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Thread: 20 mpg?

  1. #1

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    20 mpg?

    Here is my truck:
    1989 mm reg cab 2.0L carb'd 5 speed.1 owner with 128k miles.Last 6 years it sat basically as a guard dog to make think people live in a house that was empty.Owner would rive it periodically and do maintenance on it like he drove it everyday. I have owned a few of these and never had 1 that didn't leak any fluids, was this clean under the hood and the engine made no ticking noises at all.
    Anyway,i installed new plugs n wires and it runs great,fires right up but it gets what I consider sub par gas mileage.Like I said,I have owned a few and never had 1 get this low on the mileage.
    so,am I wrong to think it should get better mpg? I used 93 octane and it didn't do any better.

  2. #2


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    Octane ratings do diddly for MPG, or anything else unless you are tuned for higher octane. Dont waste your money on it.

    Technically speaking, your engine has to work harder to ignite the higher octane fuel, so your MPG may actually go down.

    According to your list of repairs, you only did 50% of the ignition. A cap and rotor should be done at the same time as plugs and wires every time. The reason why os they all carry the same load, and they all get used exactly the same amount of times. Replacing only half, then leaving the other half used makes little difference.

    You should also consider replacing your fuel filter, air filter, PCV (if equipped), and running a fuel system cleaner through. It is also wise to spray down a good carb cleaner to bust out buildup. You may also want to check yout ignition timing and adjust accordingly. Set it to spec for optimum mileage.

    If after all of that is done, and your mileage still sucks, some adjustments may be required.

  3. #3

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    I will disagree on your octane statements based on a few vehicles in my 44 years that I have owned and a 2004 Volvo s60 turbo my wife drives and how the lesser gas makes it pings and gets terrible mpg while the 93 runs smooth enough to pull 30mpg. And if you ever owned a ford that has labor knocking on 87 octane,you will know what I mean about better gas.lol

    I was leaning on the pcv and the fuel filters.I want to say the previous owner did a lot of the other things on the truck. But a once over should get it all straightened out.thanks

  4. #4


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    I build cars for a living. Just tossing in my experience lol.

    Higher octane is meant for vehicles built for it, not a substitution for proper maintenance and tuning. If the engine "needs it" just to run right (not from the factory or as a result of modification to tuning), something isn't right.

    There is a big difference between a boosted engine, a stone age small block, and a Mitsu 4 banger.

  5. #5

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    this thing was built when we had real gas and not watered down ethanol. lol

  6. #6


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    True story, but that is a completely different subject.

  7. #7

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    so,you are telling me all that is told about ethanol gas is false? that it's cheaper but you will not get as much MPG as the higher octane gas?
    You are saying all gas is the same? You are saying to me that 87 octane with ethanol is the same as 87 octane in 1989?
    And that my truck should run the same with either gas in it with no issues at all?
    I appreciate the help and advice on where I need to maybe look to get my MPG up but the all gas is the same story aint it.

  8. #8


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    Where do you see me saying that?

    At the end of the day, you are the one with the issue looking for a solution, not me. I offered up the info, and you didnt like it. Implying I am saying something when I have not, is a fools argument. It's silly.

    I have no reason to offer up bad info, but I'm not going to keep this up.
    I hope you find the solution, and I hope whatever it is that plagues your MPG get solved.

  9. #9



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    A higher octane = slower burn time in the cylinder. That reduces the chance of pinging and extends the burn time. This in turn reduces the amount of throttle you need to use to hold 55 MPH. But only slightly.
    Ethanol = less MPG vs good old gas.
    Yes it's cheaper but you will run more through the engine to produce the same HP. The only way around that is FI and an engine built for eating corn. Thats why we don't fly aircraft on corn. We use 110 - 125 octane leaded fuel and low led fuel.
    As for a low MPG if you get 18 MPG on an engine with over 100,000 on it your doing good. Compression is lower now. The feed back carb is not as good as they were when new. Wires get more resistance in them, the rubber is hard and parts just get old and worn. The technology from 89 to now is light years apart. I always say you can only get, what you can get. Drive it at 55 MPH for a week fill the tires up to the max on the side of the tire and see how much the MPG goes up. You will a couple more MPG. You must keep in mind the national speed limit was 55 when the truck was in production, and we reduced the speed limit from to save gas. Now a new KIA comes off the floor with 200 HP compared to our huge 97 HP new off the floor. More HP = less throttle to make it go. Any thing and everything you do to increase the MPG will help. Alignment, tire pressure, new grease in the bearings you name it. it can help. The easier it is to push the truck by hand the easier it is on the fuel. It goes hand in hand. My ready mix trucks get 4 MPG loaded on a good day, 6 empty. There hard to push by hand but it can be done. Breaking inertia is the hard part. It's the old, a body at rest tends to stay at rest thing. But it will coast from 55 MPH on flat ground for about 2 miles or more before it comes to a stop.
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  10. #10


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    Ethanol is crap.... It has less energy then none ethanol and you will use more in the long run then with standard gas. It also eats rubber and will loosen rust that has built up in the tank over the years. This will cause fuel filter,carb jets and bowl to fill with rust.

    Higher octane is not needed for an engine with 8.5:1 or lower compression with out boost, and no you will not get better mileage running 91 over 87 on a none computer controlled engine (Timing) as the engines now a days will increase timing till it knocks then back off the timing. On a higher compression engine or boosted you will need to run a higher octane so they don't ping under load. My 2002 F-150 I use 87 octane as the engine is only 8.5:1 compression but the D50 is running 11:1 and the bastard pings like a mother unless I run 93.

    If you run 91 you can advance your timing up before it pings and it might help with the MPG and give you a little extra power.

  11. #11


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    20-21 mpg is my average. I calculated each tank fill up just like you do. Don't feel too bad about 20 mpg.
    1989 Mitsubishi mightymax; 1990 4g63 6 bolt swap: malhe 9.0 pistons, eagle h beam rods, acl race bearings, rebuilt head, evo 3 16g, aem afpr, Maft, 3" gm maf blow through, walbro 255 fuel pump, hks ssq bov, fmic, coil over plug, FIC 750 injectors, dsmlink, apexi avcr, AEM eugo, s90 throttlebody, JMF FIAC block off plate. LT1 T56 in the works.
    Why a mightymax? Why not? 18 psi FTW.

  12. #12


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    Check the vacuum advance when you change out the cap. (use a length of line and suck on it and listen for hissing or watch advance move)
    what did your 'other' pickups get?

    E

  13. #13



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    Quote Originally Posted by Fordubishi View Post
    Ethanol is crap.... It has less energy then none ethanol and you will use more in the long run then with standard gas. It also eats rubber and will loosen rust that has built up in the tank over the years. This will cause fuel filter,carb jets and bowl to fill with rust.

    Higher octane is not needed for an engine with 8.5:1 or lower compression with out boost, and no you will not get better mileage running 91 over 87 on a none computer controlled engine (Timing) as the engines now a days will increase timing till it knocks then back off the timing. On a higher compression engine or boosted you will need to run a higher octane so they don't ping under load. My 2002 F-150 I use 87 octane as the engine is only 8.5:1 compression but the D50 is running 11:1 and the bastard pings like a mother unless I run 93.

    If you run 91 you can advance your timing up before it pings and it might help with the MPG and give you a little extra power.

    Don't forget the water it collects. Ethanol is the #1 killer of 2 stroke boat motors collecting moisture and ruining the internals of the engine. This additive is a total scam on the public and destructive to all internal combustion engines. Search your local area for fuel stations that do not yet use this product. Your mileage increases and your engine last longer without this crap.

    I have to agree with Merrill also, I have read many articles about octane and cars nowadays only require higher octane if your pinging and knocking. Then again if you have to climb in octane more then the manufacture requires, there is something wrong with your engine. In the old days, Octane helped, I agree to this also, but the biggest helper was that wonderful metal, LEAD. Oh how I miss that stuff in my engines. Octane only will help to a point then it starts to make things worse. Start burning things in your engine and getting hot spots among other issues. If your not running high performance, higher octane is usually a waste of cash if your not getting engine pings. There is one more thing that is good though. Some higher octane fuels carry detergents that clean your engine. Chevron use to have good cleaning properties in their fuels and if not mistaken their premium was the grade that they used to push detergent fuel additives. So, in tis case a higher octane would benefit your engine I'm sure.
    Cars now are not like cars we use to have. Feeding and engine it's own crap is what we deal with today. Keep shoving that exhaust back in it's mouth until it's all just clean air coming out, lol.
    What I noticed from the earlier years of cars to now is this. Come to a traffic light and watch the idling steam out of the exhaust pipes in the morning. It just barely flows out maybe a couple inches from the tip of the pipe and then disappears. In the earlier years, exhaust pipes would be thrusting that exhaust out and pulsating so hard it would shoot a foot or more out the tip of that pipe before it would loose resistance and flow away.

  14. #14



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    use sta-bil marine ethanol treatment or equivalent - it negates the effects of ethanol in the gas and can improve mileage. My 92 honda prelude went from 18 to 23 mpg in the city and now runs so quiet you can't hear it run with 221K miles on it. Yes the ethanol gas is total crap - it not only takes more to go the same distance, but they can add more water to the fuel, and when it breaks down, the alcohol turns into carbolic acad which eats the fuel system - the sta-bil marine stops that from happening with 1 oz to 10 gallons of fuel for standard 10% ethanol fuel. been running this stuff for 3 years now with no fuel system problems, and the fuel will last for a year or more vs 3 months tops without.
    Pennyman1
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    Living the D-50 lifestyle since 1980

  15. #15

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    Octane is added to fuel to prevent detonation from compression and heat and to ensure that only one ignition occurs for each power stroke. The higher the compression ratio of the engine, the more octane is required to make sure the fuel ignites when the spark plug fires and not before. A really old high mileage motor may require higher octane fuel if the carbon deposits on the pistons and head are enough to raise the compression. I have my timing set at 7 deg. BTDC . The fuel economy is not too bad in the warm weather and there is no pinging. If you try to drive these old trucks like new vehicles you will go through a ton of fuel. Newer vehicles have computer controlled timing and fuel delivery to give the best economy and lowest emissions while giving you a quick responsive ride. The only way to try and mimic that with our carburetor equipped four cylinders is to drive heavy footed which uses more fuel. On the bright side, the gas mileage might not be the greatest but I don't have a huge car payment every month.

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