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Why Do Some 4.x Engines get 26+MPG, Others <18 MPG Highway

1463 Views 5 Replies 4 Participants Last post by  steelybill
My 87 Eldo (4.1, 63,000 miles) has gotten, at best, 22 MPG on the highway. Usually it is less than that. And I get 15-16MPG around town. I am light footed too.

I see many others posting 26-28 MPG highway, and 19-20 MPG city.

I have tried to check/replace everything I have heard about on this forum, yet my mileage has not improved, other than when I finally put good plug wires on (upped city from 13 to 16 MPG city).

Only thing I haven't done is replace the catalytic converter. How do I determine if it is restricting/clogged?

Any other suggestions?

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Your fuel economy could be slightly low. What speeds on the highway are you driving? You'll get your best fuel economy right around 60 mph.

What type of tires are you using? Wide and low profile tires hurt fuel economy. You'll get the best using the stock 205 width tire.

What brand of plugs and wires are you using? You definitely want to use AC Delco plugs and wire on these engines.

Is your torque converter locking up? Make sure you're not getting that E39 code.

For better fuel economy, try advancing your timing a few degrees. Best fuel economy is achieved when it just barely pings under heavy throttle.

I've heard that a less restrictive CAT back exhaust system can help with fuel economy. Have an exhaust system made with 3" pipes and use a low restrictive muffler. The lowest restrictive muffler I'm away of is Thrush glasspack.

A restricted/clogged catalytic converter would also be associated with driveability problems such as delayed shifts and no power when accelerating.
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