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post #1 of (permalink) Old 09-07-2005, 03:51 PM Thread Starter
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Gas mileage CJ vs. TJ...What gives?

Here is a question that I have been meaning to ask for a while. I was reading in “The Story of Jeep” by Patrick R. Foster, that 1982 CJs were rated by the EPA at 23 MPG city and 28 MPG highway. Then I stopped in at my local friendly Jeep dealer only to discover that the new Wranglers only get 18/21. What gives? You would think that in 20 years, fuel efficiency would increase, not decrease. Is all that fancy TJ stuff (AC, auto locking hubs, etc.) drinking more fuel? Did the old CJs really get better fuel mileage than TJs, or are they measuring it differently now than in 1982? Enquiring minds like mine want to know!
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post #2 of (permalink) Old 09-07-2005, 03:57 PM
 
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Re: Gas mileage CJ vs. TJ...What gives?

EPA is just estimate and I don't think they were that good. There is a formula for a certian driving conditions I believe. I'll have to google it to be sure. I never hit an EPA figure yet on any new car I bought.

Best I ever saw on my 76 CJ5 was 21 MPG with stock tires. Now I'm lucky to get 19 with 33" tires.


[EDIT}
Here is an 86 sticker:


I guess they havn't changed mutch after all. [img]/ubbthreads/images/graemlins/laugh.gif[/img]
{/EDIT}
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post #3 of (permalink) Old 09-07-2005, 04:02 PM Thread Starter
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Re: Gas mileage CJ vs. TJ...What gives?

Still, if the EPA is going by the same method to calculate these numbers, shouldn't a new TJ get better mileage than a vehicle with 20 year old technology? I would think that there have been some technological improvements over the past 20 years that would boost the MPG a little. Fuel injection, for example.
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post #4 of (permalink) Old 09-07-2005, 04:13 PM
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Re: Gas mileage CJ vs. TJ...What gives?

maybe they were using nautical miles... [img]/ubbthreads/images/graemlins/grin.gif[/img]
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post #5 of (permalink) Old 09-07-2005, 06:41 PM
 
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Re: Gas mileage CJ vs. TJ...What gives?

My 82 doesn't get anywhere near those numbers. I think i'm getting about 13-15 mpg. I'll have to record the numbers next time i fill up, but my office is about 10.2 miles from my house. So 20.4 miles x 5 = 102 - it was a little bit over 8 gallons last time i filled up making it about 12.75 per gallon. Now i have had to resort to 87 octane and my jeep pings so i may need to tune it a little.
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post #6 of (permalink) Old 09-07-2005, 10:10 PM
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Re: Gas mileage CJ vs. TJ...What gives?

I'd guess the lower emissions of today's vehicles as opposed to our 20+ year old Jeeps has quite a bit to do with it.

How does the gasoline of today compare to 20-25 years ago? Is 87 octane gasoline the same regardless of era, refiner, chemist, etc... Or, do different gasolines - of same octane rating - have different chemical makeup, and thus more/less 'energy' potential?

Any petroleum engineers out there? (I guess I should have taken a few more chemistry courses @ Ohio State. :P )
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post #7 of (permalink) Old 09-07-2005, 11:26 PM
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Re: Gas mileage CJ vs. TJ...What gives?

If folks are going to lie, it will likely be about their gas mileage or sex. But surely the government would not lie.

"I did not have..." a refill?
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post #8 of (permalink) Old 09-08-2005, 04:14 AM
 
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Re: Gas mileage CJ vs. TJ...What gives?

I'm thinking that to keep an object moving your going to have to push some air out of the way and overcome so much rolling resistance. This takes Horse Power and you get HP by burning fuel. I have a hunch that the efficiency that the engine transforms fuel to power is just a small part of the equation. It helps to be more efficient but you still are going to have to burn fuel because weight, shape, and rolling resistance.


Do TJ’s weigh more than CJ’s ? Do lock out hubs save that more energy than low friction bearings constantly turning axle’s, Is a carb powered engine running at a 14.7 :1 ratio realy worse than a FI engine running at 14.7:1. Probably not. But in the real world I bet the FI engines will run circles around a carb engine as far as burning fuel efficiently during acceleration and we do a lot of acceleration. It’s just not showing up in the EPA numbers.

Also from the EPA website: http://www.fueleconomy.gov/feg/how_tested.shtml
[ QUOTE ]
Adjusting Estimates
In the 1980s, an EPA study found that drivers were typically achieving lower fuel economy than predicted by EPA laboratory tests. As a result, EPA required the laboratory-derived city and highway MPG estimates posted on the labels of new vehicles to be adjusted downward by 10 percent for city estimates and by 22 percent for highway estimates to better reflect the MPG real-world drivers can expect.


[/ QUOTE ]

So the corrected estimates for a new TJ to match the 1982 estimates will be 19.8 City and 25.6 Highway. Oh, thats the 5 speed manual 4 banger. The uncorrected figures for a 4.0 6 speed is only 15/19 or 14/18 with automatic. Suprisingly the Grand Cherokee with a V8 and automatic is slightly better at 14/19.

You know, I'm realy liking my CJ now.
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post #9 of (permalink) Old 09-08-2005, 07:11 AM
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Re: Gas mileage CJ vs. TJ...What gives?

Great post [img]/ubbthreads/images/graemlins/40BEER.gif[/img]. Physics can explain just about anything. Can you help me with my chemistry? [img]/ubbthreads/images/graemlins/wink.gif[/img]
post #10 of (permalink) Old 09-08-2005, 07:46 AM
 
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Re: Gas mileage CJ vs. TJ...What gives?

[ QUOTE ]
Great post [img]/ubbthreads/images/graemlins/40BEER.gif[/img]. Physics can explain just about anything. Can you help me with my chemistry? [img]/ubbthreads/images/graemlins/wink.gif[/img]

[/ QUOTE ]

Uhmm No, My chemistry teacher was "English Chalanged" and I could never translate fast enough to keep up with the class. [img]/ubbthreads/images/graemlins/smile.gif[/img]
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