Accurately Measuring Tires

[TrainMan]

I'm going to be recalibrating my speedometer and have a question about measuring my tire size. I have the Falken Wildpeak AT3w in 35/12.5/17. The actual measurement I recorded was 33.5 inches. However should I also account for the part of the tire compressed on the pavement? Obviously tires expand while driving. Am I overthinking? Their website says the tires measure 34.8 and I just can't see how they got that when they are not even 34".

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[kevman65]

Using the Tazer Lite, I had to go 3 sizes bigger than actual tire measurement.

I used three separate GPS devices to accomplish this.
One read appx .25 mph slower than the other two which were matched, so I called it good.

Actual measurement on my tire was 34.625" and I believe I ended up on 34.825".

 

[ShadowsPapa]

Rolling diameter can be different...... you may measure xx inches around, but as it rolls, it's not exactly that. I've done the chalk mark tests where you make a mark on the sidewall and on the pavement aligned with that mark then drive forward about two revolutions until that mark is straight down again. Measure, divide by 2 and it won't necessarily match the outer circumference of the tire.
And if you measure height, with the same PSI in the tire, a jeep that weighs a bit more will flatten the tire down a bit more. So if you have a tire that has 36 psi in it and put it on a Sport S, then measure the footprint and then move that same tire to a loaded Rubicon - the footprint will be larger, the height a bit less.
What kevman experienced was some of this. The measured size is a starting point - may be fine, you may have to adjust a bit.

 

[bastage]

Just set it close & download a GPS app on your phone... Drive a bit & if your off adjust accordingly.

 

[ThatStinging_Jeep]

you can just get a tape measure'er and put it right on the middle of the hub and multiply it by 2 and bam you have a accurate measurement when the tires are on the ground.........of course

 

[hjdca]

When you are driving the centrifugal force helps to keep the tires at the specified diameter. If you start with the specified tire diameter from the manufacturer, that should put you extremely close to the proper setting.

 

[Alabama Mud Machine]

Just set it close & download a GPS app on your phone... Drive a bit & if your off adjust accordingly.

If you already have the WAZE app, it tells you how fast you're going, and does it VERY accurately.

My tires are 37x13.50, and with 45 PSI in them, and using the Tazer JL Mini, I reprogrammed mine to 37.00" tire size, and it's dead-on. It's within 0.25-0.5 MPH tested with cruise control and a GPS. I'm very happy with the outcome.

 

[Jeeperjamie]

If you already have the WAZE app, it tells you how fast you're going, and does it VERY accurately.

My tires are 37x13.50, and with 45 PSI in them, and using the Tazer JL Mini, I reprogrammed mine to 37.00" tire size, and it's dead-on. It's within 0.25-0.5 MPH tested with cruise control and a GPS. I'm very happy with the outcome.

I second Waze app or Google maps both pretty accurate. JScan I had to set my tire size to 34 actual tire size was 33.8 and those were 315 70 17 BFG KO2s. It was within a 1 mph running 33psi. I recently went up to 44psi and noticed it's within .5 now so the extra air in the tires made a difference.

 

[ShadowsPapa]

When you are driving the centrifugal force helps to keep the tires at the specified diameter. If you start with the specified tire diameter from the manufacturer, that should put you extremely close to the proper setting.

Maybe with racing slicks but these things are so stiff, that, with vehicle weight, they still flatten when on the road. You'd have to drive pretty fast to make the circumference increase, and have a light vehicle for the weight not to overcome that force.
(You can watch tires "grow" when hitting about 80-100 mph in the burnout area)
Rolling circumference is often different from measured static numbers.

I second Waze app or Google maps both pretty accurate. JScan I had to set my tire size to 34 actual tire size was 33.8 and those were 315 70 17 BFG KO2s. It was within a 1 mph running 33psi. I recently went up to 44psi and noticed it's within .5 now so the extra air in the tires made a difference.

Yeah, the rolling diameter would increase with air pressure, and decreased footprint. Any decent app should be accurate because it's using the hardware of the phone.
The speed and location should be fine, it's the maps that are sometimes garbage in some apps (Google maps has sent us the wrong way more than once)

 

[TrainMan]

Thanks everyone. Question...i think im a little dyslexic lol. If my speedometer reads 40 but my app says 41 then i need to lower my tire size right?

 

[TrainMan]

I finally got it 99% accurate. I entered 33.50 and it's spot on. I used a highly rated app to compare.

 

[TrainMan]

Btw in case anyone wants to know I used the Rough Country calibrator. It was easy except step 1 says to plug into PC and select your vehicle. Well nothing happened when I did this. It had the latest update, but I couldn't find anything about selecting my vehicle. Well apparently it didn't matter because everything worked.

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