06Cummins

what flash did you have done???

Mark Hodowanec

I don't think you will hurt his feelings - I'm guessing that his milage will improve by 2 to 3 mpg. His rolling resistance will go WAY down. Tire is narrower than stock (255 vs. 265), can run at almost twice the pressure (if his kidneys can take it) and have a tread design & tire compound designed to give high milage.

The only hit in mpg is that the truck is now taller, and thus not as aerodynamic.

SSminnow

Without getting into a Physics war, your are still talking about more weight, rolling resistance. The tire may be slightly narrower, but since it is taller, he will have more of a contact patch from front to back. Throw out the contact patch all together, and you still have more weight to move, which in turn uses more fuel. I have owned a few 4WD trucks, and the first thing I would do after the factory tires wore out, go to a taller tire....and in turn get worse fuel economy.

D.Wiggs

One thing you are forgetting about is inertia. Once the weight is rolling, the differences in power it takes to keep the heavier weight moving is very minimal and in no way will offset the advantage of the 500 RPM drop. I should see about a 2-3 MPG increase in mileage especially since most miles are highway miles. Weight is ONLY a factor with stopping and starting and the increase in weight is not all that great, especially for motors with this much HP and TQ.

Raspy

It sounds like you are making an argument that is based completely on the fact that you want it to be true. It has been shown by others (and I have had the same results) that bigger tires that are heavier and/or taller and/or wider produce worse mileage. You are not going to get a 10%-15% improvement by going to a tire that has these characteristices. I'd love to be wrong, and maybe you'll show that I am by produucing accurate and honest results. In the mean time I can't understand where that magic number of 2-3 comes from????? What factors are you using in that calculation? If it is simply the difference in RPM you're going to be disappointed.

You're right that weight is a factor when starting and stopping. It hurts mileage on starting and doesn't help on stopping so there is a net loss. Of course the engine has the power to turn the extra weight, thats the point, it takes extra power to turn the heavier tires, thus, lower mileage. On the highway the wind resistance and rolling resistance come in to play. Maybe you think the engine is so inefficient at a slightly higher RPM that by lowering the revs slightly, even while increasing drag in another way, that the overall efficiency of the system will increase. Well, we'll see. Please post the accurate results.

The bottom line is: if your truck has a gearing problem, and by that I mean you are unhappy about the efficiency you are getting, and you've shown that is is RPM related, why try to fix it with tires that are less efficient at rolling down the road? If you want bigger tires, go for it. Bigger tires are cool. But why argue, without supporting data, that one problem will be fixed by adding another? Again, please post honest results. I'd love to see a big improvement.

Wetspirit

D.Wiggs

I will definitely post accurate results for I want people to know what can or cannot be done. I am by no means GAURENTEEING an increase in MPG, simply doing some predictions based on what I know about the engine and the information I have gathered from the CUmmins technicians. Also, I would be very interested to know what the drag increase, if any, would be. I would be willing to bet that the difference would either be (A) too minimal to matter, or (b) better than stock for the following reason. I think that the stock wheels well, give that it is so open, acts as more of a "catcher" for the wind and in less aerodynamic than a stock heigh truck with a larger, curved surface (i.e., a tire). I obviously have no wind tunnel data to prove this, but I would not be surprised if this were true. One thing is for sure, these trucks are MUCH MORE INEFFICIENT in the 500 RPM range from about 1900-2400 RPM. Just ask the guys on here how huge an MPG difference there is when they drive at the one speed versus the other.

I will definitely post as soon as I have numbers to compare.

Raspy

Exactly. It's the speed of the truck that causes the mileage drop. The tires don't make up for higher truck speed. I'm Looking forward to your report on 2-3 MPG increase with bigger and heavier tires. Especially since I've already run the test (as have others) and had the opposite result. If only it was true. We'd all be running bigger tires to increase mileage. Heck, why stop at 35s to get 2-3 MPG, go straight to 44s and get 9-10 MPG. Why not go to 60s and get 20 MPG more? Sounds good to me.

Wetspirit

SSminnow

One thing you are forgetting is it still requires extra power to keep those tires rolling. You think once they are "at speed" that it no longer requires any more power to keep rolling Like I said above, not tying to start a war, but to think that you are going to get better fuel economy with a larger/heavier wheel/tire just becasue they are taller is

SSminnow

ANY weight that you can take away from the motor requiring to spin, will help effciency, and ADDING any weight (like what you are doing) will hurt efficency. Aluminum/carbon fiber driveshafts are a perfect example of this (among others) Why do you think Pro Stock cars (which are the epitome of weight savings looking for the extra .0001 of a second in ET) Carbon fiber driveshafts, carbon fiber brake rotors, titanium LUG NUTS, etc.....WHY? Because it leads to less rotational mass, which increases efficiency (in there case, would be ET, in our case, would be fuel economy)

xyzer

"06"
My majic TSB was TSB 18-05-06 and the why it was flashed . https://www.dieseltruckresource.com/...ead.php?t=95998
Sorry it took so long!

D.Wiggs

You are missing the point. There is obviously a point of diminishing returns (i.e., why you would not go to 40's or 49's etc.); however, since one cannot run 44's, or 60's, without so much lift and increase in drag, there is not going to be enough of an increase in engine efficiency to overcome there additions, unlike the 22.5" upgrade.

D.Wiggs

This is completely false. A simple refutation by counterexample shows the flaw in your logic.

Suppose I drive my truck from full to empty on the highway at 95mph without stopping completely empty (except for me and the necessary fluids) and I go 300 miles before I have to refuel. Now, after I refuel I put a football into the passenger seat (i.e., increase the weight of the vehicle) and then drive from full to empty on the highway at 60mph. Now I go 600 miles before I have to refuel. According to your logic, I could conclude that adding ANY weight will effectively double my range and thusly must make my truck more efficient.

Is this true? Of course not. You cannot make blanket statements about these types of issues. There are many other factors, and, as I said before, my hypothesis is that the benefit of the decreased RPM and increased engine EFFICIENCY will be greater than the DECREASED efficiency the heavier wheel/tire will cause. Only time will tell since NO ONE knows what will happen; one can only guess or believe what will happen, and as everyone should know, belief does NOT equal knowledge.

Mark Hodowanec

Yor're right, lets not base the arguments on physics & logic, but just be emotional about it .

The size (in area, sq.in.) of the contact patch is independent of the tire size, but is a function of presure. For example, if the tire is inflated to 50 psi and your truck weighs 8000 lbs., than the size of your contact patch will be 40 sq.in. (50 psi x 40 sq.in. x 4 tires = 8000 lbs.). YES, IT IS THAT SIMPLE. a tire with less pressure needs more contact patch, and has to flex more every time it comes around (i.e. once per tire revolution). If the tire compound is the same, then the tire that has toflex more will be less efficient.

As far as your weight argument, it is a matter of degree. Obviously, more weight will cause more drag than less weight. Inertia has nothing to do with this. Do understand the impact of weight, one needs to understand what proportion of losses (i.e. inefficiency) load (i.e. weight) related & load independent. Don't forget that you are already dealing with a truck that weighs approximately 8000 lbs. Put on larger tires and the truck weight goes up to 8100 lbs. Do you really think that your milage goes down because of weight?

The fact that you have had several trucks and changed to taller tires and each time got less fuel economy should not lead you to the conclusion that taller tires are less efficient. Not unless you kept all other variables unchanged (tire width, construction, compound, pressure, etc.) it is not possible to make that conclusion!

When I changed from my stock tires (265/70-17) to the H2 tires (315/70-17) my milage went up by approximately 1 MPG. This is in spite of the fact that the tire is 2" wider and can only take 50 psi instead of 65 psi. If I could get a tire that is as tall as the H2 tire & take more pressure, and keep the stock tire width (265) I would get much more than just a 1 mpg improvement. This was my basis of GUESSING that the would see a 2 to 3 MPG improvement.

Raspy

Mark,

Your contention that inertia has nothing to do with drag is not right. A larger, more massive tire and wheel combo has more inertia. Therefore it is harder to get rolling. When stopping, the truck is braking, and braking doesn't add to efficiency or mileage. There is a net loss. Maybe not much, but a loss. Were looking for an overall improvement, not a small loss.

I also switched from the stock 265s to a set of H2s with 315s and my mileage dropped about 1.5 MPG. And I wasn't surprised. But I was disappointed. They are heavier and they are wider.

Not all tire switches lead to worse mileage, obviously. For instance in a truck that is geared a little low and the old 750X16 bias M&S tires are replaced with a set of very high and skinny radials, the mileage can be expected to go up. But claiming, as some seem to be doing, that the reason the trucks are getting poor mileage is because the engine is spinning to fast and they plan to correct that problem by doing something that adds resistance in a number of ways, is a little silly. As I mentioned earlier, it's trying to fix one problem by adding another. Mileage does not go up simply in inverse proportion to the reduction in RPM. If so lets reduce the RPM by 50% and increase the mileage by 50%, or better yet let's decrease RPM by 100% (0 RPM) and increse mileage by 100%. Sounds silly. Each case must be taken on it's own merits, but adding resistance in one area to lower it in another may not give much benefit. And my experience with this shows that it doesn't.

Wetspirit

Raspy

You forgot to mention the mechanism required to spin the football as required in SSminnow's example. Please explain how that will fit into your calculations. Also, in your example you were getting twice the mileage after slowing to 60. Are you now admitting that wind resistance is a big player in the mileage calcs? Of course it is. And far larger than a few hundred RPM.
An easy test is to run the truck on a given stretch of highway at a speed of about 60 in OD lockup with the cruise on. Then do the same stretch in Tow/Haul (3rd gear lockup) with the cruise on and just hit resume. In each situation with speed stabilized and identical conditions reset the computer and read the mileage for a few miles. The only differences will be overdrive and PRM. Now, what's the mileage difference? If it's lower in Tow/Haul can you fix it with heavier, wider, and larger diameter tires? Maybe with taller, narrower, and easier rolling tires you could. When that's established we'll go to an in-town stop and go test. Can we improve the mileage with lighter wheels and tires that have less inertia? Probably so.

Wetspirit


The debate rages on............

Wetspirit