a very theoretical type of calculation applied to a very complex and non linear system (a tire). A tire is far from a rigid perfectly round cylinder, it significantly changes shape at the contact patch and this will affect its effective revolutions per mile.

In the static case when your rig is just sitting still the sidewall stiffness dominates how much the center of the wheel drops relative to the road. That’s why the static loaded radius is nearly the same for a worn tire and a new one, assuming identical inflation and load.

The dynamic/rolling case has actually been studied - tread wear reduces hysteresis/squirm, so the effective rolling radius shrinks less than geometry alone suggests.

Michelin and Bridgestone technical papers (SAE, mid-2000s) measured this: a passenger radial from new to wear bars loses ~2% in free circumference, but only ~1–1.5% in effective rolling circumference.

So, long story short, your 68 is probably closer to 69.

 

[dpAtlanta]

Michelin and Bridgestone technical papers (SAE, mid-2000s) measured this: a passenger radial from new to wear bars loses ~2% in free circumference, but only ~1–1.5% in effective rolling circumference

Interesting…. I’ll have to try and find that paper(s) since I have free access as an SAE member.
Thanks!

 

[kodiakisland]

And then there are people like me.

Bronco came with 32ish tires with a speedo within a certain tolerance of being correct. Now has 37ish tires and the speedo is no longer in whatever that tolerance was. Have not lost sleep over it, or gotten any tickets.

 

[swamp2]

New with OEM Goodyears was 2 mph off - speedometer reading 50 mph and was actually going 48 mph.

That difference is intentional/by design from the OEM.

 

[SierraBronco]

Although I did not check the details of the math in the OP, I'm sure it is correct. That said that is a very theoretical type of calculation applied to a very complex and non linear system (a tire). A tire is far from a rigid perfectly round cylinder, it significantly changes shape at the contact patch and this will affect its effective revolutions per mile.

In the static case when your rig is just sitting still the sidewall stiffness dominates how much the center of the wheel drops relative to the road. That’s why the static loaded radius is nearly the same for a worn tire and a new one, assuming identical inflation and load.

The dynamic/rolling case has actually been studied - tread wear reduces hysteresis/squirm, so the effective rolling radius shrinks less than geometry alone suggests.

Michelin and Bridgestone technical papers (SAE, mid-2000s) measured this: a passenger radial from new to wear bars loses ~2% in free circumference, but only ~1–1.5% in effective rolling circumference.

So, long story short, your 68 is probably closer to 69.

Came to give accolades for using % instead of fixed mph then got distracted by the funny number.

 

[HoosierDaddy]

"Have a great day and slow it down" before he walks away and says "Geez what a nut".

these are the same kind of crazy rabbit holes I go down, glad I’m not only one!

I'll never forget the gentleman that started a conversation with me at the fuel pumps regarding the fuel mileage of my 2001 F250 Powerstroke.
It logged 21 mpg so steadily that I began to worry as that number dropped slowly into the 19's. (manually calculated every fuel up)
Then the tires need to be replaced and my mileage shot right back up to 21.

So, dude pops around the pump and asks about mileage, I answer 21

him: 21 ! No kidding!

me: Yeah, it gets that so often I've learned it changes with my tire wear.

him: You can see your tire wear in your fuel mileage calculations? (eyeroll)

me: Yep (start to explain about the drop off and recovery when I replaced the tires)

him: (slides back behind his pump to be out of direct sight of the crazy guy)

I always get a chuckle from that memory.

This pretty much sums up that conversation....

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