Poor fuel economy, but car runs great?

[King_V]

Ok, I'm not quite sure where to begin, really.

Well, first off, some months back, the battery drained - disconnected it to recharge. So, I guess whatever the computer learned in the years that I was driving it fairly regularly have been wiped.

Since then, though, I've started driving it, not daily, but regularly. I've gone through 3 tankfuls of gas.

This is 80-90% expressway driving. In overdrive, around 75MPH or so. Yeah, sometimes I creep up to about 80. I have 2.73 gears, and factory-style dual exhaust. Hell yes I'm using the AC

Yet I seem to be eking out something in the way of only 20-21 MPG. It used to be closer to 23-24, and I recall some people on this board claiming to get even more from a stock 5.0 with highway gears.

I'm not quite sure why, or what I should be tinkering with - as the car seems to run great. Starts up quickly, idles smoothly, no hiccups or anything on acceleration, etc.

Isn't this abnormally low for a 1987 SEFI 5.0 with AOD and 2.73 gearing? I could see if I was doing mixed driving, but like I said, this is almost all highway cruising.

Thoughts?

[sly]

my 88 only gets that on the highway with the AC blasting... on a good day. Typically 13-19 in that car. I would imagine if you drove 60-65, you would probably see those higher numbers though.

[jpc]

Any computer codes?

[King_V]

I'll have to re-check, but the last time I read the codes (a year ago?) - there was one related to a clogged AIR tube or something (code 44 if I recall?)

I was also thinking that I've never checked the timing on this beast - did a quick search, but while I found numerous threads on timing questions, what do I have to connect/disconnect before checking on an 87?

[BerniniCaCO3]

I'd be happy to have your fuel economy


If you want to check timing, there' a little plug right by the distributor, the SPOUT connector.
It's grey, and it terminates there. Unplug the connector, and your base timing will remain static.
If you try to set time with the spout connector in place, you'll be several degrees off, since the computer normally messes with timing at idle (I forget whether it advances or retards; I'd have to think about it).

[mercurygm88]

My 88 MGM just turned 103,000 miles, and cruising around out in the country I get 22-24mpg. On the interstate running 65-70 it drops to 19-22, to be honest I don't think thats out of the ordinary.

[1990LTD]

Look at o2s.


Check your diff fluid and make sure none of your brakes are dragging. Shot in the dark but if it runs good and the o2s are good it's a though.

[sick88tbird]

Years ago, on my first T-bird, I dropped in an HO motor and was getting piss-poor fuel economy but the car ran like a champ...was getting around 12mpg highway...changed the fuel filter and more than doubled my fuel economy. Good luck

-Don

[gadget73]

21 mpg at 75-80 mph is not what I'd consider to be poor fuel economy.

[pantera77]

This is 80-90% expressway driving. In overdrive, around 75MPH or so. Yeah, sometimes I creep up to about 80. I have 2.73 gears, and factory-style dual exhaust. Hell yes I'm using the AC

Yet I seem to be eking out something in the way of only 20-21 MPG. It used to be closer to 23-24, and I recall some people on this board claiming to get even more from a stock 5.0 with highway gears.

Isn't this abnormally low for a 1987 SEFI 5.0 with AOD and 2.73 gearing? I could see if I was doing mixed driving, but like I said, this is almost all highway cruising.

Thoughts?

Your driving a 25 year old car with the aerodynamic properties of a barn. That's more than perfectly normal.

[Lincolnmania]

sound sounds about right........i got 28 in a 89 gmq rental once but speed limit was 55 and that was 1989

[mercurygm88]

sound sounds about right........i got 28 in a 89 gmq rental once but speed limit was 55 and that was 1989

I can get that in a 55 with my 88, provided I don't have to slow down, speed up or stop. I went on a 60 mile one way trip today on the interstate and averaged 23.5mpg and I don't think I can complain about that.

[cld783]

so driving like a normal person would get you about 23-24mpg? at 65-70. pretty much what i get but the roads have to be perfectly flat.

[sly]

pretty much. Any hills, wind, changes in speed will decrease that number.

[mercurygm88]

pretty much. Any hills, wind, changes in speed will decrease that number.

yes a headwind or tailwind makes a big difference on these cars, I've gotten as high as 30mpg with a 20mph tailwind.