Sizing Tires For Calibration

[KevinM60]

I just picked up some Wildpeak R/T01s in 35”.
Wanting to get an accurate sizing for the speedo calibration I did a little checking.
The Falken website says 34.8” which I figure is a nominal size with small variations.
Measuring from the ground up showed 34” but I figured with vehicle weight on the tire they would sit a little flat depending on air pressure.
It stands to reason that the distance around the tire (Circumference) will be the same no matter the air pressure so I measured the circumference of the spare that was hanging underneath with no distortion and full access to the entire thread and divided that by Pi to get an accurate diameter. The circumference of 110” comes out to just over 35” which is probably close enough to the 34.8 that it may not matter unless my “Curse of the Perfectionist” clicks in and causes me to recalibrate to 35.0 instead of 34.8. The difference is only 2.4” per rotation.

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[fourfa]

I'd ignore the math. Manufacturer specs vary too much and in the worst cases (cough*BFG*cough) might be totally fictional. Real world rolling diameter can vary with pressure, tirewear, load, even altitude. I'd get it set up the way you want it (PSI, load etc) and compare to ground truth. Use a GPS tracking app (phone or standalone) - if smartphone, don't connect to the car (which seems to override the phone's GPS with the car's GPS). Reset one of the trip odometers, start the tracking, then go for a drive.

It'll take a few cycles, but just monkey with the tire diameter in Jscan or Tazer or whatever, until the dash odometer closely matches the phone track distance. Odometer is better than speedo, as most speedometers read high (for liability CYA reasons), and you want to average out over longer distance anyway.

 

[ShadowsPapa]

I just picked up some Wildpeak R/T01s in 35”.
Wanting to get an accurate sizing for the speedo calibration I did a little checking.
The Falken website says 34.8” which I figure is a nominal size with small variations.
Measuring from the ground up showed 34” but I figured with vehicle weight on the tire they would sit a little flat depending on air pressure.
It stands to reason that the distance around the tire (Circumference) will be the same no matter the air pressure so I measured the circumference of the spare that was hanging underneath with no distortion and full access to the entire thread and divided that by Pi to get an accurate diameter. The circumference of 110” comes out to just over 35” which is probably close enough to the 34.8 that it may not matter unless my “Curse of the Perfectionist” clicks in and causes me to recalibrate to 35.0 instead of 34.8. The difference is only 2.4” per rotation.

You want rolling circumference - don't go by tire size at all.
I have mine set so when I drove 17.1 miles by odometer (resetting trip odo) it matched GPS and measured distance exactly.
I ignored the web tire sizes and got close by measuring the installed height then tweaking it a time or two after driving it.
The web measurements won't be right at all.

 

[KevinM60]

I'd ignore the math. Manufacturer specs vary too much and in the worst cases (cough*BFG*cough) might be totally fictional. Real world rolling diameter can vary with pressure, tirewear, load, even altitude. I'd get it set up the way you want it (PSI, load etc) and compare to ground truth. Use a GPS tracking app (phone or standalone) - if smartphone, don't connect to the car (which seems to override the phone's GPS with the car's GPS). Reset one of the trip odometers, start the tracking, then go for a drive.

It'll take a few cycles, but just monkey with the tire diameter in Jscan or Tazer or whatever, until the dash odometer closely matches the phone track distance. Odometer is better than speedo, as most speedometers read high (for liability CYA reasons), and you want to average out over longer distance anyway.

Agreed that the calculation is a starting point with the mileage comparison as the fine tuning method. I usually use my phone gps with CarPlay and my 38 mile drive differed by about 1/2 a mile. I’ll do some more comparisons so I don’t end up chasing around small numbers.

 

[Escape.idiocracy]

I wing it. Measurement is a starting point but use a radar detector (uniden R4) and a speed app to fine tune at 70mph. ??? adjust the tire size up and down as needed from there.

 

[KevinM60]

You want rolling circumference - don't go by tire size at all.
I have mine set so when I drove 17.1 miles by odometer (resetting trip odo) it matched GPS and measured distance exactly.
I ignored the web tire sizes and got close by measuring the installed height then tweaking it a time or two after driving it.
The web measurements won't be right at all.

Wouldn’t rolling circumference be the same no matter the conditions? I would think The distance around the tread would be the same regardless of shape unless heat caused it to get a little larger and cold a little smaller.
That was why I started with the free form circumference and calculated the diameter instead of using the standing height from the ground. I have the AEV Procal Snap that sets by the tire diameter so I needed that number.

 

[KevinM60]

I wing it. Measurement is a starting point but use a radar detector (uniden R4) and a speed app to fine tune at 70mph. ??? adjust the tire size up and down as needed from there.

I thought about having my wife drive behind me to compare but that depends on hers being correct also.

 

[Escape.idiocracy]

I thought about having my wife drive behind me to compare but that depends on hers being correct also.

https://apps.apple.com/us/app/speedometer-simple/id939507215

This app is pretty accurate. Or at least as accurate as the Uniden, and it’s worth its weight in gold.

 

[kevman65]

When I went to 35's, I measured to the shoulder of the tread and entered that measurement.
I then used two separate, independent GPS devices not connected to the JT in any way to verify my speed.

Had to drop the tire size two selections down to get everything almost matching. It reads .2 MPH fast.

 

[KevinM60]

When I went to 35's, I measured to the shoulder of the tread and entered that measurement.
I then used two separate, independent GPS devices not connected to the JT in any way to verify my speed.

Had to drop the tire size two selections down to get everything almost matching. It reads .2 MPH fast.

So if the vehicle speed is higher than the actual speed you put the tire size smaller to get it to match?

 

[kevman65]

So if the vehicle speed is higher than the actual speed you put the tire size smaller to get it to match?

No, taller tires read slower on speedo if nothing else is changed.

After my initial measurement and calibration, I relied on GPS to give me actual speed and adjusted sizes until I got it "right".

The speedo and my radar detector GPS round to the closest MPH. My actual Garmin GPS shows me tenths of a MPH. So at 70 MPH speedo I am going 69.8 MPH on my Garmin.

 

[kevman65]

The first assumption everyone makes is that their speedo is accurate from the factory.

Mine wasn't. I have a Rubicon, so it came with 285/70R17 which is almost a 33" tire. My speedo was reading slow to start with. Verified in the same way that I calibrated it for my 35's, two independent GPS sources not tied to the JT. It was reading 4.5 mph slow. Thats roughly the difference between 31's and 33's.

 

[Hootbro]

The first assumption everyone makes is that their speedo is accurate from the factory.

Mine wasn't. I have a Rubicon, so it came with 285/70R17 which is almost a 33" tire. My speedo was reading slow to start with. Verified in the same way that I calibrated it for my 35's, two independent GPS sources not tied to the JT. It was reading 4.5 mph slow. Thats roughly the difference between 31's and 33's.

FWIW, Jscan and I assume other like tools have a speedometer error % pid to take that error out of the cluster. I had to do it with my 2021 WILLYS. Adjusting the tire size up/down could never get the speedometer cluster error % to null out.

 

[kevman65]

FWIW, Jscan and I assume other like tools have a speedometer error % pid to take that error out of the cluster. I had to do it with my 2021 WILLYS. Adjusting the tire size up/down could never get the speedometer cluster error % to null out.

I'm not anal/OCD enough to worry about the .2 MPH. It is a constant from 50 MPH to 85 MPH so I can live with it.

 

[SoK66]

I went through this when I swapped out the OEM tires (255s, BFG’s spec of 32.1”) on my ‘23 Willys for the Rubicon spec 285/70s (32.8” per BFG). Per GPS the Speedo was off by about 1.5 mph @60. I reset using Jscan based upon the actual measured diameter and the speedo was still off by about 1 mph @ 60. I was surprised the factory had set the rolling diameter of the original tires at BFG’s 32.09”!rather than a measured spec. When set the JSCAN to 32.8” the Speedo was dead on te money at 60.

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