Gas Mileage on a Run

The Director

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So, a few days back, I took a 131.15 mile round trip. (I know, absurdly precise, but it was imperative for the math behind this.)

I went from Jersey to Pennsylvania, mostly using County Roads in Jersey on the way out, and there were a few 25 mph zones and some red lights along the way. On the way back, I used Interstate 78 and 287 for the most part. (Our northeastern readers will know exactly what roads those are.) I had the windows open for the entire duration, and I did a decently high speed on the way back, enough to be detrimental to gas mileage (as far as I know.)

So, I actually managed to do 18.725 miles per gallon when I finally went and refilled the tank. (The whole trip wasn't 131.15 miles, but that was the mileage in between the last fill up to that fill up. I did that for accuracy.)

This I think is actually pretty good, given that my Fifth Avenue still has crooked steering (alignment out) and runs a bit rich. (It's a stock car. Still has lean burn and all.)

Out of curiosity, is there anyone else here that would like to share their gas mileage rates? I'd like to see how they compare. (I know, who owns a fifth avenue for the gas mileage, right?)
 

Aspen500

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My Aspen averages about 9-10 mpg but waaaaaaay back when it was still a 225/4-speed it got 16-18 mpg avg. Best ever was 22 on a highway trip. Keep in mind, at that time, the national speed limit was still only 55 mph though and at that speed with an E24 225 (90 hp), O/D was useless on anything but level road and around here that's not too common (seems like you're always going uphill or downhill).
 

Mr C

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Pretty good...I'm averaging @ 15 MPG combined on the 5th Ave that is my daily.
 

AJ/FormS

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For a daily the hiway mileage number often drops to 70% or even 60% and in winter 50%. So then 15 doesn't sound so bad.
My fancy DI 2.4 VVT Orlando/Ecotec falls to 27 typical, from 40 hiway, which being 67.5%.. It is what it is.
 

toolmanmike

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18-20 is a pretty good average. I get 19 and change with my HP 273 in my 66 and about the same with my 318 Swinger. Both have 3.23's and got that mileage with 14" or tall 15's. If I would poke along at 50 mph for hours at a time on the interstate I could probably get 20+.
 

AJ/FormS

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I have ,over the years, stumbled on data to the effect that if you reduce your cruise rpm x percent, your fuel-economy can be expected to rise by about half of that per cent.
So from 65 to 50 is a decrease of about 23%
therefore, by that yardstick, you might expect an 11.5% increase in economy, or from 18 to 20mpg.

I once decreased my cruise rpm from 2600@56mph to 1800@75 and my economy jumped from low 20s to low 30s. It would be hard to make an apples to apples comparison with those differing baselines, but it is, in any case, remarkable the effect rpm and speed have on fuel useage.
 

MoparDan

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Currently 13-15 mpg pretty much all stop and go traffic. A long time ago when the alignment was better and the engine didn't stall I once did as good as 35 or 36 mpg (figured out with a mpg calculator) on a trip from CT to North Carolina, going about 65 the whole way, stock 318 with computer
 

AJ/FormS

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Currently 13-15 mpg pretty much all stop and go traffic. A long time ago when the alignment was better and the engine didn't stall I once did as good as 35 or 36 mpg (figured out with a mpg calculator) on a trip from CT to North Carolina, going about 65 the whole way, stock 318 with computer
Remarkable!
When you say it had a computer, do you mean it was an EFI'd Magnum?
Or a LeanBurn,
Or a custom set-up.
 

Master M

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On the interstate and some town driving getting there with a/c on, and the interstate speed of 75 mph, the 1988 5th. in original condition got 20 mpg going and return trip got 21 mpg.
 

SonOfaTomP

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my mileage is garbage. sometimes i feel like my gas is disappearing into thin air. park it at a quarter tank get back in its at half. haven't quite figured out the exact rate but i reckon around 12 to 15 mpg. pleeeeease tell me how i can get 30 mpg in a Diplomat. most of my driving is on country roads and city/suburban streets
 

AJ/FormS

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my mileage is garbage. sometimes i feel like my gas is disappearing into thin air. park it at a quarter tank get back in its at half. haven't quite figured out the exact rate but i reckon around 12 to 15 mpg. pleeeeease tell me how i can get 30 mpg in a Diplomat. most of my driving is on country roads and city/suburban streets

Are you being serious?
 

toolmanmike

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The first thing to check when figuring mileage is not the as gauge but the speedometer/ odometer accuracy. I am shocked at how many people don't know how to check mileage.
 

mgbeda

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Took a 305 mile round trip last weekend in my '87 5th. Got back and filled up with 15.01 gallons. So 20 mpg, in 100+ heat with AC on full. I was impressed.

-mB
 

The Director

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All these 20+ MPG I'm seeing here makes me confident that I could probably pull it off on my next trip. I recently got my steering straightened out, (FINALLY!), and it seems to drive better with that and the general alignment, so, I might be able to squeeze a couple more miles. Far drives are part of my goal, and I'm figuring that it starts to become very economical after 22 or so miles per gallon. Consider the math:

If we use an equation of r = cm (r is range, c is tank capacity (in gallons) and m as mileage)

15 MPG: 18 x 15 = 270 miles range (in theory)
18 MPG: 18 x 18 = 324 miles
22 MPG: 18 x 22 = 396 miles
25 MPG: 18 x 25 = 450 miles

Notice that the difference between 15 and 22 is enough to add an extra 126 miles to the range of a tank. That 126 miles could mean crossing the next state border or something, in a longer trip. And, less fuel ups would mean better timing.

It gets deeper than that. I'm about to state the obvious, but the cost is noticeable. However, we could break it down to this:

Take the average price per gallon, and use a slightly higher percentile. Maybe at about 15% higher than usual, for a conservative estimate.

In my case, I use $2.47 as an estimate, even though I've been managing to get gas for $1.95 since April, because that was the very high end of what I've ever paid for gas in the last 18 months or so. (I use these high estimates, because it's nicer to see it cheaper when things are lower in price, rather than the other way.) Anyway, we use that number, and divide by MPG. This is literally the cost per mile, on average.

15 MPG: 2.47/15 = $0.165 (can round to 17 cents, but using this to be precise.)
18 MPG: 2.47/18 = $0.137
22 MPG: 2.47/22 = $0.112
25 MPG: 2.47/25 = $0.0988

Why have I put all this in writing here? Because if gas gets expensive again, or for those of us who just want longer ranges out of tanks or just to save money, this should provide some reference. For those of you getting between 18 - 22 MPG, you're not doing bad. I included that 25 MPG reference because if there's a way to get to that, it's not only cheap, but it will go far. This is what we can look to for getting more out of an M-body in terms of gas. Personally, it's very satisfying to see the numbers really high.... it just feels accomplishing.
 
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