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Over the years, I’ve tried several tire combos and read literally thousands of posts of what guys got and can make an easy generalization:

If you put substantial larger or smaller outside than OEM diameter tires, expect to see a reduction in MPG. The boys at the factory do have a handle on this.

Bigger tire, counterintuitively get worse MPG than OEM-sized rubber because of increased rotational moment of inertia. ROI is the measure of how effective a wheel is at being a flywheel (storing rotational kinetic energy). Before you accelerate the truck, you must accelerate the tires first. ROI goes up with the square of the radius, so if you go from 31.7” OD OEM rubber to 305s you will see a 15% increase in ROI.

This would not have any effect if you drive the same speed all the time, but that isn’t the way anybody drives. People start and stop, speed up and slow down. Every time you do the rotational kinetic energy state of your tires must change. That means you either wear your brakes or burn fuel.

Likewise, tires or a smaller OD get worse MPG because they make your engine rev up more at a given road speed. Higher engine RPM generates higher engine frictional HP and that increases fuel burn.

If you change tire size, get your speedo corrected. If you don’t correct your speedo, you are driving your ECM crazy and you are lying to yourself.

Always air your tires up to the maximum pressure on the sidewall of the tire. These trucks are VERY sensitive to tire inflation. Soft tires can cost you 1.5-2.0 MPG depending on season. Tire inflation varies itself a lot in colder months. I run very high pressure in my 235-85E tires but I don’t recommend it unless you are as crazy as I am.
 
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Hi Dave,
I listen very closely when you speak. I have a set of 265/75R16 E's on my late model 1999. Stock according to the sticker on the door frame is a set of 235/85R16 E's as you are running. I do not know if the speedometer has been adjusted, but probably not. The 265 is 30 mm wider than the 235. The aspect ratio has been adjusted to increase the diameter of the 235 slightly. Do you think/ or know if this would make a difference in MPG? If so, how significant?

So far I have gone to the 203 STAT and put in a set of gages. These seem to help
as the cab is warmer and I can take even more foot out of the machine with the help of the gages. I pull a 5th wheel, so the tonneau would have to be a roll up when I am not towing.
Thanks for you help.....Ghyslain
 

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Discussion Starter #3
265-75 and 235-85 are identical for MPG and they are exact at this truck's sweet spot.

Get a roll up that is not a pain to put on. If its a pain, you won't use it.
 

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Back when I was selling Class 8 trucks we would spec them out using component specifications like engine (max torque at specified rpm), transmission ratio, esp final top gear ratio, rear end ratio and tire diameter.

We had a computer program (I think it was either a Kenworth or Caterpillar program) to provide final top end speed , cruizing speed and provide a maximum efficiency range.

I was looking at the torque specs on the 7.3; 6.0; 6.4 and 6.7. Both the 7.3 & 6.7 are rated max torque at 1600 rpm while the 6.0 & 6.4 are rated at 2,000 rpm.

Based on the torque specs for the specific engines, maximum fuel efficiency should be acheived close to the max torque level/rpm for the specific engine.

Depending on what your goal is; for this topic/thread we'll assume MPG. You need to decide on what your hwy cruising speed is going to be, calculate your engine rpm at that speed using transmission final gear ratio, rear end ratio and tire diameter.

Am I making sense?
 

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I was looking at the torque specs on the 7.3; 6.0; 6.4 and 6.7. Both the 7.3 & 6.7 are rated max torque at 1600 rpm while the 6.0 & 6.4 are rated at 2,000 rpm.

Based on the torque specs for the specific engines, maximum fuel efficiency should be acheived close to the max torque level/rpm for the specific engine.

Depending on what your goal is; for this topic/thread we'll assume MPG. You need to decide on what your hwy cruising speed is going to be, calculate your engine rpm at that speed using transmission final gear ratio, rear end ratio and tire diameter.

Am I making sense?
The max torque ratings are interesting. I knew that the 6.0's were different compared to the 7.3's, but the new data of the new engines are news to me.

I don't think that maximum mpg is necessarily achieved at max torque. That RPM still creates an amount of frictional and even aerodynamic losses in the engine that might not be necessary to propel a particular vehicle at speed. Thus, operating at an RPM below max torque can produce better MPG's.

As an example, I think that Dave's package operates around 300RPM's below a 7.3's max torque of 1600 at 70MPH resulting in improved MPG. In my Jetta, I swapped out my fifth gear for a taller one that reduced my RPM's at cruising speeds, and I gained MPG even though I'm operating below the max torque RPM that I was before.

I think if one is using the vehicle to work (to tow, to haul, to operate in a manner of capacity) that it's easier to recognize max torque and all for the ease of "usership" for employees and such... Thoughts?
 

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I like you tank but I have to disagree with that. Total crap. Let's all just agree that we need a truck, to haul or tow, safety, or whatever. Now that it is established that we need a truck it would be foolish to not seek to minimize operating costs associated with that truck. Doing something to improve mpg even 2 mpg on our trucks is at least 12%. That 12% is enough to feed my family dinner for two nights (6$ large pizzas) every time I fill up.

The answer is not "buy a prius". A prius does not haul and tow. A second mpg vehicle is also not the answer since the costs of maintenance and insurance exceed the fuel savings for those of us that don't drive high miles.

We are performance enthusiasts. Performance can be measured in more than one way. MPG while moving an 8000 lb vehicle is a performance measurement that I like to improve.
 

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00 Excursion 4x4. 8600lbs 325/60/20 (roughly 35.25"). 3.73s 15.8mpg pure city and 20mpg highway.
 

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Discussion Starter #11
A few years back I experimented with LT225-75x16E tires instead of the LT235-85x16Es the truck came with. That resulted in a slight loss of MPG.

But that was before I got my GV on and sorted out. At 70 MPH I was revving 1,685 RPM with 235s and 1,825 RPM with the 225s.

Now with the GV (and its 0.8:1 drive ratio) I turn 1,325 RPM @ 70 MPH. With 225s I’d expect about 100 RPM higher. Still 35% slower engine speed at the same road speed than stock.

With the 235s I run EGT right at 600 degrees and can’t seem to coax it down much. Trials with 225s show temps 125 degrees cooler. Lower temp indicates lower fuel burn.

Lower temps are also good for extending engine service life.

If you look, all the cars that get outrageously good MPG have teeny little wheels – generally 14 inchers. That’s because they are designed to be urban cars. Lots of stop-and-go driving.

Your drive train doesn’t accelerate the truck away from a light. It accelerates the wheels/tires. The tires grip the road and that equal and opposite reaction thing accelerates the truck. You have to pour kinetic energy into the wheels before they move your truck.

The size of the “flywheel” is measured by the moment of inertia of that wheel. Before I stated the moment of inertia goes up with the square of the diameter.

My bad. Moment of inertia (according to my Marks handbook) goes up by the FOURTH POWER of the diameter. So 225-75s have a moment of inertia 30% less than that of 235-85s. Also the tires weigh a third less.

Thus I will expect the truck to require less fuel for stop-and-go driving (about a third of my driving) and the penalty for 100 additional RPM may be offset by cooler engine temps.

We’ll see.
 

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Dave, we check mileage every tank [hand calculated]. We switched from our bridgestones to michelins for longer wear mileage. Unfortunately we noticed a 1 mpg decrease in mpg. Have you ever noticed certain brands get better vs worse mileage?
 

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Discussion Starter #13
Y'know, I drive a lot...but not that much. Not enough to do any brand vs brand testing.

Tire manufacturers do internal testing on the rolling resistance of their products. The OEMs have a big interest in this as tire rolling resistance is a factor in EPA mileage estimates.

OEM factory engineers have access to tire manufacturers' rolling resistance data but we don't. Otherwise everyone who gave a damn about MPG would buy one brand and model and nothing else.

This can be a trap for those not aware that tires are multifaceted things. It stands to reason that a low rolling resistance tire may have less overall traction than a higher resistance tire.


All that said, there are some indicators we can use to make decent guesses at tires that will give good mileage.

Generally, a quieter tire will give better MPG than a noisy tire. That makes sense as generating sound requires energy (as rock musicians know). Any energy that goes into sound is not energy used in moving your truck over the real estate.

Since the half-ton truck market is subject to EPA mileage estimates and those take tire rolling resistance into account, it would be a fairly good guide to find what brand and model tires go on new half-tons that get the best MPG. Get the E-rated version for your truck and you should do OK.
 

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i like the noisy tire vs quiet tire comment!
i might add, a tire that gets warmer than another tire OR one that sends mechanical vibration through the hub (out-of round) is wasting your invested energy as well.
subscribed!
 

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this thread has been an interesting read.

my van came with 245/75/16s, that I switched to 215/85/16s when I got a cheap almost new set, now i'm running 235/85/16s because I have got a lot of them on parts trucks I've picked up cheap.

I did get my best fuel economy with the 215s but only aprox 1 mpg. I usually run my tires at 80psi as I'm usually loaded with stuff or pulling my flatbed trailer.

I check everytank with a calculator unless I forget to reset the tripmeter. I correct my mileage for calculation with a GPS, 215s were exact, 235s speedo reads 4% low.

traction=noisy tires do reduce the mileage, but for my needs on a lot of loose surface trailer towing and winter roads it is necessary.

best tire I ever had for mileage was a Michelin XPS rib on my 96 F250 , I ran them front and rear and no winter driving. wear like iron and ride like iron, but they added almost 3mpg to my highway driving when I took off the uniroyal allseasons. both were 235/85/16 E .
 
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