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Old 06-11-2002, 18:52   #1 (permalink)
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Diff Ratios and Fuel economy

I went back to SA on the weekend from Canberra, the car return 9.33 litres to the 100 or 30 mpg over 2500kms. I'm still not impressed with the fuel economy the car it is an el 5 speed maunual futura running with a 3.23 diff, pacemakers headers fitted.

The car does 2000 rpm in fifth at 100 km/h. What rev's would it do at 100 km/h if I went for the auto 3.08 diff and what kind of fuel ecomony benefit would I gain? Am I chasing very dimishining returns? could I consider going lower than 3.08 maybe a 2.9 diff ratio, does one exist?

any other advice on how to improve fuel economy appreciated, would like to get 35mpg from at highway speed.

cheers

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Old 06-11-2002, 19:49   #2 (permalink)
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If your doing 2000rpm at 100km/h with a 3.23 ratio, you
will be doing 1907rpm at 100km/h with a 3.08 ratio.
OR be doing 1796rpm at 100km/h with a 2.90 ratio (if it exists).

Each of the above steps is approximately a 5% change. In a perfect world fuel economy would improve accordingly.

That is 9.33l/100km to ~8.9l/100km (3.08 ratio) to ~8.4l/100km (2.9 ratio).

In imperial from 30mpg to ~31.5mpg to ~33.4mpg.

All theoretically of course, ignoring all other associated factors.
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Old 06-11-2002, 19:57   #3 (permalink)
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Wowzers ! 9.33 L/100kms !! You've already beaten me hands down mate! I get *attrocious* economy in my EL wagon - trip computer says 14.5 L/100kms, but it's on gas too, and in reality I get about 20 L/100kms (something's not right atm). I would be damn happy with that kind of economy especially in an I6, presuming that is what you drive ??

Cheers,
Steve
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Old 06-11-2002, 20:10   #4 (permalink)
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Yes it is an I6, while it is good I would like to improve it further. My consumpution rate goes up when I have the bike rack on the rear with my bike by a litre a 100. My not on gas, can't afford to loss the boat space.

The type of driving I'm doing I reckon i could have the engine reving at 1500 rpm in 5th at 100 and still have enough power.

Did the xf come out with a 2.77 diff and would it fit?

cheers

Craig
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Old 06-11-2002, 20:33   #5 (permalink)
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I was getting, at best, 8.5l per 100k's in the XR6, with a 3.45:1 ratio gears.

Most happy with that.
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Old 06-11-2002, 20:44   #6 (permalink)
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That fuel economy isnt that bad.

Sitting at 2grand at 100km/h is ok. If you go lower, you will have to gear down when you come across a hill.

Also with a higher diff ration, your acceleration will drop off.
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Old 06-11-2002, 20:57   #7 (permalink)
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I spend about $25 on fuel, get about 500kms roughly on long runs, about 250-350 on city runs depending on how i drive

(i6, lpg)
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Old 06-11-2002, 23:24   #8 (permalink)
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me too ..
but thats one gas...

filled up for 32c a litre today :)
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Old 06-12-2002, 00:00   #9 (permalink)
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Lucky for LPG

I wish I had those figures i was getting 35litres to 100 kays when i had the 2.75 init now i have a 3.55 im yet to try but it wont be good im sure
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Old 06-12-2002, 00:11   #10 (permalink)
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My suggestion for better consumption at high speeds.

Go down to the wreckers and pick up an EAI 2.77. The LSD should cost around $150.
Better still, he would probably be more then happy to do a clean swap for yours.
Grab the controll arms as well.

Bolt it up to your car, and you will have some seriously low revs.
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