Tire math maddness!!!

[JasonCzerak]

ok. this is fustrating!



ok so I'm at http://www.1010tires.com/TireSizeCalculator.asp?action=submit and I input 295/70/17 right? I get:



Section Width: 11. 61 in 295 mm

Rim Diameter: 17 in 431. 8 mm

Rim Width Range: Unrecognized tire size.

Overall Diameter: 33. 25 in 844. 55 mm

Sidewall Height: 8. 12 in 206. 24 mm

Radius: 16. 62 in 422. 14 mm

Circumference: 104. 4 in 2651. 7 mm

Revs per Mile: 625. 5



ok. here's the fustrating part. I took chalk, marked the bottom of the tire. drove on my drive way, re-marked more chalk, drove. . measure the distance between the 2 marks. and I got 101 inches... where did the other 3. 4 inches come from in this calc?



That puts things off a bit. with 101 inches, that makes it at 627Revs per mile.



Yes, that's splitting hairs. here but how does this work out? could this be a basic off set that an off road tire would have veruses a plain radial of this size? Or are these numbers caculated with possible tire expantion at high speeds.



I'll prolly just use 626 revs per mile and split the difference

 

[GO 4LO]

The calculation is assuming that the tire is manufactured at the exact size stated on the sidewall. Tolerances exist, and most tires run on the small side. Additional, when you put a load on your tire (like the weight of your truck), the distance between hub and ground shrinks, meaning the rolling radius is smaller than the theoretical radius. Heat will make a difference too... .

Chris

 

[biggy238]

He is dead on it. They account for tire variation (load radius for one)



626 sounds good to me. what are you working on doing?



Also, some manufacturers use the Carcass diameter and you would need actuall tire diameter, tread included, to be exact. (interco WAS bad for this. I don't know now)

 

[JVolpe]

Tire pressure and rim width can also contribute to the variance. A lot of variables that a generic calculator does not take into account.

 

[EBung]

Measure the back tires pumped up to 60psi and I bet you get a different reading.

 

[JasonCzerak]

Well, what does not make sence to me, if I mark the tred and roll and make 2 marks on the ground, that distance will never change (less less tred wares off, but that's a different story). So, if I'm at 10PSI, or 65psi, that distance the tred travels is the same, (think tracks on a dozer) I understand that if you measure from ground to the top of hte tire, that diamater is going to change with load on the tire, so that can't be an accurate way of measureing, you would have to measure that distance, unloaded.

 

[Paychk]

What you are saying is true... for a solid tirel that doesn't flex any. But the tire has flex in the side wall and the tire flattens out, this flat spot accounts for the difference you are seeing in math. No the actual circumference of the tire doesn't change, but the effective circumference does change because of the change between the rolling (tire loaded) diameter and actual (tire unloaded) diameter.

 

[Cattletrkr]

Take your spare and do the measurement.

 

[JasonCzerak]

Well, I got my ECU corrected. I have a friend here in Detroit area that works at a dealership and we went for a cruise. What I did prior was fun. here's the deal.



GPS located on windshield, right above the edge unit. I don't have all of the numbers here so I'll be general. I calculated the numbers for the edge unit, based on 101 inches (measures on the ground with chalk experiement. Went for a drive. This was about 615 Revolutiosn Per Mile. the GPS was a bit faster then the Edge calc. so I did 103 inches, or able what 610 RPM's would be. closer, the GPS, and edge unit were EXACT with this setting. . it was 103. 7" to be exact. I would guess I had 1/4 of a MPH error in there.



Off to the dealership. plug in the scan tool. correct the RPM's to 610, for for a drive. What I saw was the edge unit was WAAAAY of, like 7mph off. the speedometer was much close, it was slow by 1mph, as apposed to 3. Interesting. Math, based on what I put in the edge unit, 610 should be perfect?



so based on the what 1010tires.com said for the 295/70/17, i have 104. 4 inches of tire circomference. So 63360 / 104. 4 = 606RPM's. I changed the DRB tool to that, and bingo! GPS and speedo right on. infact, the real speed is about 1/2mph lower then the guage, I suppose 605 would be a bit closers. but this is getting fustrating.



if I'm at 606 to be right on based on GPS. How is 625 even close based on what 1010tires.com calculator.