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PostPosted: Tue Aug 18, 2015 1:07 am 
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Joined: Tue Jul 29, 2014 10:36 pm
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Year and Trim: 1991 Bonneville LE
I've heard of 3800 powered H bodies getting 30MPGs more times than I care to recount. From different people driving different cars.

I've only recorded my economy since I've gotten my 91 Bonne, I've driven it over a year and over 15 thousand miles. The best I've achieved was 25 on a road trip.

Some things I've considered was the introduction of ethanol in modern gas, from a bit of research I've come to imagine it would make up to 1.5 MPG difference, and the fact that I always run with the A/C on Max Setting. I wonder how many MPGs I lose by running it.

I've been getting 20MPG lately and that involves 75% Highway with 25% City, A/C always on of course.

I used to think something was wrong with my car, but it only has 111k on it, good vacuum readings, new ignition, coil pack upgrade, new 02 sensor, maf cleaning, air filter, fuel filter, etc.

Is the A/C and ethanol the culprit? And have any of you people gotten the 30mpg, and if so under what conditions.

Thanks

Edit: I forgot to add my final drive ratio it is 2.97 (RPO F79)

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Last edited by Keepin'her_going on Tue Aug 18, 2015 12:37 pm, edited 1 time in total.

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PostPosted: Tue Aug 18, 2015 6:15 am 
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I'm not sure what a 91 is capable of but I'm curious, what is your final drive ratio?

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PostPosted: Tue Aug 18, 2015 7:17 am 
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Joined: Tue May 27, 2008 8:53 am
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Location: Las Cruces, NM
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I also believe ethanol plays a fair bit into it too. I believe they didn't have the computer tweaking air fuel ratio for e10 until 94. I have even noticed drivability concerns that went away with ethanol free. Also some mods will help too.

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PostPosted: Tue Aug 18, 2015 9:59 am 
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Joined: Thu May 22, 2008 8:09 pm
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Location: Michigan
Year and Trim: 2002 Buick Park Ave
AFAIK the 30mpg+ experiences are typically with series 2 engines. The best I was able to achieve (in a 92 C body, S1 L67) was 29 mpg. So I think your 91 numbers are right where they should be. My series 2 personal best was 34 mpg for an all highway trip. But the FB page was talking about drafting, and I don't care that much for the already bubbling paint on the hood, so I thought I would give it a try. 29-31 is more normal for that trip.

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PostPosted: Tue Aug 18, 2015 10:04 am 
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Location: Dearborn, MI
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I think the pre-'92 cars are worse off on E10, when I had my '89 Electra the best I had ever gotten was 26 on E10. I vaguely recall getting 28 out of that car without E10, no clue on the FDR that car had. At a 50/50 mix it would get 20-22, and 75% city was down to 18-20. I wouldn't think A/C would cost you that many mpg though.

30mpg was achievable on the '86-'87 LG3 vin 3 motors, and then on the L27 and up. LN3 gets close, maybe in the right car with the right gearing you could get 30.

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PostPosted: Tue Aug 18, 2015 10:25 am 
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Location: Portsmouth, England
Year and Trim: 1992 SSEi
I have seen 30 mpUSg but only on long highway journeys at a steady 70mph. No air con in my case.


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PostPosted: Tue Aug 18, 2015 12:37 pm 
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Year and Trim: 1991 Bonneville LE
Sorry, I forgot to add my final drive ratio it is 2.97 (RPO F79)

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PostPosted: Tue Aug 18, 2015 1:10 pm 
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Location: Las Cruces, NM
Year and Trim: 1993 Buick Park Avenue
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Most of the heavy fuel mileage hitters have the 2.84 fdr also. Speed and terrain has a fair amount to play as well.

In my PA, I have never been able to crack 25 mpg with a 2.84 fdr. In town mileage is worse because gearing is working against it. A/c hurts pretty bad too. The Regal, with a 3.06 fdr pulled 25 mpg on its first trip out through the mountains, and it needs a tuneup, and suffers from a pretty hectic misfire under load. It consistently pulls better in town fuel mileage though.

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PostPosted: Tue Aug 18, 2015 8:39 pm 
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The only way I get below 30 on the open road is if I travel over 80 mph. The roads are mostly flat around here though.

3.05 FDR, Series 2, Overkill tune with extra cruise timing.

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PostPosted: Sat Aug 22, 2015 8:10 am 
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Location: Space Coast, or at least it used to be
Year and Trim: 03 SLE, 95 SE, 95 TS SE
When my 91 was alive... 2.84 ratio.. I could easily do more than 30 on the road with the ac on.. I recorded 31 mpg with a couple of runs in the triple didgets.. I had a hard time believing it so I recorded milage on the way back and I pulled 33-34 keeping it at 70-75.. the 2.97 ratioshould run 29-31 mpg without much hassle. My dealer service writer at the time had a 90 Bonneville SE with The 2.97 ratio, even he said 30mpg was easy to do on the road..

The Vin C really should be capable of 30+ with the 2.84 and 2.97... 3.33 will tear that wall down, 3.33 might get 27 if you try hard..

Something as simple as an injector flowing a little much, an O2 reading a tad off could affect mileage and bring it down.


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PostPosted: Sat Aug 22, 2015 8:48 am 
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Year and Trim: 1999 Buick Lesabre Custom
When we drove my LeSabre to Chicago last year, I averaged about 32mpg on regular 87. When we took it to Miami for our wedding, I lucked out and found multiple gas stations that had ethanol free 100% gas. On that trip, I averaged almost 36mpg. 99% of those trips were on the highway, anywhere between 55mph to 75mph with the cruise control set.

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PostPosted: Sat Aug 22, 2015 11:28 am 
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Location: Farmington Hills, Michigan
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My 97 se is capable of 30mpg when it's all freeway. My old 94 Regal would get close too on long drives when sticking closer to 60 mph. both N/A but the regal had a 3.06 FDR.

the 99 averages 24 on the freeway.

I blame ethanol for the less than expected mileage; back in the 90's and early 2000's ethanol wasn't as much of an issue.

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PostPosted: Tue Aug 25, 2015 5:34 pm 
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Year and Trim: 2000 Galaxy Silver SE no mods
On my 2k Bonnie I got new tires and Gabriel ready struts put on and achieved 32MPH. I also got rid of my hard shifts because of the new ride height.


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PostPosted: Wed Aug 26, 2015 8:42 am 
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Year and Trim: 1997 Buick Park Avenue Ultra
I used to get 28-29 mpg in my '95 series I supercharged park avenue. The best i recorded was 31.3 mpg on a 240 mile trip on E10, 95% highway and a few thousand feet net elevation loss through the mountains. I can't specifically recall, but I was probably using A/C as I often do. I haven't done hand calcs on the series II supercharged so I can't speak for it, other than to say that the computer read 27.6mpg for my recent 1000 mile trip with A/C on and ample cruise use between 60-80mph.

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