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post #1 of 9 (permalink) Old 01-08-2011, 03:47 PM Thread Starter
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Summer mpg mods?

So I'm thinking I can help mpg with some summer driving.
'97 F-250 4x4, converted to full F-350 suspension w/D60 and rear blocks.
I'm thinking install '97 F-250 front springs that sag and gain a few inches of drop.
Run smaller front tires with low RR tread and gain a few more inches of drop.
Run smaller rear tires with low RR tread. Front could be smaller than rears, I just wouldn't use 4 wheel drive in the summer. Hardly ever do anyway.

Any thought if the drop and tires would be beneficial?
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post #2 of 9 (permalink) Old 01-08-2011, 04:58 PM
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Re: Summer mpg mods?

Unless you compensate for the smaller tire with numerically lower gearing, you will lose MPG to a higher-revving engine (at any given road speed) and increased engine friction. More than you will gain from a small lowering of the truck.

31 inch diameter tires are optimum. The factory engineers specify wisely.

I did lower my front an additional inch by replacing the 235-85x16s with 225-75x16s. Being a 4x2, I could do that. At the same time I use drop I-beams and shackles to slam the truck 3 inches in the front and 6 inches in the back. Adding the inch in the front, I lowered it 4 in front and 6 in the rear. This resulted in a solid one MPG gain.

2000 F-350 SC 4x2 6 speed
3.08 gears Gear Vendor Overdrive
Slammed 4"
Rugged Air Dam
315,000 miles
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post #3 of 9 (permalink) Old 01-08-2011, 05:03 PM
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Re: Summer mpg mods?

Jim, I would suggest not running the f250 front springs due to the pogo stick ride that comes with it. Ask Cat rebal about it.

02 f250
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post #4 of 9 (permalink) Old 01-09-2011, 12:55 PM
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Re: Summer mpg mods?

Quote:
Originally Posted by Dave Whitmer View Post
Unless you compensate for the smaller tire with numerically lower gearing, you will lose MPG to a higher-revving engine (at any given road speed) and increased engine friction. More than you will gain from a small lowering of the truck.

31 inch diameter tires are optimum. The factory engineers specify wisely.

I did lower my front an additional inch by replacing the 235-85x16s with 225-75x16s. Being a 4x2, I could do that. At the same time I use drop I-beams and shackles to slam the truck 3 inches in the front and 6 inches in the back. Adding the inch in the front, I lowered it 4 in front and 6 in the rear. This resulted in a solid one MPG gain.
Great post, Dave. Can you expand a little more for me?

Loosing MPG with higher rev's... Ok, each engine cycle has an amount of friction, more RPM's means more friction. Is there a point where the RPM's are too low? What RPM do you shoot to cruise at on your set up?

Lowering... That's for less air getting under the vehicle in addition to the air dam? Less from the side also I would have to assume too.


Just got a new to me E350 (6.0PSD, 5spd auto). I have interior work to do, but I'd like to aim toward improving mileage also. Have to build a baseline on her yet. I will need tires at some point. 245/75R16's are stock, considering 215/85R16's for smaller contact patch and a similar height, and I'd like to think about different rear end gearing than the stock 3.55's... But does anyone make anything for the rear end anymore that is taller?

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 #5 of 9 (permalink) Old 01-09-2011, 03:10 PM
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Re: Summer mpg mods?

I know if it were me I wouldn't go any narrower than a 235 tire for safety an peace of mind. At some point your going to need some traction. I would also think those gears are about perfect. After all it is shaped like a brick.

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post #6 of 9 (permalink) Old 01-09-2011, 03:29 PM
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Re: Summer mpg mods?

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Originally Posted by fordfan30 View Post
I know if it were me I wouldn't go any narrower than a 235 tire for safety an peace of mind. At some point your going to need some traction. I would also think those gears are about perfect. After all it is shaped like a brick.
Well, it is a brick. So, side to side traction isn't really a huge need. A narrower tire will push through snow and wet better than a wider tire. And it's narrower to cut through the air also. 225's were standard on my bus also.

Are the gears perfect? Can't say, but I think Dave's F250 has taller gearing. Can't remember what his RPM's are at, say, 70, but I'm betting its a few hundred RPM's below 2000...

The brick part is good and bad, isn't it? It's almost a flat front, almost...!...really the worst part is the drag created at the back. My Excursion could do almost 19MPG with 4WD, towing mirrors, taller and wider tires than stock. 3.73's for stock gearing, and with 285/75's, I would have liked to have had taller final drive gearing so that I was spinning fewer R's on the interstate. A van with 3.55's with 245/75's ends up with about the same RPM's and speed when compared to my Excursion's set up.

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 #7 of 9 (permalink) Old 01-09-2011, 05:06 PM
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Re: Summer mpg mods?

OK. Engine frictional power. Somewhat of a misnomer. Only a tiny part of it involved with metal parts sliding against each other. Plain bearings are pretty darned good. The oil pumps do have a small load imposed, but the big enchilada here is "pumping losses" - that is the power needed to pump air and exhaust gas through the engine and turbo.

Just like vehicle air drag, the pressure differential needed to pump a unit volume of gas goes up with the square of velocity. Beyond a certain threshold (somewhere around 2000 RPM) the pressure differential becomes dominant. This shows up on the dyno as reduced torque. It shows up as increased fuel flow for a given flywheel torque.

This is why these trucks are so sensitive to engine speed. At 1300 RPM (thanks to 3.08 gears and the GV overdrive) my truck incurs about 50% of the engine frictional HP that a stock truck (3.73 gears/31" tires/2000 RPM @70 MPH) has to deal with. Engine frictional HP and aerodynamic drag of the truck go up (beyond 70 MPH/2000 RPM) with the CUBE of the speed increase. That's why the guys whorun 75-80 MPH don't get the MPG of guys who run 70.

2000 F-350 SC 4x2 6 speed
3.08 gears Gear Vendor Overdrive
Slammed 4"
Rugged Air Dam
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post #8 of 9 (permalink) Old 01-10-2011, 12:45 AM
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Re: Summer mpg mods?

Quote:
Originally Posted by Dave Whitmer View Post
OK. Engine frictional power. Somewhat of a misnomer. Only a tiny part of it involved with metal parts sliding against each other. Plain bearings are pretty darned good. The oil pumps do have a small load imposed, but the big enchilada here is "pumping losses" - that is the power needed to pump air and exhaust gas through the engine and turbo.

Just like vehicle air drag, the pressure differential needed to pump a unit volume of gas goes up with the square of velocity. Beyond a certain threshold (somewhere around 2000 RPM) the pressure differential becomes dominant. This shows up on the dyno as reduced torque. It shows up as increased fuel flow for a given flywheel torque.

This is why these trucks are so sensitive to engine speed. At 1300 RPM (thanks to 3.08 gears and the GV overdrive) my truck incurs about 50% of the engine frictional HP that a stock truck (3.73 gears/31" tires/2000 RPM @70 MPH) has to deal with. Engine frictional HP and aerodynamic drag of the truck go up (beyond 70 MPH/2000 RPM) with the CUBE of the speed increase. That's why the guys whorun 75-80 MPH don't get the MPG of guys who run 70.
Oh, that's great info, Dave!

Makes sense on the pumping velocities. Didn't recognize it, but I know about moving through wind from racing. Unfortunately, motorcycles, for road racing, look aerodynamic, but they aren't.

Ok, so you have 3.08 gears in the rear end. And the GV overdrive? What does that make your final drive ratio? Somewhere around 2.37?

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 #9 of 9 (permalink) Old 01-11-2011, 08:29 PM
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Re: Summer mpg mods?

Skinner tires (same psi) don't give you a smaller contact patch. It's just narrower.

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