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SUVs Jeep Liberty and Jeep Liberty Diesel Jeep Liberty CRD Diesel Real World MPG
148 messages, Last post on Jun 27, 2009 at 4:36 PM
You are in the Jeep Liberty and Jeep Liberty Diesel Forum. Your Hosts are steve_ & tidester
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Replying to: rjsj (Sep 10, 2006 3:08 pm)  I just wanted to add my 2 cents on MPG data and calculation. You travel a great distance just to burn through a tank of fuel. Maybe 400 miles? The equation in your CRD's computer, one way or another will be based of tire diameter. If you think about how many times that tire turns in 400 miles, you can see how even a small error in tire diameter can effect how far your CRD thinks it has traveled. I would use the roll-out method. With the wheels on the CRD, roll it forward and measure the distance traveled for exactly one revolution of the wheel. Use a simple formlula to solve for tire diameter then convert your number to whatever flavor a CRD computer wants to see. My Hypertech programmer asks for tire diameter in inches. It's simple. Roll-out is a superb method for the given situation. If you have worn down tires, your tire size according to your side wall is no longer accurate. Use a roll-out and it will tell you the diameter of your bald tires or brand new tires. It doesn't matter. Double check that with GPS. But don't stop there... Triple check it with one of these handy 1-2-3-4-5 mile checks on the interstate. Do you guys have those? Here in the pacific northwest we have them. I think they are fascinating. No matter what car I'm driving in, I'll come up on an I-5 mileage check and I'll zero my trip odometer right when it starts. 5 miles later I look and I'll be damned if not every time do I end up with 4.9 or 5.1 or something. It's never 5.0, right lol? But what I look for is to see if it's in the same positive/negative direction that the GPS is suggesting of my cars speedometer. And it always is  ... as a mechanical engineer ... that sort of thing makes life tollerable lol. Also don't forget the inherrent inaccuracy of measuring translational distance traveled by revolutions of a wheel?! Things slip. Especially tires! And significantly when it comes to surfaces any other than warm tarmac. As you can imagine, your computer sees your wheel spinning more, which thinks you've traveled farther, so what does that mean? You'll get inflated MPG numbers when driving on snow and forest roads. Also, consider the inaccuracy of exactly how full your tank was when you started measuring and after. Even if you controlled it (you are the filler) and you tried to fill it the same everytime, you know it's impossible to fill it the same everytime, so that your fuel amount for the distance traveled is accurate. Even the reporting device. They usually only report to 2 or 3 decimal places on the receipt. To minimize this error, you would sum up many tanks of fuel over many miles traveled. Lastly, the fuel itself. Same brand? Same pump? Biodiesel? How much? This is also a variable and will affect combustion efficiency and BTU per unit. Did you drive the same drive, on the same fuel, from the same station, in the same conditions on this tank as you did the last. You can minimize the bias by collecting and summing a volume of data and then calculating. In other words, tank-to-tank is nice but look at these contributors high-low tanks records. Some are improving 40 or 50% from worst tank to best tank. That's crazy. It would be fascinating to do a proper error analysis such that you could say... "My 2005 CRD gets 24.1 MPG +/- 1.8 MPG". If you keep a spreadsheet, don't forget to track fuel price. It's fun (sometimes) to watch what that has done to you over the last several years. It's also interesting to see what really makes a difference on your MPG numbers (if you are charting). I switched jobs once going from a 70 mile roundtrip highway commute to a 34 mile roundtrip city commute. In my Jetta TDI I dropped from steady 42MPG to steady 37MPG. I'm convinced it was the job change (my commute is over 90% of my driving).
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Replying to: evolDiesel (Dec 30, 2008 11:47 pm) And how large would that be? One full revolution of slipping per mile driven won't even get you up to two tenths of a percent error in your distance measurement. And that is assuming the slippage is all in one direction. tidester, host SUVs and Smart Shopper
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Replying to: tidester (Dec 31, 2008 9:31 am) Consider 3% as a conservative number. I'm staying in 4WD Full Time to reduce this value and I'm not regretting my choice
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Replying to: caribou1 (Jan 01, 2009 12:47 pm) I suppose that's possible if ALL your turns are in the same direction (i.e. right vs. left).  tidester, host SUVs and Smart Shopper
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Replying to: tidester (Jan 01, 2009 3:07 pm) Just look how tires tend to wear; wear is an indicator of unmatched rotation versus trajectory
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I get great hwy mileage (28 60, 27/65, 2670) but around town its 18.5. Are there any tricks or suggestions on improving city mileage? |
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Replying to: f4windex (Aug 26, 2008 1:43 pm) |
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Replying to: winter2 (Jul 14, 2006 9:25 am) I bought my wife an 05 Limited which she dearly loves we have found that fuel choice makes a difference. With the cheaper brands like Citco our mileage drops and by using better fuels Like Shell the 1 to 3 cents more per gallon pays off. Other friends have tried this with their diesel's and are now using brand name fuels too. 65% highway 35% city 24 mpg to 27 mpg all highway up to 31.4. Pulling our Layton 19LT Camping Trailer 17 to 22 mpg We love our little Jeep. |
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