New 19.5s for 6.7 Dually

[Sasquatch]

Additional "unsprung weight," and other esoterica, have me worried about the new 19. 5 mags I've ordered for my dually, and the commercial-style tires they'll be wearing.



The retail tire center I've ordered them through has been helpful, for years, in directing us when we've had purchase questions, but there are conflicting "pros and cons," here, so I'd appreciate TDR member input.



I've measured my new dually's rims, and they're approximately 18. 5, now, so the additional size of the new 19. 5 rims shouldn't dramatically affect "the look," insofar as the wheel-well-filling appearance of the new rims and tires.



The tire store tech told me that the height of the new tires/rims will be pretty much the same as the old setup, but I'm not sure this makes sense. If the rims are an inch bigger, to start with, why wouldn't the new mounted tires be also marginally larger?



The salesman said that the new, 14-ply tires (Toyos) are "substantially heavier" (he 'guesstimated' they weigh 85-90 pounds), but that my new mag wheels were also substantially lighter (and stronger) than the factory steel wheels.



He said the net weight difference would be, maybe, 30 pounds more per corner.



Can anybody speak to whether or not this (additional weight) is something to be concerned about in terms of lagging acceleration, a hit to fuel mileage, or towing, ride-comfort, or other issues?



What decided me, initially, is that these tires are "guaranteed" for 150,000 miles (by the store) and the purchase price includes free-rotation, free balancing, free "road hazard" (for life of tires), etc. , etc.



This store is very reputable, so I take them at their word, and the cost of rotating and rebalancing 6 wheels/tires (seasonally) is not insignificant here in mountain-country Colorado, where we swap sets of tires (snow/road) twice a year. The store handles mounting and changing snow/road tires free of charge, too, which is a very nice service (so long as you purchase the wheels tires from them).



Ride comfort is a concern, but durability is more of an issue, since this truck sees a lot of construction sites, and we live and work along gravel roads most of the time.



Thanks for your advice and comments

 

[Wolfy]

... there are conflicting "pros and cons," here, so I'd appreciate TDR member input.



The salesman said that the new, 14-ply tires (Toyos) are "substantially heavier" (he 'guesstimated' they weigh 85-90 pounds), but that my new mag wheels were also substantially lighter (and stronger) than the factory steel wheels.



He said the net weight difference would be, maybe, 30 pounds more per corner.



Can anybody speak to whether or not this (additional weight) is something to be concerned about in terms of lagging acceleration, a hit to fuel mileage, or towing, ride-comfort, or other issues?

Hi Sasquatch (Sassy?)



Regarding "accelleration":



It's up to you how concerned you might be, but you will suffer a double-whammy in the accelleration dept (if the tires are heavier AND of greater diameter.



Not so esoteric,... basic physics.



The greater diameter will give you a higher effective gear ratio, ... not what you want for accelleration.



The greater weight, even if if distributed in the same proportions as your current set-up, will simply mean more mass to accellerate (AND stop) (both in the linear and the rotating sense), . . i. e. , more momentum to overcome in attempting either.



With the change to lighter wheels, but a net increase in combined wheel/tire weight, a greater percentage of the greater weight is further from the axis of rotation. Anything further-out (the heavy tire) from the center of rotation, is moving faster, because it has a greater distance(circumference of the traveled circle) to travel in making a revolution, than something closer to the axis(lesser circumference).



If there is more weight than before(the heavy tire again) which has to be made to travel at this higher speed, ... there will certainly be more momentum in the system, resisting both starting and stopping.



Your new combination has less weight(light wheel) traveling the slow circles, and more weight (heavy tire) traveling the fast circles, when compared with the standard set-up.



The double-whammy is the anti-accelleration(or de-celleration) effect of 1. the higher gear, and 2. The greater rotational momentum. (Actually there is a 3rd element, ... the additional GVW of the total number of pounds that the 4 wheel/tire combos add to what the engine has to move. )



To picture what is happening, consider trying to rotate a barbell(In a flat plane like a compass-needle),... the first time with all the weight in the middle, ... then with half the weight at each end.



The weight-in-the-middle set-up will be much easier to start and stop.



"Gas" mileage?



Anything that makes the engine work harder to get things moving, will hurt, to whatever degree . Your rig is 120 # heavier (30 at each corner), and you are going to have to put your foot further into it, to get the heavier weight moving forward (and rotating) at the speed you are accustomed to. Whether any greater-diameter "high-gear" advantage while cruising can cancel-out the mileage disadvantage every time you accellerate (or from being more 'loaded",) depends on the specific ratio-change vs. the above-mentioned "costs", and what percentage of your driving is "cruise" vs. stop-and-go(again).



Your brakes are not going to appreciate the greater rotational momentum (or the greater GVW), when it is time to stop,



More "foot" again, to stop the way you do now.



So, accelleration, brake-wear , and probably mileage, will definitely not benefit from this exchange. How large the difference will be, in any of these areas, can only be determined by careful testing,... something seldom done.



But the principles mentioned here are sound.

Mother Nature cannot be fooled.



The appearance and durability differences will have to be great enough to make it "worth it" to you.



HTH,

 

[DPKetchum]

Its a dually so the added weight then is not 120. Its 180.

 

[Sasquatch]

Thanks, Wolfy,



I grasp the basic physics involved, I guess, and wouldn't be surprised to pay some sort of fuel penalty for the greater weight and energy expended to start/stop the truck.



I don't think the outside diameter of the tires/wheels (at least according to the tire tech) is much different than my current setup.



What's more an issue, for me, (as long as the aforementioned fuel penalty isn't ridiculously bad... ) is the extent to which these curvy mountain roads toast tire flesh; we're talking pork chops in a Pirhanna tank, here. My factory-issue tires are already showing significant wear, and the truck only has about 10,000 miles on it.



I live and work at altitude (8,000' plus), and am almost always traveling around a corner, going up, or going down. Lots of the roads are little more than gentrified logging trails, and tires are routinely expected to deal with stone and ice shards that would make a Spartan turn-tail and run.



I went through six sets of BFG 285/75/R16s in a little over seven years on my '97 2500 CTD, and that's expensive enough to contemplate moving up to commercial "meats," even if it means I've gotta hit the hip a little more frequently at the fuel island.



I love this truck, and haven't had any of the discouraging issues that other members relate: to the contrary, the truck has been problem-free, and I'm getting about 15 mpg in light city (okay, there are no "cities" out here -- Picture Mayberry, RFD inundated with millionaire tourist skiers) and about 17. 5 to 18. 0 on the highway.



We have a lot of snow out here, and most of the valley plow vehicles are duallies running commercial tires -- this notwithstanding the fact that srw vehicles are reputed to be better (smaller "contact patch" and less rear-end float) than the 3500s. I figured there must be some appreciable benefit to dealing with snow and ice in 3500 4x4 drw, or these operators wouldn't be running them.



I guess I just want to hear some reassurance from guys that are already running the 19. 5s and commercial setups, and see how they feel about the fuel and ride-comfort trade-offs -- particularly if there are some who've already made the switch and are driving the new '07 3500 6-speed rigs.

 

[Sasquatch]

Yo' Ketchmeister,

I "heard that," buddy.

One of the anticipated "realities" of dually ownership at the tire store: Multiply EVERYTHING by 6, instead of 4.

 

[Jeff Knight]

Hey Sasquatch,



We will be in your neck of the woods in a couple of weeks. My stepson lives in Parachute CO. We've been there 3 times in 2. 5 years. I know what you mean about the side road out there. Good luck with your decision.



Jeff

 

[Sasquatch]

Hey, Jeff,

Parachute is about 65-70 miles from us, and it's a great town. I'm about 20 miles northwest of Aspen and about 3 miles north of hwy 82. Very close to the little town of Basalt.

Colorado is an amazing place to live.