Tire Size and MPG - PowerStrokeNation : Ford Powerstroke Diesel Forum
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post #1 of 15 (permalink) Old 12-05-2011, 09:44 PM Thread Starter
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Tire Size and MPG

Over the years, Iíve 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 31.7Ē 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 isnít 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 donít 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 donít recommend it unless you are as crazy as I am.

2000 F-350 SC 4x2 6 speed
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post #2 of 15 (permalink) Old 03-04-2012, 03:15 PM
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Re: Tire Size and MPG

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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post #3 of 15 (permalink) Old 03-04-2012, 07:31 PM Thread Starter
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Re: Tire Size and MPG

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.

2000 F-350 SC 4x2 6 speed
3.08 gears Gear Vendor Overdrive
Slammed 4"
Rugged Air Dam
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post #4 of 15 (permalink) Old 06-02-2012, 05:21 AM
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Re: Tire Size and MPG

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?

1995 PSD 230k miles, 5spd, CC, 3.55 gears, Long Bed. Tymar, Ranch Hand front & rear bumpers. Otherwise stock. 22mpg
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post #5 of 15 (permalink) Old 06-03-2012, 05:47 PM
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Re: Tire Size and MPG

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Originally Posted by Deererainman View Post
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?

Dave
2005 E350 6.0PSD
2003 Excursion Limited 4WD 7.3PSD - Sold
1996 E350SD dually 7.3PSD - Gone!

2002 VW Jetta TDI - stock
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post #6 of 15 (permalink) Old 06-03-2012, 05:54 PM
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Re: Tire Size and MPG

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post #7 of 15 (permalink) Old 06-03-2012, 06:41 PM
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Re: Tire Size and MPG

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Originally Posted by TANK'S 250 View Post
Hey, if you want to continue spending your man card money on fuel taxes, by all means continue... Man card status might be better saved on generating wealth, reason, and the like, but that's up to you.

Enjoy reading the MPG forum! Must be like porn?

Dave
2005 E350 6.0PSD
2003 Excursion Limited 4WD 7.3PSD - Sold
1996 E350SD dually 7.3PSD - Gone!

2002 VW Jetta TDI - stock
2007 Escape Hybrid
1999 Kaw KLR650 - 17/42 gearing
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post #8 of 15 (permalink) Old 06-03-2012, 10:19 PM
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Re: Tire Size and MPG

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Originally Posted by TANK'S 250 View Post
I agree but so far ive seen no complaining. Just people using big words and putting something together, now if they have a superduty pushin some real power getting 30s mpgs I want to see that. Or really high 20s
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post #9 of 15 (permalink) Old 06-04-2012, 05:47 PM
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Re: Tire Size and MPG

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Originally Posted by TANK'S 250 View Post
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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Lowered rear end to F250 height, 4" exhaust w/ muff, foil delete, AIH delete, Ford AIS intake, tru-cool trans cooler, autometer under dash pod and gauges, Mile Marker hubs, 04 headlights, soft tonneau

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post #10 of 15 (permalink) Old 07-08-2012, 08:16 PM
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Re: Tire Size and MPG

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