30 mpg highway??!
 

Guys I am curious after getting this good of gas mileage.

I got 28 mpg on a 150 mile highway trip to college cruisng at about 4k in 5th at 65-75. Is this normal? I was suprised when my gas gauge barely moved from 3/4 the entire trip.

At the end of the day I had over 230 miles on the tank before it got low enough (below 1/4) to make me fill up. This includes after I got to campus, taking my car into the mountains and doing a 30 minute run at 5k+ pretty much the whole time. Then the next day I went on a 40 minute run in the mountains with some friends and filled up that night.

I don't believe I'm running lean. It wasn't getting hot except when I was trying to keep up with them damn turboed cars uphill. It doesn't stutter or anything like that and after getting 12 city this suprised me, like a lot.

lol, i had the same thing happen in my MGB, i got to like 160 miles or so, and it was more than half tank, and i was wondering which one was lying, the odometer or the gas gauge!

turns out that i really did get 30mpg, weird because it does like 7mpg in town

Its possible. How did you determine you were getting 30mpg? What was your methodology for doing
the measurement?

I got 30mpg at first with my hogged out nikki with the stock primary jets. I now get 24mpg all day
long, even in the mountains. Sometimes it will dip to 20mpg in the mountains depending on my foot.

I used the gas gauge and odometer. When the gauge settled at half on level ground it was at 200 miles. So I just divided 200 by 7 since I knew it was completely full before I left. Not the most accurate but it works.

oh if you actually put gas in it, you'd probably not get 30 mpg. you need to actually put gas in it to get the correct amount, and then you can do the maths, unless your PJ and correct the odometer too

What do you mean? I reset the odom every time I fully fill up, which I did before I left.

I used to get 27+ with a 12A and have seen as much as 32mpg. There is a happy spot in the engine's powerband/chassis power consumption curve/carb tuning right in the 70-75mph range where everything comes together. Drive slower, gets worse economy.

Need to check your odo for accuracy... meaning check it against mile markers. Mind where the odometer is when you pass one mile marker and then check it against the mile marker 10, 20, 50, 100 miles later. Mile markers are generally very close to correct but error happens due to whatever reasons so you need to average.

When I did 32mpg, i got GRILLED over it :) The funny thing is, I had 4.78 gears and some other error-inducing things, so the odometer actually reported over 40mpg...

you need to get the amount of gas as well when you fill up, not guessing that the halfway mark on the gauge equals 7 gallons. it may or may not make a difference (probably will though), but it gives you a more accurate calculation.

dude these things are supposed to get 9mpg, and a bridgeport idles at 4000rpm, and you can't drive it on the street, and it doesn't make any power below 12,000rpm!

Yeah I know, my bridge port idles at 5000 and i have to shift at 17000 and the fuel consumption is so huge i need to shut it off at gas stations because the gas pumps can't keep up with the engine

(actually i've done 29mpg with the bridge port... normal is about 22-23)

The gas gauge is not what you'd call linear, and the tank is not what you'd call square.

Measured gallons over measured distance is the only way to be sure.

High 20's on the highway at steady speed is not that unusual for a 12a in decent tune. Low 30's quite doable.

Last trip I made out to Vegas and back I was getting very happy mileage, held a steady 80 and better the whole way, windows down (I have no AC ATM).

Around town in stop and go traffic, yech. Half or less.

The solution for poor city fuel economy is to avoid city traffic as much as possible :) I'd happily drive 5 miles of highway to avoid one mile of city.

i've been doing something like this. my morning commute is like 3 miles round trip, and its not a drive, its more like intermittent parking, mileage is bad, in any car. on the way home i've been taking a longer route, but it skips the worst of the stop lights, mileage seems better, slightly, or less worse.

When I filled up around 230 miles it pumped 9.65 after some extra squeezes. This still isn't accurate though because of the air bubbles when filling, there's no way to tell when it's actually full.

Which us why I prefered the odometer way bc I knew it was full when I used gas tanks to fill it.

The position of the guage is not accurate, at all. You have to note your mileage at each fill up.
Also fill up on level ground or at the exact same pump location each time. When you fill the tank, do
it identically each time. Then average the results over a few fillups to get a true idea of mileage.

The way you are doing is not accurate and you probably didn't get 30 mpg. In fact your last post
shows you getting 230/9.65 = 23.8 mpg for that fill up. Which is ok mileage.

Yeah I realize that it was different then my previous calculation but either way isn't really accurate. I don't care enough about gas mileage to do a completely accurate test I just was suprised that I got anything over 18.

Well you cared enough to post about it here when you thought it was 30mpg!

Seriously, though - I set up an experiment many years ago which did a Six Sigma Design-of-Experiments taking into account seasonal factors, oxygenated fuel additives, city/highway driving, and level of performance - and found that the single biggest contributor to overall fuel mileage was city vs. highway driving. In other words, the more you go and the less you stop the better your mileage will be. Recognize (without boring you all) that this also takes into account all the other variables like whether I was refueling with oxygenated fuels (made no statistical difference), driving it in the summer vs. winter (not statistically significant), and whether the car had been upgraded with performance parts (no difference). All the math was there to support it.

Part of the analysis was finding a VERY accurate method of refueling and drive testing to make sure I was getting consistent test results (decreasing measurement system error). This had me refueling at the same pump, on the same day of the week, at the same time of the day - to remove any extraneous variables. Also, after each fillup, I'd drive the same road course in an out-and-back fashion to avoid terrain features affecting fuel mileage. The best way to get the most accurate reading is 1) lots of data points, and 2) fill your tank completely, drive it for greater than 150 miles, and then refill the tank completely.

Divide the miles you drove on the odometer by the exact amount of gasoline you had to REFILL the tank with, and there you go.

You'll always have some measurement system error from the tires, wind direction, AC use, et al - but over time and by collecting your mileage readout, those factors are muted. I still track fuel mileage just so I can tell when compression starts to go (mileage will drop considerably, and quickly).

Performance parts upgrades always made fuel economy better for me. Oddly enough, ditching the Mazda fuel pump for a Holley unit was usually worth 4mpg by itself. I figure the fuel flow was "smoother" so it disrupted the carb less.

i did a small analysis on mileage in my vert, and found much the same as Mr Duck, city mileage was bad and freeway was good.

the thing a ran into though, was that my test loop was too short, something like 50miles, so the refill amount was under 2 gallons. at 2 gallons and 50 miles, you need a couple decimal places of accuracy on "full" and i'm not sure why but the tank/pump isn't that accurate. so my highway mileage varied way too much.

i found the fewer miles you drive the more accuracy matters, to wit; 230miles in 9.65 gallons = 23.83mpg. 230miles/9.75 = 23.58mpg and 230/9.55=24.08.

vs my test 50miles/1.92 gallons = 26mpg, 50/2.02 = 24.75mpg, 50/1.82= 27.47mpg

Long Duck,

i've been logging mileage data on the Impala since last December and the plan is to compile it for a full year. i haven't been nearly meticulous as you described you were. for example, i don't necessarily go to the same station, let alone on the same day, at the same time. i do however, record the fuel to the same 3 places on the pump/receipt and i make notations on things like A/C use, maintenance and anything i deem not ordinary. so far, my findings are precisely in line with yours.

i can attest to having the same experience, Peejay.

Anecdotally, I would agree that performance upgrades which increase horsepower (free-flow exhaust), decrease friction (light flywheels, light driveshafts), and decrease rolling resistance (+2 wheels, low profile tires) do increase overall efficiency which can be tracked through fuel mileage - but the statistical math didn't support this as a critical factor - in MY experiment.

That's not to say that it wouldn't be statistically significant with other factors being considered. In my examples given above, you should definitely see a slight bump in fuel mileage if you increase performance in those ways, as it allows the fuel you're burning to do more work with less energy loss and that would be seen at the pump. There's a strong argument that you burn less fuel to do the same amount of work (acceleration, distance) with a more efficient vehicle, but in the end we're talking about sportscars and acceleration requires burning fuel.

Big wheels/low profile tires always hurt, not help.

Putting 205/50-15s on my bridge port knocks highway economy to 17mpg corrected... put 215/70s back on it and it's 24mpg corrected, same conditions.

If they are still 15s, that makes sense because the 50s reduced the tire size and will
force the wheel to spin more times per mile, thus reducing your mileage. Low profile
tires don't always mean reduced mileage, it depends on the overall diameter of the
rolling stock combined.

You missed the part where "corrected". I care about real miles driven not whatever fantastical figure the odometer says. With 4.78 final drive and who knows what transmission is in the car this month, the speedometer/odometer is never remotely accurate. Need to correct error.

Hell, I'd say just use the GPS but there's error in GPS as well. It doesn't track altitude changes much. There isn't any flat ground in Ohio.

I didn't miss it, what I'm saying is your revs are higher for any given speed and so
your mileage will suffer because you chew up more gas cause the motor is working
harder.