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Old 07-19-2011, 01:57 PM   #1
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454 Gas Mileage

I have an 87 Allegro 33' MH total gross weight is under 14k and I was shocked at the low gas mileage I am getting. I just filled up and I'm averaging under 5.5 MPG's. I don't expect to get great mileage and was hoping for 7 to 8. During the last 2 trips I ran the generator about 10 hours with min. load (mainly recharging the batteries). I don't pull a toad.

I feel I drive with a light foot. Freeway about 58 and 2 lane road I try to keep it under the posted speed limit by a couple of miles per hour. I ease out of stops and ease into stops. I use my tranny to slow me down and keep it in 2nd until I'm at about 35 -40 MPH.

What can I do to improve my mileage?

Thanks
Arnold
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Old 07-19-2011, 02:14 PM   #2
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I had a 83 33' MH with the 454 and added a $$$$ Banks Exhaust system plus a $$$$ US Gear overdrive unit.

When I traded it off it's average MPG was 5.96 pulling a 2,600 lb trailer.

Love my 10 plus MPG ISB MH.
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Old 07-19-2011, 02:58 PM   #3
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The only thing I can think of is to slow your engine down by changing your rear gears, which could lead to a bunch of other issues, changing the tranny (another option that could be a bigger headache than its worth), or maybe look into one of those auxillary transmisisons like Gear Vendors under/overdrive transmissions the most awarded auxiliary transmissions.

Given that the engineers at GM might have known a thing or two about the 454 performance, chances are your rear gears are somewhere around 4.56s. To see the kind of significant mpg increases you'd like, I'm betting 4.11s wouldn't cut it (my dad had a '87 3/4T 2wd GMC, 454 V8, 3spd AT w/4.11 and only got 8 mpg running empty or towing 9500# 5er), and 3.73s might negatively impact hill-climbing performance despite having that huge engine.

Given the age of your RV, I'm guessing the tranny is a TH-400 (3-speed auto). The GM 4-speed OD at that time, the 700R4, was not rated for use with the big block V8, and even then had some issues early on. I haven't been able to find if it's successor, the 4L60, was ever mated to the big 7.4L (I'm thinking not), but that might be a remote, and I stess the remote part, option. The later 4 speed AT's all required electronic controls so I'm betting these are out too.

The Gear Vendor looks like a neat product, but at $3k+ before install, it might be hard to justify. Despite having read the info on their site, I still can't figure out how you'd "split-shift" it. Seems to me a standard two-speed rear end with 4.56s and 3.73s, or maybe 3.52s, would accomplish the same thing for less money. Oh well, what do I know...
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Old 07-19-2011, 03:51 PM   #4
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You may remember back when your motor home was new being able to buy a RV cam shaft. Edlebrock was one company that I remember selling a RV package.
A bit of research into 80's performance for your 454 may be helpful.
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Old 07-19-2011, 05:51 PM   #5
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Something has got to be wrong. I have a carbed 454 in my 95 Winnebago Brave and we just got back from a 2700 mile trip. I ran the Onan most of the time driving and drove between 60 and 65. For the whole trip, idling time, city traffic, the whole kit and kaboodle, we got just over 8 mpg. I wanted to find out what a real average was and not just a tank full. See if the exhaust smells like gas. If you are driving mostly in big hills and mountians, that may explain it.
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Old 07-19-2011, 08:24 PM   #6
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Quote:
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Something has got to be wrong. I have a carbed 454 in my 95 Winnebago Brave and we just got back from a 2700 mile trip. I ran the Onan most of the time driving and drove between 60 and 65. For the whole trip, idling time, city traffic, the whole kit and kaboodle, we got just over 8 mpg. I wanted to find out what a real average was and not just a tank full. See if the exhaust smells like gas. If you are driving mostly in big hills and mountians, that may explain it.
1990 was pretty much the last year for carbs. Some were throttle body some were carbs that year. After that throttle body, unless someone changed it over.
GM that is.
I had my carb gone over and my distributor recurved this spring. I went from 7-7.5 to 8,75mpg. Not pulling a toad.
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Old 07-19-2011, 08:45 PM   #7
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There are many things to look at. There is a damper on the exhaust which could be stuck. New plugs, wires and a dis re-curve would help. Rebuild the carb. Re-set the timing. I have a 35' pulling a trailer and I only get 7.4 mpg. Something is wrong somewhere with yours. Good luck!
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Old 07-19-2011, 08:52 PM   #8
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I have a 83 30ft Pace Arrow with the Banks Power Pak installed, but the PO told me not to expect more than 5 MPG, we will be heading to Texas soon and will post the real mileage we get during that 1200 mile trip.

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There are many things to look at. There is a damper on the exhaust which could be stuck. New plugs, wires and a dis re-curve would help. Rebuild the carb. Re-set the timing. I have a 35' pulling a trailer and I only get 7.4 mpg. Something is wrong somewhere with yours. Good luck!
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Old 07-19-2011, 10:47 PM   #9
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You should be getting at least 7 mpg or something is wrong. A lot of 454 owners are reporting 8 -10 mpg. I'm going to have my dis. re-curved and see if that helps. If I got 5 mpg I'd sell it....
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Old 07-19-2011, 11:21 PM   #10
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Quote:
Originally Posted by CharlieTwo View Post
Something has got to be wrong. I have a carbed 454 in my 95 Winnebago Brave and we just got back from a 2700 mile trip. I ran the Onan most of the time driving and drove between 60 and 65. For the whole trip, idling time, city traffic, the whole kit and kaboodle, we got just over 8 mpg. I wanted to find out what a real average was and not just a tank full. See if the exhaust smells like gas. If you are driving mostly in big hills and mountians, that may explain it.
You have fuel injection and an overdrive trans, as well as a timing controlled by a computer. The early 80's system is no match without tweaks.
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Old 07-19-2011, 11:43 PM   #11
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We had a 1993 Winnebago 34 Foot vectra It had headders on it ran streight pipes no mufflers just the Cat converters; It sounded a bit loud When a tunnel or near a berrier, ok on the open road Ran it at 58MPH Towing a car. Tires inflated 95 lbs. ram air filter. It would average 9.5 most all the time Unless The speed was increased/mileage was down. Now that had the Over drive Trans in it; You got to remember that is a truck engine . and would only haul a load one way. inThe motorhome it is loaded all the time;
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Old 07-20-2011, 07:41 AM   #12
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The quadrojet carbs had a problem with a leak in the lead plugs in the bottom of the carb. This alowed gas to be drawn in when the engine was running that contributed to poor milage. A good carb shop would know about this and could repair it and probably help you with your problem. Good carb. shops are getting hard to find because all they teach the younger generation is fuel injection and engine managment.
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Old 07-20-2011, 08:07 AM   #13
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Check your plug wires--at nite-- for sparking. In older units like this I would replace the coil. I have done just that in my earlier days when they were filled with something that looked like plastic or roofing tar. Mileage and power increased dramatically on the higher mileage vehicles. Hotter spark means better combustion and increased power, resulting in increased MPG.
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Old 07-20-2011, 01:30 PM   #14
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The quadrojet carbs had a problem with a leak in the lead plugs in the bottom of the carb. This alowed gas to be drawn in when the engine was running that contributed to poor milage. A good carb shop would know about this and could repair it and probably help you with your problem. Good carb. shops are getting hard to find because all they teach the younger generation is fuel injection and engine managment.
Don't forget "Nozzle drip".....
Nozzle drip is just as bad as leaky well plugs, Just take a peek at or around idle speed and each drip is a big waste...

Jim
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