Look again... even BFG, its manufacturer, says its actual diameter is 32.7" at the bottom of the screen shot.
Re-gearing for 33's
- Thread starter JML
- Start date
Both hub height and tire rpm have always been in sync perfectly, despite vehicle weight or tire pressure. This is on tires that are not significantly worn anyways.
My main point for Jerry was that the revs per mile calculates to a different (smaller) diameter than the "advertised diameter" (the 32.7" in this case). If you do the math on 640 rpm, it comes out to 31.51". That is spot on and has always matched GPS tests as well.
The only methods I've found to be incorrect are going off of the advertised diameter and doing anything dumb like measuring the literal tire diameter on the vehicle in any dimension (horizontal, vertical, etc). Hub height x2 is fine but unless the tire is way worn down, going off of the rpm is very accurate in my experience with 10 or so different tires I've tried on.
I don't really care, I go by what the tire's manufacturer says. They certainly know the precise diameter of the tire they manufactured.
Jerry, you are not paying attention. I know what BFG lists the advertised diameter as. It is not correct.
BFG lists that exact tire as 640 revs per mile. 5280/640 = 8.25 feet per Revolution. 8.25 X 12 = 99” per rev.
99/3.14159 = 31.51”. THAT is the correct diameter for a new and not heavily worn 33” BFG. Not 32.7”.
You do care. As for going by the tire manufacturer’s specs, so do I, I go by the revolutions per mile that don’t calculate out to the advertised diameter. The advertised diameter has never ever been known to be correct.
Hub height x2 will always be accurate to the vehicle at the time of measurement. It doesn't matter if the tires are brand new or worn bald. Just like the verification will always be accurate to the vehicle at the time of measurement.
Understanding this means we don't need to know or care what the manufacturer says about their tire measurements.
This has gotten stupid. Take it up with BFG, go tell them they're posting incorrect information lol.
That will be correct when there is no pressure on the tire from the vehicle. Andy
I was curious about advertised vs true tire diameter a while ago and did a little experiment. Tires are BFG KO2 33s at 26psi. Measuring ground to hub center using squares to be precise got 15.5 inches at all 4 tires or 31" diameter. Making a mark on a lug and marking the street then rolling ahead 1 revolution measured 98.75 inches repeated several times. The actual ground measurement of 1 rotation of 98.75" calculates to a diameter of 31.45" I didn't try different pressures to see if the actual ground distance changed.
Conclusion: measuring to hub center should have resulted in a rolling distance of 97.34" but actual measured distance is 98.75" . The difference of 1% isn't much but it isn't zero either. My Jeep with the 33s and 4.10s and NV3550 shows the exact RPMs in the calculator when I use the diameter calculated from measuring 1 revolution on the ground.
Conclusion: measuring to hub center should have resulted in a rolling distance of 97.34" but actual measured distance is 98.75" . The difference of 1% isn't much but it isn't zero either. My Jeep with the 33s and 4.10s and NV3550 shows the exact RPMs in the calculator when I use the diameter calculated from measuring 1 revolution on the ground.
I have Goodyear Wrangler Duratrac 33x12.5R15
The nominal diameter = 33"
The published diameter from Goodyear = 32.8"
Using the calculations from mechguru that I linked to before:
R = 32.8/2 = 16.4
SLR = 0.96 x 16.4 = 15.744
Rolling Circumference = 2 x PI x SLR = 98.92
The rolling diameter is therefore = 98.9 / PI = 31.48"
Look at that.. it's almost exactly 2x my hub height of 31.5".
So I have taken the published diameter and calculated 2x hub height from it.
Andy
The nominal diameter = 33"
The published diameter from Goodyear = 32.8"
Using the calculations from mechguru that I linked to before:
R = 32.8/2 = 16.4
SLR = 0.96 x 16.4 = 15.744
Rolling Circumference = 2 x PI x SLR = 98.92
The rolling diameter is therefore = 98.9 / PI = 31.48"
Look at that.. it's almost exactly 2x my hub height of 31.5".
So I have taken the published diameter and calculated 2x hub height from it.
Andy
Regardless of the back and forth in here, the grim Jeeper website is a great tool to estimate your RPMs…and you want 5.13s for your 33s and 42RLE.
If you had a front Dana 44 I’d recommend getting 5.38s for 33s because it only revs about 150 more RPMs than 5.38s around 70mph.
You can’t run 5.38s in a Dana 30 so you want the next best option which are 5.13s. There is no practical debate worth having on this that’s your best answer.
I’ve seen guys regret not going low enough because they thought lower RPMs would be better for the highway to not overrev (that’s a mistake bc it’s a dog and doesn’t want to get up to speed on the highway…you would be wasting your money). I’ve yet to see someone say they went too low.
With 5.13s you will be around 3000 at 75mph. That’s where u want to be and you would gain nothing by doing 4.88 other than having less drivability and being unhappy with your choice. Best to do it right and move on with life.
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With the RPMs from the Torque app and ground speed from GPS what would be the reason? The difference @ 70 between hub distance and 1 rotation calculations is about 30 rpms.
Next time you drive on the road/hwy, note the rpms and speed very carefully and jot them down at various speeds.
Then go play with Grimm jeeper calculator and plug in different tire sizes to see what matches what you see on your jeep and report back
You will see that what @jjvw said (2x hub height) is pretty darned close. If you don't believe us, ask Blaine who will confirm that you are incorrect.
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Doing it 6 times carefully and having them within 1/8 isn't terribly inaccurate. Pretty easy to see when a mark on the side of a lug is touching the road. Not worth debating over 30 rpms.
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