MPG difference between 6spd & 48RE
05-20-2006, 04:35 PM
#31
Originally Posted by 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.
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.
I meant any ROTATING weight/mass, or any mass/weight required by the motor to spin/turn....that is why I specifically used item's such as a driveshaft, brake rotors, wheels/tires, lug nuts, etc....for my example. NOT by adding the weight of a football in the front seat of my truck
Also, by adding 100 lbs to a truck that already weighs 8100 as mentioned above, does it decrease efficiency, sure does. Technically it will hurt it....would you be able to tell? Probaly only if we got into a .00001 or so in MPG as in 18.75432 MPG as opposed to 18.75433 or whatever. All I am saying is that you are adding more ROTATIONAL MASS in a heavier wheel/tire combo (and the associated friction that goes along with it) and to expect a 2-3 MPG increase is just not going to happen. This debate will go on long after we are gone, and you have your opinion, I have mine, and I can leave it at that.
Oh, by the way....I wish I could have the 19.5 wheel/tires on my truck....not because they help fuel economy, but becasue they look cool...and safe to assume that is why you really bought the 22.5" anyway, as if they looked like crap, even if they did help fuel economy, you would not have bought them.
05-20-2006, 07:59 PM
#32
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Originally Posted by SSminnow
and safe to assume that is why you really bought the 22.5" anyway, as if they looked like crap, even if they did help fuel economy, you would not have bought them.
05-20-2006, 09:34 PM
#33
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To make it simple, when I was a kid, my brothers and I had a slot car track, and they always beat me racing. I found that if I put larger tires on my car, that I could smoke them in terms of speed on the track, and smaller tires gave me a better hole shot off the line. The larger tires made my car much slower off the line, and I had problems with burning motors up, the smaller tires gave me a better hole shot off the line, but the car wound out too quick. Going to the larger tires may give you better cruising mileage, but what is it doing to your tranny at take off, it will take much more torque at take off to get all that weight rolling. Best thing to do is keep the stock tire and rims, and untape the brick from your foot 
. My 05 is running 18.5 to 19.0 empty, and the last 3 weeks loaded with pumps and trailer relocating to our new shop, 17.0 to 17.4, and all I have done is lose the muffler. I have seen it go as high as 21.0 if I feather it and that was running 70 mph on the hiway. But I have also found that the mileage will vary depending on where I buy fuel, the Exxon fuel gets me the best, Wally World mid range, where I normally buy fuel, the worst, and here latley, Exxon has been .09 a gallon cheaper, wally world is .13 cheaper. So to get 19 mpg and save .09 a gallon, the savings will add up. I can get fuel .15 a gallon cheaper, but the drive eats up the savings.
. My 05 is running 18.5 to 19.0 empty, and the last 3 weeks loaded with pumps and trailer relocating to our new shop, 17.0 to 17.4, and all I have done is lose the muffler. I have seen it go as high as 21.0 if I feather it and that was running 70 mph on the hiway. But I have also found that the mileage will vary depending on where I buy fuel, the Exxon fuel gets me the best, Wally World mid range, where I normally buy fuel, the worst, and here latley, Exxon has been .09 a gallon cheaper, wally world is .13 cheaper. So to get 19 mpg and save .09 a gallon, the savings will add up. I can get fuel .15 a gallon cheaper, but the drive eats up the savings.
05-21-2006, 06:01 AM
#35
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Wetspirit,
Rotating inertia will be a factor only in accelerating & decelerating, so only enters the calculation in stop & go city driving. Even then, the impact of the increased inertia of the larger tires has to be compared to the other inertias - your still spinning alot of gears, axles, driveshafts, etc. Then have to evaluate the inertia relative to the mass of the truck that is being accelerated & decelerated. Finally, stop & go milage also has idleing & some driveing at sustained speed components & does not consist 100% of accelerating & decelerating. Take all of the above into account, and you will see that while the inertia does have an impact on city driving, it is not going to be a measureable differance.
As far as engine speed, it's safe to say that there is an optimal (from efficiency) speed that the engine should be run at. I'm guessing that it is somewhere around 1500 - 1600 RPM. At 70 mph with stock tires & gearing the engine is spinning around 2100 RPM. Certainly beyond the engines best efficiency point. Larger tires do allow the engine to get down to a speed range where it is more efficient. One way to drop engine speed down is to reduce road speed, but how many of us are willing to drive 55 MPH?
Bottom line is that lager diameter tires do reduce engine RPM, which if all else was equal, increase milage, but have more weight, inertia, and raise vehicle height (more drag) all of which reduce milage. Tire rolling resistance may increase or decrease - depends on width, tire construction, pressure rating, etc.
So the efficiency determination is quite complicated after all. Your results may vary...
Rotating inertia will be a factor only in accelerating & decelerating, so only enters the calculation in stop & go city driving. Even then, the impact of the increased inertia of the larger tires has to be compared to the other inertias - your still spinning alot of gears, axles, driveshafts, etc. Then have to evaluate the inertia relative to the mass of the truck that is being accelerated & decelerated. Finally, stop & go milage also has idleing & some driveing at sustained speed components & does not consist 100% of accelerating & decelerating. Take all of the above into account, and you will see that while the inertia does have an impact on city driving, it is not going to be a measureable differance.
As far as engine speed, it's safe to say that there is an optimal (from efficiency) speed that the engine should be run at. I'm guessing that it is somewhere around 1500 - 1600 RPM. At 70 mph with stock tires & gearing the engine is spinning around 2100 RPM. Certainly beyond the engines best efficiency point. Larger tires do allow the engine to get down to a speed range where it is more efficient. One way to drop engine speed down is to reduce road speed, but how many of us are willing to drive 55 MPH?
Bottom line is that lager diameter tires do reduce engine RPM, which if all else was equal, increase milage, but have more weight, inertia, and raise vehicle height (more drag) all of which reduce milage. Tire rolling resistance may increase or decrease - depends on width, tire construction, pressure rating, etc.
So the efficiency determination is quite complicated after all. Your results may vary...
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