What is the FE sweet spot for... - Fuelly Forums

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Old 10-04-2007, 04:47 PM   #1
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What is the FE sweet spot for...

does anyone know the FE sweet spot(hwy) for GM 3.8 v6(fwd) cars? my 95 olds apparently is not scan gauge compatible. will be testing my wife's car(same motor) for this soon, but since she drives very little hwy and her car is rwd, there prolly is no info benefit in it(at least for my car).

keep in mind that i'm NOT willing to drive 55mph on the 70mph FL turnpike. just want to know if 75 is more efficient than 70 or if 65 is MUCH more efficient than 70,etc.

any help would be appreciated! rh77 and anyone else that has owned/owns or rented one of these vehicles thanks in advance for your info.
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Old 10-04-2007, 05:04 PM   #2
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I used to rent Park Aves a lot around that time for work up north. I'd go 60 - 75 mph and get get around 28 MPG US. No idea where the sweet spot was, but it's an impressive powertrain for economy.
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Old 10-04-2007, 05:24 PM   #3
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Test it. Find a gas station and two a few round trip runs on the highway w/o traffic and the cruise set on 65/70/75.
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I think if i could get that type of FE i would have no problem driving a dildo shaped car.
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Old 10-04-2007, 06:44 PM   #4
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It really depends on the drag coefficient. The best FE seems to have been just above TC lockup. These cars like low RPM cruise. From the 3.1 in the Century or Beretta to the 3.8 in the Park Ave, Grand Prix, and LaCrosse, the best FE came from an average cruise speed around 50.

A good compromise might be 60 mph, but you'd have to test it out. Depends on the car...

RH77
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Old 10-04-2007, 07:00 PM   #5
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the best way to know would be with a scan gauge, unfortunately i think that the sweet spot on an american v6 is pretty low on the rpm range i know they probably throw that top gear out pretty far so i think that rh77 has a pretty close estimate.

the slower the better. and i bet you will get significantly better fuel economy going 65 instead of 75.

i understand you want someone who speaks from experience, but my advice to you is don't go to slow, try to keep it at the speed recommendation(limit) of 70
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Old 10-05-2007, 01:14 PM   #6
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thank you all.

50mph does sound about right according to rpm(well below 2k). interesting tho, 70mph is ABOVE 2k(maybe 2200) w/cruise control, while without, it is right at 2k.
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Old 10-05-2007, 01:37 PM   #7
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TC

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interesting tho, 70mph is ABOVE 2k(maybe 2200) w/cruise control, while without, it is right at 2k.
TC lock and unlock is pretty touchy on GM 4-speeds -- can you tell if the TC is locked at 70 with the CC on? That may be the issue. TomO mentioned this fix in another post -- firms-up the shift and helps prevent damage.

RH77
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Old 10-05-2007, 01:51 PM   #8
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TC lock and unlock is pretty touchy on GM 4-speeds -- can you tell if the TC is locked at 70 with the CC on? That may be the issue. TomO mentioned this fix in another post -- firms-up the shift and helps prevent damage.

RH77
very good link! i can feel my shifts, but not sure how to tell if TC is locked or not. how can i know? BTW, using amsoil AT fluid, so temp problems are not likely.
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Old 10-05-2007, 01:53 PM   #9
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You can tell by comparing the tach to speedo ratio...
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Old 12-30-2007, 05:51 PM   #10
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Bowtieguy,
As a rule of thumb, most engines achieve their best fuel economy at an RPM corresponding to a piston speed of 5 to 6 m/s (16.4 to 19.8 ft/s). Piston speed (ft/s)= 2*stroke(inches)*rpm/720.

Piston speed = 2 x Stroke in inches x rpm / 720

For the 3.8 GM V6 this is between 1740 to 2100 RPM.

Cheers , Pete.
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