Hope this is Right

[JEly]

OK... ... ... ... . went to the dealer I bought the truck from on Monday for my speedo recalibration and they were unsure if it could be done so I went this morning to the in town dealer and 5 min. and 22. 50 later it was done.

Now when I ask him what number he used he said 587... ... ... I told him about the TDR and 601 number and he said this is how he got his number

He measured the circumfrance of the wheel which was 9' 1" (he used 9') and divided that into 5280------feet in a mile-----and came up with 586. 6

I dont know if this is accurate since I dont have a GPS



Any Thoughts



Thanks

JIM

 

[Vaughn MacKenzie]

JEly, using the diameter numbers listed on BFG's website I come up with 584. 5 revs/mi. But on the same spec page it says 601 revs/mi. Something doesn't jive on BFG's website.



I think your dealer did it right based on what he actually measured so you should be OK. Check it against some mile markers.



Vaughn

 

[rbattelle]

Tires aren't quite round when they're rolling at speed, so maybe that's the reason for the discrepancy? Plus there will almost certainly be some slippage, increasing the revs/mile.

 

[Northerner]

I must have been sleeping . Can DC do a recalibration for 315,s now? This would be good news .

 

[JHardwick]

I'd say 601 is the right number.



What that tech did in theory was correct. But, when you set the truck down the axle centerline becomes closer to the ground because of tire squat. This effectively creates a smaller diameter and more revs/mile.

 

[How'dHeDoThat]

Not sure I agree....

Originally posted by JHardwick

I'd say 601 is the right number.



What that tech did in theory was correct. But, when you set the truck down the axle centerline becomes closer to the ground because of tire squat. This effectively creates a smaller diameter and more revs/mile.

If the tire was an oval and it made the truck lower to the ground, would it turn less revolutions? I think not...



If you roll an egg that is 4 1/2" (oval)diameter and a ball that is 4 1/2" diameter they will turn the same amount of revolutions to get from point A to point B.



Think about it... every linear inch of that tread will have to run out over road surface to make a revolution. So wouldn't it be X revolutions squatting or not?



James

 

[JEly]

I did a mile marker to mile marker check and it is very close and this is the best way as of yet I can check. I know in 4 miles I was a 1/10 over..... 4. 1

If the number is 601 thats fine but for me to have the tech change it I need some sort of proof rather than me just saying that is what it should be... ... ... ... ..... he has been to tech school

I did go to BFG's web site but couldnt find anything that talks about it so I guess I am stuck with this for now

 

[Vaughn MacKenzie]

Well if you are off 0. 1 every 4 miles then to correct it would mean dividing 587 by 4. 0 and multiplying by 4. 1. And guess what. . . the answer comes out to 601.



At 0. 1 error every 4 miles I wouldn't lose too much sleep over it, most odometers aren't that accurate anyway.



Vaughn

 

[JEly]

Doing the math you did would it be worth going back to the dealer and showing the tech and asking him to put in 601 for the number if he would or just leave well enough alone

 

[JHardwick]

Re: Not sure I agree....

Originally posted by How'dHeDoThat

If the tire was an oval and it made the truck lower to the ground, would it turn less revolutions? I think not...



If you roll an egg that is 4 1/2" (oval)diameter and a ball that is 4 1/2" diameter they will turn the same amount of revolutions to get from point A to point B.



Think about it... every linear inch of that tread will have to run out over road surface to make a revolution. So wouldn't it be X revolutions squatting or not?



James

An egg is rigid and has nothing to do with a piece of rubber filled with air which is flexible. Dynamically, a tire has less circumference than what it measures statically with no load.



What is important is the diameter correct? Lets break that down to a radius which is the axle center line to the ground. Now, we have a solid axle and a tire at each end. 1 tire has 100PSI of air and the other 10PSI of air. Now add a load, the tire with 10PSI will squat lower and give you a smaller radius ..... no?



Now, I want to see you drive this axle straight with both tires maintaining 100% traction ... ... it's impossible.

 

[rbattelle]

I agree with JHardwick. Tires are not a perfect circle, it's as simple as that. I would go to 601 rev/mile.

 

[mgonske]

Originally posted by JEly

I did go to BFG's web site but couldnt find anything that talks about it so I guess I am stuck with this for now

Try this BFG pdf file on the web. Check the 3rd-to-last row, and the next-to-last column. The difference in rolling radius vs. static radius others have described is the reason the tire manufacturers publish the revs per mile numbers. You could always print this data sheet and show it to the tech if it would make you (and him) more comfortable.

 

[Vaughn MacKenzie]

Originally posted by JEly

Doing the math you did would it be worth going back to the dealer and showing the tech and asking him to put in 601 for the number if he would or just leave well enough alone

Another way to think about it is how soon do you want your warranty to expire? If you're rolling up miles more than 2% faster than actual that adds up over time. If you have a lot of miles left in your warranty it would be worth the effort to correct it. Your odo is running fast, and will run faster as your tires wear. To buy some extra miles I would make them do it again, and take a printout from BFG showing their error and maybe if you're lucky they'll redo it for no charge (probably a long shot).



Vaughn

 

[David Muench]

I've verified the 601 number with a GPS, it's dead on for me.

 

[JEly]

I printed the info out on the BFG site and will take it with me first thing Monday morning and get it changed.



Thanks again

JIM