I thought my new bigger.....

[JVolpe]

Here is a tire calculator, different online calculators return different answers.



http://www.1010tires.com/TireSizeCalculator.asp?action=submit

 

[JHardwick]

here is the math get out your calculator.



33. 5" diameter tire converted to feet for easy math = 2. 792' Diameter.



2. 792' X 3. 1416 (pie) = 8. 7702' tire Circumference.



5,280' (ft in 1 mile) / 8. 7702' = 602. 032 rotations per mile.



hope that helps

This math is correct, but does not take into account tire loading and squat. 33. 5" may be the measurement with the tire laying on its side, but mount it and put a load on it and it may roll like say a 31. 5" tire.



The easiest way I've seen to measure this, is to chalk mark the tire and a corresponding mark on the ground. Roll the truck forward say 10 revs of that tire and make another chalk mark. Measure the distance of the chalk marks and divide by 10. Plug that number in the above formula.

 

[JVolpe]

The question of what is the correct RPM setting for bigger tires is very confusing at best.



For BFG Rugged Trail 265/70R/17 that came stock on my 4x4 the BFG documentation says 658 RPMs.



For BFG TAs 315/70R/17 the BFG documentation says 601 RPMs.



I researched many forums and used many calculators when I changed to the BFG 315s. I PMed user that used a GPS system to calculate his exact RPMs on a similar 2500. He came up with 605. At least 2 or 3 of the online tire calculators came up with 605. 3 RPMs for the 315s.



I had mine programmed for 605 RPMs by the dealer. I have done my interstate mile marker checks many times for straight 10 mile stretches. OD has never been off more that 1/10 mile. I have made a 92. 5 mile trip 3 times that I know the mileage and still am within 2/10 of mile.



Can not tell you what the variations are caused exactly, but I think that mine are as close as I can get. Just my experience.

 

[KFette]

Tire Calculator

Just for S & G's, thought this was a neat calculator... nevermind that it says "Miata", it still works.



http://www.miata.net/garage/tirecalc.html



KFette

 

[HootieF]

Do you know someone with a hand held GPS?

 

[JVolpe]

I cant find who I PMd before. I dont have one yet, but it is on my Christmas list for me to buy me.

 

[BlueRam2500]

When I put my 35x12. 50R17 BFG Mud Terrains on my truck, the manufacturer gave me a revolutions per mile of 594 or 598, I am not sure-it was definetly not over 600. At 70MPH with the 4. 10's, the tach is a 2000 RPM's. Not sure if this helps you in any way.

 

[JVolpe]

BlueRam2500,



598 is the BFG Documented RPM for 35x12. 50R17 BFG Mud Terrains.



I assume that that number is based on the optimum rim width for the specified tire and recommended inflation for vehicle load.



I suspect that anything within a couple digits is going to be close enough.

 

[Beast2B]

C, D or whatever... go by the weight rating and pressure on the tire. My 37" BFG are rated 3525 at 50psi. I tow 12k rated trailer loaded to about 9500lbs. The tires don't get hot to the feel or per a heat sensor. I have 25k miles on MUDDERS towing about 5k miles (at least 15k miles left). Sway is WAY more evident than stock, but it's because of the sidewall heigth and tire width versus the wheel width more than the rating. Use a little common sense and as long as the specs are met, you are WELL within the safe capacities... everyone knows the ratings are so low to meet liability issues!

 

[JHardwick]

everyone knows the ratings are so low to meet liability issues!

Liability issues they are! Don't cause an injury accident and be found overloaded for the truck or tires ... ... ... ..... it'll be turned into a criminal case.

 

[DPelletier]

Not trying to be negative, but the load range C 35psi tires aren't even enough for the front tires of an empty cummins equipped truck IMO. I know lots of you guys are running D rated tires on your trucks and if you make sure you are within the tire ratings and IF sway isn't a big issue, you can get away with it. Personally, I'll never use anything less than an E on any of my 3/4 ton or heavier trucks, but I will admit that the larger 315's etc look better than the tiny stock E's and that there aren't alot of choices out there (yet)



Dave

 

[White Ram1]

Well, All I can go on is what is on the tire side wall in raised letters, 2450 load rating at 35psi. We'll see what happens with a car trailer and a car in it once I get enough miles to pull, only up to 200 miles since I picked it up. Unloaded the tire don't even seem to hold any temp. at all.



I did get in to the dealership on Monday. I had the Mopar remote starter installed. While it was there I asked them to reset my Rev's per mile due to the new tire size. I started to show them the calculation listed in this thread and I was informed they have a computer that tells them what to set it at.



So I pick up the truck after lunch and I drive off. I turned on my handheld GPS and when the GPS said I was doing 70 the speedo said I was doing 85. I took it back to the dealershiop this morning. I asked what rev's they set it to. They looked it up on the ticket and said they set it at 603 rev's per mile.



We called Dunlop customer service, but they haven't got back to us yet for their recomendation of Rev's per mile.



So we reset it this morining to 597 rev's per, due to someones comment in this thread, and when the Speedo said I was going 70 the GPS said I was going 68. At this point I'm sure 598 or 599 will get it pretty dead on.



But in the a. m. I will go back and have it set to 598, drive around with the GPS and if it's good, then I'm done.



BUT... . I'm still amazed at how little differance in the RPM's there is between the stock 28. 5 tall tire and the 33. 5 tall tire. At 70 via GPS the RPM's were at 1850ish, and with the stock tires at 70 the RPM's were 1950ish to 2000. I was hoping for a bigger drop in RPM's.



Anywho... Thanks for the input guys, at least yall pushed me in the right direction for the corrections I needed, which I would have been clueless about coming from the PSD days.

 

[DPelletier]

Well, All I can go on is what is on the tire side wall in raised letters, 2450 load rating at 35psi.

The front axle of my truck weighs 4730 lbs empty. 2450 x 2 = 4900 lbs. 4900 - 4730 = 170 lbs. I hope you're a light guy who likes to travel alone!



Anyways, I'm not trying to be a smartazz, but you screwed up. Those tires are too light duty for your truck. Sorry.



Dave

 

[AKAMAC]

Whiteram1,

The problem you will have is steering response. When you have an overloaded tire your side wall will flex, this will cause under steer. That is the effect of turning the wheel and not turning immediately. Think of an underinflated tire. The tire installer/seller should know of the inherent weight ratings associated with HD P/U's. Go talk with them and find something safer.



Welcome aboard, Mike

 

[White Ram1]

D,

No sweat, I don't think you're a smart azz. I'm sure you're just on of those dudes who always thinks he's the smartest guy in the room. I know plenty of your kind. But as you grow older just remember Two plus Two isn't always four. :-{}



I'll report back to ya at 40K miles and let ya know if I'm driving on the rims.





AKAMAC,

No need to explain understeer. Here, check this out. www.SandersMotorsports.com It's just a job, but it's what used to pay the bills! Pretty much retired now-a-days other than a one here and one there deal. Believe me, I know plenty about understeer, push, tight, loose and free. And even some terms we can't use here.



And I know plenty about crew chiefs who think they know how to set up a car, what tires to use, what tire pressure to run, and how many laps we can make it on a tank of fuel just because the calculator said so! Yea right! :-laf



See ya in turn one!

 

[JHardwick]

I thought my new bigger TIRES would drop my RPM's.



I went from stock size to 35's, but at 70 MPH I'm still dead on 2,000 RPM's before and after the change of tires.



I'm new to the Cummins game, maybe there is something in the computer I need to modify to drop the RPM's???????



Any input???

Let him go :-laf

 

[DPelletier]

Let him go :-laf

Yeah, fair enough. Ya try to help some people... ... ... ... ...



Dave