Tires&mpg: tall for tall gearing, or small for less rolling resistance?

[pwknapp]

I was running Truxxus M/T's. Very capable here in the snow and stuff for huinting. Very soft tread. Two hunting seasons, about 20k miles. Last winter they definitely did not have the traction when new.

 

[Dl5treez]

I haven't gone away from stock yet, but have researched some for MPG and agree with somewhat tall/skinny. Don't forget tread -- I think there was a Cummins paper that said 60 - 70% of rolling resistance is in the tread. Go to a summer ribbed pattern, that's a big savings. I think concensus was Michelin XPS Summer Rib was excellent as skinny/min tread/tough/E range, but pricey. Search on XPS, I think there's a thread or two out there on tires for MPG.

I didn't forget tread, just thought it was so obvious that it didn't need to be presented... ... . guess I was wrong. :-laf



High traction treads have more rolling resistance, but "high traction" does not necessarily mean aggressive looking tread.



(It's hard for me to explain my thoughts without leaving out important details... . )

 

[Diesel Nut]

I ran the Truxes MT's on my 02 and I did see an increase in milage with the same size (285) BFG MT. The tread was WAY soft on the truxes MT's. Tread composition (hard or soft) makes a differense as well as the aggressiveness of the tread. AT's usually net better milage, bot fuel and wear than more agressive MT's. Mt 19. 5's are a pretty streetable tread that wouldn't do much anywhere else except dry, firm dirt or gravel. But, even with a fairly soft compound they have been running for 70K and will run another 50. Different type of tire, yes, but the tread and size play into it. Larger tires of a streetable tread will not produce as much resistance IF they are properly inflated because they turn slower, which helps procuce less heat, and they have more tire to dissipate heat. I think. It sounds good, right? Someone correct me if I am wrong.

 

[Sarge]

Does the 19. 5" Ricksons increase the tire height? Someone told me a while back that the 19. 5 wheel didn't help because the tires you could get were a shorter profile. I never was able to verify or dispute that. What is the experience of the members?



I have considered Ricksons due to the 4. 10's on the Dually, and I can't find any taller tires than stock that will fit on the dually wheels... ..... Sarge

 

[Matt400]

I would think the added weight of the 19. 5's would use more fuel to spin up.

Smooth tread, narrow lightweight tire on an alloy wheel that keeps RPM's at 1600-1800 would give the best mpg IMO.

 

[EricBu12]

MY set... .



Before, I had the stock 265-16 "E" tires..... Did good mileage around 20-22



I bought the ricksons Forged aluminum rims that are called "eliminator" uses the stock hub cap and are 8 lug.



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inside of the rim



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The tires I have are Michelin XDE-MS 245/70R 19. 5"..... "F" rated... . 12 ply Rated at 4080#... @ 95 psi



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The 19. 5 is on the truck, the tire on the left is the stock 16"



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Here is the tread of the tire. It is 21/32 nds deep and should last 90K miles



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and lastly, the backspacing is out a little but since the section width is skinnier, it is very flat to the side.



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So, now the Diameter is about 33"... . BUT, the width is the same as a 265-16 tire... . . I bought these tires for hauling a 36' 5th wheel but the added benefit of easier rolling resistance... .



Now, on that... RR ..... I have read that The majority of drag of vehicles from 0-55 is caused buy mechanical drag and rolling resistance.....



Anything over 55, the wind resistance starts increasing exponetialy. Can't remember but it was a certain percentage of extra energy or MPG per 10 mph over 55. So If you are a slower driver, the better tire will work good.

 

[Sarge]

Thanks for the pics eric... ... . now if I can justify buying SIX of them..... ouch... ... .

 

[EricBu12]

For the Dually, the Forged aluminum won't work..... they have the steel rims... some are chrome or powder coated... the neat thing is they are cheaper, the bad thing they are heavier by twice.



I bought the tires locally at a shop for $265 ea. and mounting and ballance was free.



5 tires and rims was $3200 total

 

[Hohn]

MY set... .



Before, I had the stock 265-16 "E" tires..... Did good mileage around 20-22



I bought the ricksons Forged aluminum rims that are called "eliminator" uses the stock hub cap and are 8 lug.



So, now the Diameter is about 33"... . BUT, the width is the same as a 265-16 tire... . . I bought these tires for hauling a 36' 5th wheel but the added benefit of easier rolling resistance... .



Now, on that... RR ..... I have read that The majority of drag of vehicles from 0-55 is caused buy mechanical drag and rolling resistance.....



Anything over 55, the wind resistance starts increasing exponetialy. Can't remember but it was a certain percentage of extra energy or MPG per 10 mph over 55. So If you are a slower driver, the better tire will work good.

Eric, any difference in MPG with the Ricksons over the 16" Michelins?

 

[EricBu12]

In towing? Yes! I was like getting 7-8 mpg towing my raptor trailer..... with the 19. 5, about 10-11



In solo, I was lowering the pressure for comfort... . with the higher prices of fuel, I aired them up and the ride is rougher, but it rolls much better so I 'can' get over 20-22 solo but no long distance solo yet to see but I notice a difference in how it rolls though... .

 

[TJany]

Hohn-I run 315/75 R16 Cooper Discoverers on a stock truck (well, K&N and 4 inch straight) on the highway if i keep my foot out of it (67-70 mph) i can run 19. 5 hwy and 15. 5 local (not hard off the line)... that being said, here in the east its flat compared to where you are, i would imagine the motor will be lugging to hard for you to see that kind of mileage, but who knows without tryin it... i have 3. 54's



Tim

 

[Hohn]

Well, I already clear 20+ hwy, so I'm not anxious to make a change in something in search of MPG only to LOSE it instead!!



jlh

 

[Loren]

Hohn,



First, a question: Do you know that smaller tires have lower rolling resistance than larger ones? I would not assume that this is the case. In fact, it seems to me (with no data to support it) that a larger tire at the same pressure will have lower rolling resistance.







Here's what I have found about engine RPM vs. fuel mileage from installing an auxilary US Gear overdrive transmission in my truck. I think this data was included in my article on the subject that was in the magazine a while back.



6-direct 6-over

70 mph 1875 rpm 1550 rpm

75 mph 2000 rpm 1675 rpm





6-direct 6-over

70 mph 20. 8 mpg 23. 6 mpg

75 mph 18. 8 mpg 20. 2 mpg

(Sorry about the table. I can't get the columns to line up. The "6-direct" should go over the middle column and the "6-over" should go over the right column. )



This shows that reducing RPM can give a significant improvement in fuel mileage. In my case, I was running 3. 54 gears and Rickson 19. 5" wheels, hence the very low RPM numbers.



When I originally switched from the stock 16" wheels to the 19. 5" Ricksons, I found that my gearing changed by only about 4%, and the mileage improvement, if any, was not significant. A 4% change is the equivalent of 1600 RPM dropping to about 1540 RPM.



If I had gone with tires much bigger than those that came with the Ricksons, they would not have fit in the wheel wells.



I do think that the concerns about lugging this engine are overdone. Certainly you do not want to actually lug it, but Cummins' own data shows peak torque at 1600-1700 RPM for the 12-valve engines. That means that they tested it at full throttle at that RPM. As long as you shift down when the engine can no longer pull the load and RPM is decaying through 1600 RPM, I doubt that you can hurt the engine. (My opinion only - Procedure not approved by Cummins, Dodge, or anyone else. )



My evaluation of your situation is that you are already getting excellent fuel mileage. Change your configuration at your own risk.



Loren

 

[Bob4x4]

That 1600 rpm when towing may not be bad for the engine but to tow at that rpm-if you have decent torque can play he77 with the rest of the powertrain. I have a customer who is anal over fuel mileage he pulls rvs for a living now. He insisted in pulling at 1600 rpm for the mileage,but he has 3. 54 gears with a gearvendor the 6 speed doesn't come out of 6th very often when you have 800lbft of torque. He has broken his nv241hd twice now we have a 271 in it so far so good. :{



Bob

 

[Cooker]

I agree with Bob. At 1600RPM the engine may be happy but the drivetrain is ****** (especially with a load)

 

[Hohn]

Hohn,



First, a question: Do you know that smaller tires have lower rolling resistance than larger ones? I would not assume that this is the case. In fact, it seems to me (with no data to support it) that a larger tire at the same pressure will have lower rolling resistance.

Well, as I reason it through, it would seem that larger tires would have to have MORE rolling resistance. I could be wrong, but follow my reasoning and see if you agree.



What causes rolling resistance? Rolling resistance is rooted in the fact that a semi-rigid ROUND tire is attempting to conform to a (mostly) flat rigid road surface. It must deflect to do so.



Think of applications where low rolling resistance is a main design goal in a wheel/tire. On racing bicycles, tires are very very thin, and run very high pressure. The small size reduces the contact patch area, and the high pressure increases tire rigidity, BOTH of which lower rolling resistance.



Same thing for commercial OTR tires. They are relatively thin for the height (compared to a 75 aspect ratio LT tire), and run high pressure for rigidity.



So now let's think about how that tire deflects (or deforms) to conform to the road surface. First, we can guess logically that the more rigid a tire is, the less it will deflect for a given load. This rigidity can come from inflation pressure or from carcass design. So, we can see the effect of tire DESIGN on rolling resistance.



The effect of tire WIDTH is important as well, because it determines contact patch size for a given deflection. So say you have a contact patch that's 2 inches long by 4 inches wide. If we widen it so that it's 2 inches long by 8 inches wide, then we've just DOUBLED the rolling resistance. Now, in the real world, it would be slightly less than double, because the same tire pressure spread out over twice the area would make the tire more rigid for a given load, and reduce rolling resistance somewhat, so it would be less than double.



The effect of tire HEIGHT (or diameter) can go either way. On bicycles, a taller tire rolls better. But were talking about a tire that's very narrow and high pressure. The lower rolling resistance comes from the fact that the tire has to deflect less, in terms of distance.



You can see this for yourself if you picture an infinitely large tire. Such a tire would not have to deflect at all, because relative to the flat road surface, the tire is flat as well-- thus no need to deflect.



Before you go and think that a taller tire has less rolling resistance, realize that there are other factors that MORE than cancel this out.



First, in most commonly available 75-series sizes, a taller tire is wider. Wider means more area must deflect, and thus more resistance.



Second, a taller tire needs LESS tire pressure to wear properly, and thus decreases tire rigidity. However, there is a mitigating effect that that tire doesn't need as much rigidity because of the larger contact patch.



So in the end, it boils down to one pivotal question: is it better (wrt rolling resistance) to have a larger contact patch deflecting LESS, or a smaller contact patch deflecting MORE??



I believe that it's better to have a smaller contact patch deflecting more. Anecdotal evidence for this is strong. As you increase contact patch size, area increases faster than the amount of deflection decreases, so your left with more resistance. For example, if you had an 8 sq in patch that had to deflect 1", you'd think that doubling area to 16 sq in would drop deflection to 1/2", but it doesn't drop that far-- thus, a net increase in rolling resistance. I think of this in terms of the VOLUME of an imaginary space made by the difference between the the tires deflected shape and its normal (no load, perfectly round) shape. The greater the volume of this imaginary space, the more rolling resistance.



Also consider steel train wheels, designed for the ultimate in low rolling resistance. Tiny contact patches, and essentially ZERO deflection. Great for load bearing, horrible for any kind of traction. They're surprisingly not that large, given the loads on them.





OK, that's my thinking. Tell me where I'm right or wrong.



jh

 

[TJany]

ok, while agreeing with your thinking above, theres also another aspect to think about. I agree that a taller tire with a smaller footprint will yield better mileage, however i think it would only be in a highway use scenario. a taller tire is going to weigh more, and because of the larger diameter, take more force to get rolling etc... therefore in a stop and go setting taller tires could hurt your mileage... theres got to be a fairly defined line somewhere, at which point the tire is to tall to benefit mileage... maybe if i had paid attention in differential equations i'd be able to figure that one out...



Tim

 

[RustyJC]

... theres got to be a fairly defined line somewhere, at which point the tire is to tall to benefit mileage...

I think we see evidence of this in this and other threads where the 4. 10 trucks get better mileage with taller tires but as the tires get taller and taller on the 3. 54 trucks, mileage starts dropping after some undetermined point. Of course, this thumbnail analysis ignores tread width, tread design, tire compound, amount of "lift" on the truck, etc.



Rusty

 

[Boondocker]

How are you really going to use your tires?

I certainly eluded to the concerns just expressed by Tim and Rusty in my post (#5). That is, how are you really going to use your tires? For instance, if you know that you do 95% of your driving on the highway at nominal highway speed with a predictable load, then by all means bias your tire selection to that requirement. In another scenario, if we're talking about a guy who drives four miles to work and home for lunch every day, through snow in the winter time, then that's a significantly different requirement that may not adequately be met by a tire biased to the long haul highway scenario.



Clearly, the vehicle manufacturers equip their vehicles with tires that are the most suitable compromise of a wide range of driving requirements and conditions, not to mention cost. Naturally, the OEM compromise may not be the best match for you, but most likely, you still need to compromise. The trick is to bias your compromise closest to your actual use profile. That being said, my use profile must be pathetically average since I intend to replace my tires with OEM spec.



Neil

 

[Hohn]

ok, while agreeing with your thinking above, theres also another aspect to think about. I agree that a taller tire with a smaller footprint will yield better mileage, however i think it would only be in a highway use scenario. a taller tire is going to weigh more, and because of the larger diameter, take more force to get rolling etc... therefore in a stop and go setting taller tires could hurt your mileage... theres got to be a fairly defined line somewhere, at which point the tire is to tall to benefit mileage... maybe if i had paid attention in differential equations i'd be able to figure that one out...



Tim

I ignored tire inertia, because this is something that you recover when you coast.



But you're 100% right, around town all that extra rotational inertia not only hurts MPG and acceleration (regardless of gearing), it hurts braking performance, too.