Rolling Resistance? Please define

[CAgnoli]

Guru's,

As I read these posts, I see reference to a common term: Rolling Resistance.



Excuse my ignorance, but what exactly is rolling resistance?



Lastly, My truck has the factory tires, and 3:73 gears. I would reason that if I put taller tires on, and had the speedo recalibrated, that I could see an increase in mileage, with the trade off being a loss of low end acceleration?



Am I right, or does this thinking not apply. I pose this question, because I was under the impression that a truck that makes this much power, could attain 70MPH at a lower RPM, and not use more fuel? OR does the extra power that the engine makes, require additional fuel at lower RPM's?



I'm Confused Maybe I should just stick to computers, at least they make sense to me



Thanks!



Chris

 

[EEngel]

CAgnoli,



Rolling Resistance is most commonly looked at as the energy required to overcome the friction between the tire and the road surface. Since the 315 has a wider face than the 265, it requires more energy to turn it over.



A good example is an ice skate. The load of the person is distributed across a pair of 1/4" blades. If you push the person, it takes very little energy to move them. If that person had 2x4's on their feet, it would be much more difficult to slide them. Note that this example is not 100% accurate due to the higher friction coeffiecient of the wood, but serves as an example.





As for the energy required to turn over the tire. Well, it takes more energy to get a larger tire spinning. The mass of the tire is further from the center point of the power.



A good example of this, well head off with the kids to the local playground. If you have your or some unknown kid sit on the outside edge of the merry-go-round and you spin them upto xx rpm it will take much more energy than if the kid moves to the center.



Just trying to help...

 

[CAgnoli]

Okay, I'm beginning to understand the rolling resistance. Now in answer to my second question, even though the engine RPM's should be lower at a rated speed with larger tires, the amount of fuel required will be higher?



Sorry, I'm a little thick.

 

[LRoy]

No simple answers...



As a very rough rule - especially under light loads - lower engine RPM will give less fuel consumption. The faster the engine spins, the more energy is expended on overcoming internal friction (piston rings vs walls, pumping losses, etc).



I haven't done any controlled tests - but for a given speed - say 60mph - the overhead seems to show that 6th is noticeable more efficient then 5th.



But - raising the vehicle (due to taller tires) would usually increase air drag. Wider tires would usually increase drag and rolling resistance. Look at road bicycles - to lessen rolling resistance they use extremely narrow and high pressure tires.



Leonard

 

[SRadke]

Dropping rpm at cruise helps to a point. Anything below 1700 with a Cummins will hurt your mileage IMO. That's like the bottom of the "sweet spot".



On the other hand if you have a large load (realtive to engine power) many turbocharged engines become inefficient at lower rpm because they don't get enouh air.



In general with a Cummins pickup try to keep your rpm between 1700 and 2000 for the best milage. If you're cruising empty at 2300 you would benifit from taller tires despite it taking more power to get them spinning.



-Scott

 

[EEngel]

Getting taller

CAgnoli,



I guess what you really need to look at is your time on the highway and your time in the city.



The larger tires will lower your rpms but take more juice to get them going.

 

[CAgnoli]

I drive roughly 1000 miles per week. Almost 100% of which is highway, empty, at 70MPH. Basically, I'm just making sure that by increasing my Truck's visual appearance (Taller tires, fender flares) that I don't kill my wallet in the process.



Today, I fill up every 535 to 565 miles. I'm averaging almost 18mpg in the truck with 5200 miles on it. If I can put the taller tires on it, and still see 18mpg empty, with the cruz on, I'm content. If not, I'll just live with the smaller tires.



I really enjoy the truck, and the only additional modifications I'd like to perform would be a larger fuel cell, to minimize my stops. (Considering it takes me almost ten minutes to fill the tank to the top of the fill neck... . but I get an extra 60 to 80 miles..... so I do it)



Thanks for all the input. Maybe I'll wait and see what results people are getting in four or five months, with 10k + miles on thier trucks, with similar gearing.



One last question, manual locking hubs..... if I see that group purchase, does it make sense in my position?



Thanks!

Chris

 

[SRadke]

It would make sense but from what I gathered in that thread they sound cost prohibitive. Minimal mileage gain and quite expensive as hubs go.



-Scott

 

[Sherman]

Originally posted by CAgnoli

I drive roughly 1000 miles per week. Almost 100% of which is highway, empty, at 70MPH. Basically, I'm just making sure that by increasing my Truck's visual appearance (Taller tires, fender flares) that I don't kill my wallet in the process.



Today, I fill up every 535 to 565 miles. I'm averaging almost 18mpg in the truck with 5200 miles on it. If I can put the taller tires on it, and still see 18mpg empty, with the cruz on, I'm content. If not, I'll just live with the smaller tires.



I really enjoy the truck, and the only additional modifications I'd like to perform would be a larger fuel cell, to minimize my stops. (Considering it takes me almost ten minutes to fill the tank to the top of the fill neck... . but I get an extra 60 to 80 miles..... so I do it)



Thanks for all the input. Maybe I'll wait and see what results people are getting in four or five months, with 10k + miles on thier trucks, with similar gearing.



One last question, manual locking hubs..... if I see that group purchase, does it make sense in my position?



Thanks!

Chris

You may be a candidate for an aux box by gear vendors or us gear,they will knock your rpm's down by 20-25 % which would give you 20 to 25 % fuel savings,in your particular case with such high unloaded freeway mileage you should look into the aux box,I have done a lot of research in this area and am mulling over a us gear aux box now ,you can PM me if you are interested.

 

[CAgnoli]

Is there an aux box for the 2004. 5? It seems that all of the tweaks and goodies that worked on 2003, 2004 models, are somewhat defunct for the "600"??



Thanks for all of the valuable input.



BTW what impact does such a device have with the factory warranty?



Chris

 

[Sherman]

Originally posted by CAgnoli

Is there an aux box for the 2004. 5? It seems that all of the tweaks and goodies that worked on 2003, 2004 models, are somewhat defunct for the "600"??



Thanks for all of the valuable input.



BTW what impact does such a device have with the factory warranty?



Chris

Does not affect the warranty

 

[SRadke]

I believe the same unit should work because they still use the same transfer case but they are a little spendy.



-Scott