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Old 03-19-2008, 08:40 PM   #1
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Default RPM, MPH

Curious where the other GXP owners are with the RPM-MPH range.

2500 rpm @ 78 mph w/21mpg on the info center
2200 rpm @ 68 mph w/22mpg
2000 rpm @ 60 mph w/25mpg

I went from the 3800 powered '04 Grand Prix to this '05 GXP & knew my mileage would suffer. But I also thought the Grand Prix'* 3800 was turning a little slower rpm at these speeds.

So, my question to other GXP owners, ..., Are the numbers above comparable to your car?!?
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Old 03-19-2008, 09:39 PM   #2
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I would say those numbers look to be about right...

the 4.6 in the GXP Bonneville has a 3.71 gear ratio... So At 78 you will be turning some revs...

The GP if it was a GT would most likely have had 3.29 Gears...

If it was a GTP then it would most likely have 2.93 Gears..

With the 3.29 I would expect 2000 Rpms or so at 70 Mph

With the 2.93 Gears I would expect 1800ish at 70 Mph..

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Old 03-19-2008, 09:46 PM   #3
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Those are quite consistent with my GXP.
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Old 03-19-2008, 10:44 PM   #4
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Pretty close with mine as well. I wish we had another gear to improve milage!
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Old 03-19-2008, 10:46 PM   #5
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I think the GXP'* would have been fine with a 3.29 Ratio as opposed to the 3.71 ratio...

I feel this would have made for a better longer 2nd gear with the way the 4.6 winds out...

And it would have meant better overall MPG'* at highway speeds... Just my thoughts

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Old 03-19-2008, 11:59 PM   #6
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What kinda miles do you have on your engines? I get consistent 25+mpg easy on the highway with a 4.0 V8 Northstar. (3.71 geared 4T80E)

It gets better consumption then my 3.29 geared GTP. I felt free to stay well above 2000+ RPM and still get good mileage with the Aurora, I don't dare go over 2000 RPM in the GTP.
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Old 03-20-2008, 12:11 AM   #7
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well Jim I also look at it like this... They have .6 Litre more displacement to deal with, so that will drink up a bit more fuel.. Lets say your motor was a 245ci( I know I am close) and the 4.6 is a 280ci ... There is a bit of a difference in terms of how they will use fuel..

I do think that the 3.71 Gear ratio was nuts on GM'* part.... I think 3.47 wouls have been well beyond what they needed.... I think the 4.6 could have turned a 3.05 - 3.29 with good numbers all the way around..

And I haven't asked, but how many miles do you all have on your motors? Are they even broken in yet?

And yeah Jim when you drive that GP.... You really burn off fuel when you get into BOOST

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Old 03-20-2008, 12:12 AM   #8
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My GXP has been going on 600 miles trips to Cincinnati regularly and seeing 23.5 give or take a couple tenths @ 70-72Mph on cruise control. Maybe its the Castrol Syntec oil, or the fine wire copper sparkplugs ( AC 441 LT ), or that I only use Premium Fuel. But I seem to be getting a bit better mileage.
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Old 03-20-2008, 09:21 AM   #9
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These rpm ranges seem about right for me as well. However, at 73mph I see anywhere from 27 - 28mpg.
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Old 03-20-2008, 01:50 PM   #10
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Thanks for all the updates.
29k mile car, 3 months left on warranty, but it'll be over with on the miles in a couple weeks, ...
I just thought it would be a bit lower on the RPM level at those cruising speeds.
Wondered if maybe I wasn't getting a lock-up or something.
Guess it'* good to go!
Later, ...
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