meilers

What with all the threads about MPG on the forum lately (and premium gas still well over $2.15 here) I thought I would add a spin to the discussion. Many of you in those tankful/mpg threads mention that driving like a granny (shifting below 3k, avoiding boost like the plague) helps your MPG, and logic dictates that this is so -- after all, to go faster you must burn more gas, right?

However, I'm suspicious that the WRX turbocharged engine is actually more efficient at higher RPMs and engine heat than we give it credit for. To test this theory, I spent a week driving in Super Granny Mode(tm) as I like to call it, getting passed by Civics and Hundais left and right and pretending like my car would explode if it got over 3.5k rpms. The one thing I did do which negatively affected gas mileage was run the A/C (come on, it is Arizona in August.) My result? About 288 miles per tankful (a tankful being 13.1 gallons, when the yellow light goes off) for a total of 21 mpg, with mostly highway driving all by myself (185 lbs) in the car.

Then, on Monday, I took my brother (200+ lbs) on a 116-mile scenic drive through the Superstition mountains. Not only did I climb 4,000 vertical feet during the trip, but the majority of it was twisty sand and gravel roads (a side note: my Goodyear F1 GS-D3 tires, max performance summer, handle EXTREMELY WELL on sand!) and I spent the whole time hard on the throttle, eager to show off my car and my driving skillz. I did a few things that were foolish, such as passing other cars UPHILL at 80+ MPH in fifth gear, and I also spent about 15 minutes at 110 MPH on a stretch that I know has no speed traps (nor were there any other cars at all). I also had the A/C full blast for about 1/4 of that trip. This is a totally stock car, with the exception of a Perrin foam filter and 17" wheels and tires.

Guess what -- I got 285 miles out of that tankful. I've got no explanation as to why, but I am dead certain that I will never waste another day of my (or my WRX's) life granny driving... Thoughts?

Eleazar

I really really wish I had some scenic mountains to drive through

How far did you push the gas down most of the time when in granny mode? I find that in general with most cars, light on the gas even mid-range rpms is better. 3/4 throttle but shifting early is bad for mpg.

Wingless Wonder

iTrader: (7)

Driving style does make a difference. First, I calculate actual fuel economy because 'guesstimating' by the tankful can vary by a significant amount.

City driving granny style doesn't help as much as maintaining a steady speed on the highway. That's because even short-shifting (without lugging) is still shifting and like a carbureted vehicle the ECU dumps a shot of fuel to cover that hole in intake charge velocity when the throttle is released and then opened during a shift. It just isn't going to make a big difference when you have to shift a lot in traffic situations.

On the open road, however, my mileage is significantly improved by keeping the speed to posted limits or slightly higher depending on the flow of traffic. I have come close to getting 30 miles per gallon (29.9) and was just a shade under 400 travelled miles on one tank of gas and the low fuel lamp was not even on, yet! More than a few full-throttle blasts to pass convoys of trucks will seriously reduce these numbers.

Is it worth the cost savings to drive like this? Probably not, if you like to drive the way your subie was intended. My feeling is that I don't have to stop for gas as often, which is more important to me than arriving at my destination a little bit sooner. My friend has a Suzuki Hayabusa and while he can get better fuel economy than my WRX (when he's not hard on the throttle), his limited tank capacity makes him have to stop for a fillup much more frequently on trips.

--
0==WW==0
"…axles of evil…" - george w. bush

BlingBlingBlue

I bought this car expecting ~20 mpg, which is exactly what I get. I have experimented with granny driving on a few tanks. The best I got around town (my commute) was just under 24 mpg about 3 mpg better than my normal. I got 26 on the freeway once with the cruise control set at 65 most the time - from San Jose to Calistoga, which has a couple hills to climb. So...driving style can improve mileage, but I bought the car to enjoy. If I had a long commute, I'd be in a TDI.

meilers

Perhaps I'd better define my granny driving style... Yes, I do caclulate actual fuel economy, not guesstimate (no careful reading points, WW! :-) ). Granny driving is no more than 1/4 throttle, shifting before or at 3k (except for first-to-second, which needs 3.2k to not bog), avoiding excessive braking and minimal passing on the highway. Using cruise control on the highway portion of my drive I was able to get 23 mpg out of a few tanks; however, this is almost impossible to do, as the Phoenix traffic is very choppy and uneven (many people going different speeds, from 55 to 85) that you can't just sit in a lane and cruise.

To give you an idea of what real granny driving is like, when I am driving that way I get passed by almost everything on the road, and pulling away from a stoplight I might as well be driving an Echo. It is truly a shameful way to drive, but $2.30 premium is enough to humble any man.

However, what I am really shocked about, and what no one has remarked upon, is how I still managed to get 20 MPG out of that tankful blasting through the mountains -- I fully expected to drop to 16 mpg, having spent so much time on boost and heavy on the throttle; I even did some drifting on the sandy corners, kicking up dust from all four wheels. Frankly, I was very impressed with how the car held up, stayed cool (temp gauge never moved) and used fuel.

namakemono

iTrader: (3)

i did drive at higher rpms and that is when i got just about 280 miles per tank, now that i started laying off the high rpms and shifting early i'm getting closer to 350.

sneezerboi

iTrader: (9)

Yeah, my last tank in my FXT lasted about 340 miles while shifting @ 2500 rpms. I still had more gas left as I filled up right when my light came on.

meilers

All three of you are missing the point entirely. The point is that I was able to go on a 116-mile mountain drive, on boost almost the entire time, and was able to still get 20 MPG. THAT is what we are talking about.

sampson

I don't own a boosted engine, just a 2.5n/a.

Many times gas mileage estimates are off when you calculate on take volume due to the fuel guns stopping at different points in fueling.

I live out in the hills in central NY and drive constantly on steep grades and in the country roads. The weather here is rather extreme during winter.
I also drive about 40K miles a year for my job, so I'm driving an easy 200 miles a day on the weekday.

I've noticed after my car was well broken in the mileage stayed fairly consistant no matter how I drove it. I get a solid 26mpg all around. Mabey a touch more up to 30mpg when I do only highway driving, and a little lower, around 24-25 when I do mostly stop and go driving.
I've driven hard through the mountains all day, keeping the tach at around 4,000rpm all the time, and still managed 24mpg, with 3 people in the car.

So driving style does have something to do with it, but the engine has a range of mpg in which it will almost always fall.

Cheers,
Matt

joshuagore

iTrader: (1)

I believe more people see a large difference in gas mileage when they are on the throttle because most poeple on this forum dont have stock wrx's. So if your wrx isnt stock and your getting on it chances are your rich an awefull lot.

So if your afr is rich there is no way to burn less fuel per mile than if its stoich.

dynamicprojeKtz

iTrader: (23)

i get 250 a tank no matter how i drive. its about 20mpg. pretty much what i expected from this engine. i have a prodrive catback and samco intercooler hoses.

BlingBlingBlue

Dude, I think that is pretty strange as well. Unfortunately, I don't have an explanation for you.
I will make a comment about the temperature needle - the only time you're going to see that move is when the car is warming up or if it's quickly on it's way to overheating. The spot in the middle where it stays actually represents a fairly large range of normal operating temperatures. Most factory coolant temp guages are like this these days.

meilers

Yes, I'm well aware of this, and "dummy gauges" like this are nothing but an annoyance -- what good does it do me if the gauge alerts me AS I am overheating, not BEFORE? I talked to the salesman at the dealership about it just before I bought the car, and he noted that he was glad the car didn't have an exact temp display, because people would constantly be bringing the car back to the dealership to get it checked because of normal variances in the operating temp; he clearly saw it as a plus. :P

meilers

Actually, this isn't true! I am going to sound like some sort of fuel gun geek, but the way that the shutoff mechanism works is totally mechanical (not electronic in any fashion) and is therefore totally reliable, as long as you use the same type every time.

My uncle worked for 20 years for a company called Pride Cast Metals in Ohio, and their primary business was casting the handles, bodies and joints of fuel guns. In the models they made (whcih are used by AM/PM, Sunoco and others) the shutoff works like this: by law, the path from the underground fuel tank to the tip of the fuel gun has to be 100% airtight, but you have to let air into the big fuel tank in order to get gas out of it, or you will be creating negative air pressure. So, each fueling station has a small hose (which they call a vacuum line) inside the big hose, and each nozzle of the gun has a tiny metal tube cast right into it which hooks to that small line. DO NOT put the nozzle near your face to check this, just trust me!!

The way it works is simple: when gas fills the tank and begins to fill the delivery pipe itself, the gas covers the end of the nozzle and shuts off the air returning to the big tank, creating a momentary vacuum; that negative pressure pulls a "ball valve" into place and causes the nozzle to shut off. This setup also has the added benefit of helping to "vacuum" gas fumes right back into the underground storage tank, which is environmentally a good idea.

So, the only time the gun will shut off is when the end of the nozzle is 100% covered by gas, and if you make sure to put the nozzle all the way in, this should be nearly the same level every time. I'm certain there are dozens of ways to design this system, as is the case with any engineering idea, but AFAIK all of them use a mechanical principle like this because clearly having any electronic mechanism immersed in a stream of potentially explosive gasoline is a bad idea.

Whew! I hope that was useful for someone, I just had to share my trival knowledge of fuel-delivery systems.