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Old 09-05-2007, 07:26 PM   #1
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To get more FE...drive faster?

Hello all, I've been lurking but this is my first post.



Today I got my best tank ever-31 mpg

It's my highest. Second highest was 30 mpg. I've been keeping track of tank mileage for quite some time now. Here's the kicker. The 30 mpg tank was on a trip back from my brother's place, the 31 mpg tank just occurred on the way back to Findlay Ohio from Detroit. Both tanks happened at a rough average speed of 75-80 mph. You shoulda seen me slingshotting around semis...

Obviously this flies in the face of conventional wisdom that says you must drive slow to get good mpg. In fact, I've been driving like a grandmother lately for just that reason, and I think my best tank in the last 2 months has been 26 or so. Tonight I just wanted to get home yesterday, so imagine my surprise.

The car may have something to do with it. It's a 95 Buick Skylark 4 door 4 cylinder automatic.

Has anybody ever heard of this happening? I've spent the last 3 months staying at or below the limit on the highway, and now I feel silly.
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Old 09-05-2007, 07:47 PM   #2
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You should get a ScanGauge if you don't already have one. There are many variables when driving. With a ScanGauge you could easily test steady state MPG at varying speeds... or see short term averages for P&G techniques.

My Accord's steady state fuel economy is much better over 60 than under 50 MPH. I think it would do crazy P&Gs at a low average speed.

Anyway, your situation could have involved a 15 mph tailwind... maybe it was just more steady state, maybe it cleaned some carbon? It's tough to tell.

Does it have an ODBII port? It should...
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Old 09-05-2007, 07:57 PM   #3
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no OBD2 started in 96-97 should be a OBD1 car
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Old 09-05-2007, 08:02 PM   #4
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that might be becuase those were all highway that might be why your getting such high fe. Do you do all highway on those 26mpg trips?
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Old 09-05-2007, 08:31 PM   #5
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it could be that the engine is fully warmed, or that your automatic transmition is locking up at the higher speeds, and not alwas at lower speeds, or it could be the time spent behind semis, on long trips your tires warm up rasing the presure, your engine warms up fully, along with the fluid in your tranny, so it simply could be the long trip.
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Old 09-06-2007, 01:18 AM   #6
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Yeah, there's LOTS of variables that could have made that difference. Next thing on my FE todo list is to select a good test course and run it at different average speeds (60-65-70 or whatever) just to get some science on the thing. I kinda wonder if the higher RPM/ speed gets the slushbox working more efficiently.

I'd LOVE a friggen ScanGuage, but I'm definitely OBD1. Exactly how do I get myself a SuperMID?
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Old 09-06-2007, 04:42 AM   #7
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Quote:
Originally Posted by 2TonJellyBean View Post
My Accord's steady state fuel economy is much better over 60 than under 50 MPH.
Both in top gear? I find that difficult to believe.
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Old 09-06-2007, 05:10 AM   #8
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My T-bird's FE is best at 45 (Around 45-50 mpg), it gets worse slower, and it gets worse higher, until you hit around 63, then the FE jumps up to 40-45, then starts dropping off again as you go higher still.
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Old 09-06-2007, 05:34 AM   #9
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I don't know a lot about turbo motors, but I wouldn't be surprised to see a different shape curve on a speed vs. mpg graph in top gear, compared to a naturally aspirated motor like the Accord's.

I suspect a turbo motor's efficiency changes much more dramatically with varying load levels.
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Old 09-06-2007, 05:41 AM   #10
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Speed isn't the reason behind this, it's engine RPM. Your engine is far more efficient at whatever RPM you are at 80MPH than at 60MPH. Engines have a best efficiency RPM, and running in it can more than counter wind resistance, to a point. Ideally, you'd want to regear your car so that you hit that engine's sweet spot at a lower speed, which would allow the engine to run at its most efficient without the wind resistance.

Two examples, had an 89 Camaro RS 305, with an ADS superchip in it, no other changes. Running 93 octane, it would give me 30MPG at 80MPH on cross country driving. If I drove slower, mileage dropped off, if I drove faster, mileage dropped off considerably. Second example, had a 79 GMC halfton, with a mildly hopped up 350. At 60MPH it pulled down a steady 14MPG. I changed the TH350 3-speed auto trans out for a 700R4 4 speed overdrive, and mileage dropped at 60MPH to about 12MPG. If I ran 75 or so, it got 14MPG again. The truck had 2.92 rear gears, and with the 700 the engine lugged a little at 60MPH. Had I switched from the 2.92s to 3.23s, I'd most likely have been seeing 17MPG at 60MPH since the truck would have been back in its optimal powerband at 60MPH instead of 80MPH, and I'd not have been fighting nearly as much wind to boot.

Hope this helps.
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