New Mastercraft Tire Sizes for 3rd Gen Trucks

[TBrennan]

I jacked up my truck and measured the new tires to get 109. 625" in circumference. That was pretty close to the calculated of 109. 579". Using this measurement I should be using 578 revs / mile.

 

[Matt400]

I took a look at some Toyo's 34. 8" and they are listed at 596 also just like the BFG's you mentioned so it looks like the folks at Mastercraft may have misread the info or its simply listed wrong.

 

[TBrennan]

Just wanted to give an update on these tires as I've been using them for a few weeks. So far I'm very happy with them. They grip MUCH better on the dirt roads than the stockers I had and the truck actually rides better with them. I may have lost 1 mpg but I cant be sure until I get the correct rvs/mile into the ECM. These are "only" load range D at 3000 lbs each (although there are E rated tires that are only rated 40 lbs more) but since I don't generally tow or carry anything too heavy I think I'll be fine. If they provide any sort of lengevity they will be a great tire. I really like their performance so far, I think they are as good as the Goodyear MT/Rs, the true test for me will come when we get some real snow.



Now if they will make a C/T-C version (3 ply, E rated) we'll have an even better tire. Or maybe Goodyear will make a 35x12. 50R17 version of the MT/R.



Still working with Mastercraft to see if they can give me a better revs/mile number.

 

[JHardwick]

I jacked up my truck and measured the new tires to get 109. 625" in circumference. That was pretty close to the calculated of 109. 579". Using this measurement I should be using 578 revs / mile.

That method just wont work!



Once you set the truck down what happens?



The tire squats right?



I true 35" tire has a radius of 17. 5", or like 109. 9 something circumference right?



I'll bet you a paycheck and a beer the true 35" tire doesn't roll down the road with the exact centerline of the axle at 17. 5" off the pavement!



Tire pressure, wheel width, truck weight ... ... ... even new tires to old tires effect revs to mile somewhat.

 

[Matt400]

I don't understand that... are you saying that as a tire squats its circumference changes? The distance around the tire stays the same if its aired up or not and that surface area has to roll or its skidding.

I'll bet you a paycheck and a beer the true 35" tire doesn't roll down the road with the exact centerline of the axle at 17. 5" off the pavement!

I do agree you would win that bet.

 

[TBrennan]

JHardwick,



At first when I started thinking about this I had the same thought as you but as Matt400 correctly points out, the circumference of the tire does not change no matter what deformation it goes through for the most part. Unless the tire is skidding on the pavement or spinning on the rim (neither of which happens under normal conditions), one revolution of the axle moves the truck forward exactly one length equal to the circumference of the tire.



Now tire wear can reduce circumference over time. Truck weight and psi probably modestly effect the effective effective diamter.



I think I need to find a large paved area and do what someone suggested. Mark the tire and the pavement beneath it, move forward ten revolutions and make a new mark. Measure the distance on the ground and divide by ten for an accurate effective cirumference.

 

[JHardwick]

I think I need to find a large paved area and do what someone suggested. Mark the tire and the pavement beneath it, move forward ten revolutions and make a new mark. Measure the distance on the ground and divide by ten for an accurate effective cirumference.

I suggested this in another thread.



Better yet, weld your axle solid (Lincoln locker we used to call it), and put one tire at max pressure and the other at minimum pressure ... ... ... ... . the axle is gonna want to roll to the side with the least amount of air because it's "rolling circumference" has changed! We call this stagger in round track racing.



Lay both of these tires on their side and measure circumference ... ... . may not have changed size at all!



Rolling, or dynamic circumference can't be measured with the tire on it's side ans a string wrapped around it!

 

[Matt400]

Better yet, weld your axle solid (Lincoln locker we used to call it), and put one tire at max pressure and the other at minimum pressure ... ... ... ... . the axle is gonna want to roll to the side with the least amount of air because it's "rolling circumference" has changed! We call this stagger in round track racing.

Thats odd... I would call that creating a pull to the side by increasing rolling resistance. You can easily do that with out welding, set your air at 40 on the left side tires and 80 on the right and you will pull left for sure. The only time I have ever seen a condition of "rolling circumference" change while in motion is on a dragster where extreme wheel speed causes the tire to grow.

 

[realsquash]

You guys should put a yardstick on the ground and mark the ends of it. Roll the truck over it but not off and see if the stick moves forward or backward...



Andy

 

[JHardwick]

I posted this and deleted it. Our sprint tires would do the same. 104" with 9# air on the right and 93" with 4# of air on the right ... ... ... under speed, we didn't really know what they were, but we knew they stood up and gained several inches!



Look, here is a simple test:



Use the chalk mark method, but do it with no weight on the tire and then mount it on the truck and do the same test ... ... ... come back to me when you have the results ... ... ..... i'll be waiting on that beer, lol.

 

[Matt400]

Use the chalk mark method, but do it with no weight on the tire and then mount it on the truck and do the same test ... ... ... come back to me when you have the results ... ... ..... i'll be waiting on that beer, lol.

Too much work... plus this is fun right here.



I like Andy's view. .

You guys should put a yardstick on the ground and mark the ends of it. Roll the truck over it but not off and see if the stick moves forward or backward...

I don't see how the stick could move and if it doesn't move then the tire travels the same distance aired up or down.

 

[TBrennan]

JHardwick,



I never said an unloaded tire and a fully loaded would roll the same, but the difference would be small and certainly not as much as would be expected from the flattening of the tire under the axle and using that as the "real" radius of the tire.



An unloaded tire would roll on it's center more than a loaded down tire which would slightly effect the diameter. There would be a small difference but I'd be suprised if it was of much significance.



I'll send you a case of beer if you make a nice machined valve cover for us 3rd gen's. Oo.

 

[Matt400]

Well I guess I gotta pull my foot from my mouth here, I do remember having once owned a dual wheel Motor Home how important matched air pressures were in the duals and that not having them matched will cause the higher inflated tire to drag the lower inflated tire as it will travel farther so I started looking for the info and found this:



Matching pressures is especially crucial on dual assemblies because an inflation mismatch greater than 5 psi means that the two tires in a dual assembly are now significantly different in circumference.

But, because they’re bolted together, they have to cover the same amount of road in a single revolution. So, the larger tire drags the smaller one. Very fast or irregular wear – especially on the tire with less inflation – can be the result.

In one test we made, a 5 psi difference created a 5/16” difference in tire circumference.

In a single mile, this 5/16” difference causes the smaller tire to be dragged 13 feet. In a typical year’s usage of about 100,000 miles, that comes out to 246 miles.

Again, it doesn’t sound like much until you remember that the tire is not rolling an extra 246 miles, it’s being dragged. In other words, it’s as though you spun the tire against the pavement for 246 miles! At 55 mph, that would be about 4-1/2 hours of wheel-spinning.



Sometimes we forget what we learn, worse yet start running off at the mouth to boot!

 

[Ol'TrailDog]

Thanks Matt for the info. Thats a real useful warning to us running duallies.

 

[TBrennan]

Well, I just got an update from Mastercraft. They admitted the 613 number was incorrect and the correct number is 593. That puts it about exactly half way between the original 613 and theoritical 578 based on diameter alone.



Assuming this is accurate, the theoritcal number isn't that accurate and the chalk test with the tire mounted would probably be the only way to measure with any accuracy. I was way off on assuming the difference wouldn't be that great. I knew the 613 number wasn't right though since other tires about the same size or slightly smaller used lower numbers, which got me started on this whole thing.



I'll have to get the number in the PCM and see how accurate it is. It sounds about right given other numbers for other tires about the same size.

 

[JHardwick]

I'll take that Budweiser now :-laf



I did look into the billet valve cover briefly. Looking at a $500 chunk of aluminum before I ever sink an endmill into it

 

[TBrennan]

JHardwick,



I was definately off on guessing the effect of a tire loaded down by vehicle weight but the main point I was trying to make is that you can't go by the center axle to ground distance as an effective diameter either. That is what I thought you were implying, which would be equally wrong. It's a combination of factors that apparently are only easily measured in real use.



$500?? Ouch. Anyone ever get that Cummins version working with out breather set up?

 

[JHardwick]

My point wasn't necessarily that you could measure the axle centerline height and use that as the radius, but rather the rolling radius would make a pretty noticable change in the number of rev/mile as opposed to wrapping a tape around an unloaded tire.



There are only so many inches in circumference no matter how low the tire is squatted as somebody pointed out earlier, but there is a difference in the rolling radius and tire srcub involved and hence tire wear.



I guess I dont get that beer :-laf

$500?? Ouch. Anyone ever get that Cummins version working with out breather set up?

hmmmm, can you explain?

 

[TBrennan]

That should have read "our" breather set up. Apparently there is a nice metal Cummins valve cover already made for the ISBe engines but the breather setup is different than the ISBe as implemented for the Dodge trucks. Someone was working around this with some modifications but I don't remeber if they solved the whole issue.

 

[Matt400]

My point wasn't necessarily that you could measure the axle centerline height and use that as the radius, but rather the rolling radius would make a pretty noticable change in the number of rev/mile as opposed to wrapping a tape around an unloaded tire.

Probably where things got off track was when I posted- Wrapping a string around two same size tires but different brands always shows a different circumference

I didn't mean to use that as a way to determine tire RPM.

I guess I dont get that beer :-laf

PM me your email address and I will send you one of these:

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