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Old 09-25-2013, 04:26 PM   #1 (permalink)
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More evidence aerodynamics is king for gas mileage

My 95 Jeep Wrangler 4.0 has been a real lesson in general efficiency. I've found that the largest gains in gas mileage has to do with reducing drag. I reduced tire size and weight with no real difference. Removed the roof cage with a 10% increase. Drive slower with another about 10% gain. Added a stock fit K&N filter with no change. With what seems to be no change but I will report more on this later. Dropped 10% from neglecting to watch tire pressure from 40 to 32 lbs. Added ethanol fix to gas without change. Again this I will hold judgment on because it was the same time I missed the tire pressures. I do these changes not for saving money primarily but need. Needed new tires because they were bad on wet roads. Needed new exhaust from rust and stainless won't. Was going to add a matching set of headers and free flow cat but my exhaust manifold is rust free at this time.
I wasn't going to bother with watching gas mileage because it is a purpose built vehicle. But I have a friend, who tends to be a BS master, said he would average 25mpg and a max of 27mpg. Someone else wrote on his JY that hit the 30 mpg mark on a long trip. So I started to wonder how efficient was the 4.0. Can't find anyone strange enough to put this engine in an aerodynamic body until I see the older AMC Eagle. But I was wrong and the drag coefficient is rated higher at .55.
So I guess I will play with belly pans and dams. If upon keeping tire pressure at 40 psi I gain over 24 mpg I will need to reconsider the cat-back and or the gas treatment.

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With my Wrangler I average 40 mpg. Triumph Speedmaster 60 mpg + Jeep 20 mpg = 40 mpg!
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Old 09-26-2013, 11:29 AM   #2 (permalink)
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I think for city driving, (lots of 0-40 MPH stop and go traffic) weight and drivetrain efficiency play the largest roles since there is lots of acceleration. High speed cruising (50 mph+ without towing anything and on a flat grade) is mostly limited by aerodynamics and your engine's low load fuel consumption. When introducing an incline, weight and drivetrain efficiency start to become more important.

Do you remember how much weight you saved from the new wheels/tires? Car and driver gained between .4 to 2.2 MPG from reducing tire wheel size/weight. See here:
Effects of Upsized Wheels and Tires Tested - Tech Dept. - Car and Driver.

It is unclear in the C&D study whether those gains were the result of a lighter rotating mass or a rotating mass with a smaller radius. They should have tried to test only one new variable at a time.

Then again, C&D tested with a Golf, which is a bit smaller car so a change in tire size may lead to more drastic changes.

10% FE from 8 lbs. of tire PSI is seriously good bit of FE for free. Makes me wonder why auto manufacturers set the pressures so low? Is it a safety or comfort issue?

Roof racks/cages are notorious for decreasing FE. Since about the only thing past 50 MPH past cruising is aerodynamics related, no surprise there.

I wonder if removing mudflaps would make the the vehicle more aerodynamic.

Anyways, good luck with your FE endeavors. I've got a 94 Camry 3.0 and I'm averaging around 22 MPG which I hope to improve.
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Old 09-26-2013, 12:56 PM   #3 (permalink)
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Dropped about 60 lbs total with smaller tires. Lost about 3 inches width each tire. I believe I read somewhere that a wider tire can have a lower rolling resistance. They may have shown that with the poor on road performance.
As we all know Wranglers are not know for ride comfort. I have noticed a real decrease in ride comfort with returning to 40 psi.
Recently I let go of my 89 taurus sho. I've broken 31 mpg on long trips with that v6 250 hp engine. Cd of .31.
Mud flaps? You mean the plastic fender extensions? If its a bolt on bolt off I'm game to try it.
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Old 09-26-2013, 03:00 PM   #4 (permalink)
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For what it's worth, keep in mind that the Jeep Wrangler places dead last on Ecomodder's list of drag coeffients--so it's got a lot of room for improvement:
Vehicle Coefficient of Drag List - EcoModder

Also, as has already been pointed out, keep in mind that driving style will play a big role. Faster speeds = aero plays a bigger role. This chart illustrates that:

The chart is for a semi truck, but the same concept applies.
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Old 09-26-2013, 05:43 PM   #5 (permalink)
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thanks D Dave. Yep I'm quite well aware of this. But some sites claim Land Rover Defender is in the .5x as the AMC Eagle .55. I have seen the Wrangler in the .45 range. All with a grain of salt. Still not comparing frontal area. What this short wheel base block of steel has shown me today was how much rolling resistance can be felt from 40 psi to 32 psi. I thought I somehow messed up my cat during a throw out bearing change because I seemed to have lost low end torque after. Now that I have the tires back to 40 psi I can go up hills in 5th at 35 mph again. I coast down hill faster and further too.

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