Better MPG when driving like I stole it?

[Canada Kev]

I just discovered something a bit unusual. Well, I think it is. I'm just wondering if others have experienced this and if it's normal.

I got back from a road trip to see my best friend a couple weeks ago and just managed to get my gas receipts entered into my spreadsheet (yeah a bit OCD, but whaddarya gonna do?).

It was about a 450 mile trip and on the freeway, I managed 140ish kph (about 85 mph). On the second half, the speed limit is 60 mph and I usually am about 5-10 over, but with several higher speed bursts passing all the holiday RVers. I got just over 32 imperial MPG, which is 27 US MPG.

While I was there, I took a 120 mile side trip (60 there, 60 back) to see Darrin's parents. He drove his own car and because he took his cat, we had to drive sort of sanely (she gets car sick otherwise and barfs everywhere). We drove right on 60 MPH or slightly above the whole way there and I mostly did the same on the way back. I got slightly LESS fuel efficiency at 26 US MPG.

When I went home, I pretty much drove my car like I stole it, following the same route I took going down. I got even better mileage than going the other way at about 27.5 US MPG.

So this makes me wonder... When I drive faster and often in passing mode, I get good fuel mileage, but when I poke along at 60, I get worse mileage. Not that I'm complaining, but what's up with that?

[MongooseGA]

Be very careful with the acknowledgement of this occurrence. In a similar situation, my buddy and I were tooling around in the '46 Plymouth I had in high school (it was an antique then, I'm 22) and the gas gauge kept creeping up and up towards full. We joked that we were "making gas" and getting "infinity MPG". Not sooner than I let the words out of my mouth the car clunked to a stall. Tank was dry.

If your car knows you know, it'll bite you.

[Canada Kev]

My car doesn't have internet access, and I never speak this sort of thing aloud within earshot of her.

[KFC911]

Any chance that ethanol "enhanced" gas is distorting your numbers? As a data point, I've only checked my mpg once many years ago. Over a 500 mile interstate trip, I averaged close to 80 mph with a couple of bursts to triple digits and as I recall I got close to 28 mpg. I did that just out of curiosity, 'cause I normally I rarely get out of 3rd gear on the "backroads", and don't give a rip about mileage . I assure you that I get NOWHERE close to 28 with my "normal" driving habits. BTW, my mpg check was prior to this Ethanol crap...I'm sure I can't do that now .

[wayner]

A buddy of mine bring his class of students to compete in a Briggs & Stratton mileage car competition at brainard. He gave up
Building fuel sipping engines. He found that if he builds a dragster engine it is more efficient (while in the narrow power band).

Then, they drive it full throttle + coast throttle coast.

This gets them much better numbers than if they soften the cam and lean out the carb & driving steady throttle.

[Drisump]

Kc 911 brings up a good point, on a recent trip in my wifes bimmer, every leg that we were able to get straight premium gas, the fuel economy was 3-4 mpg better. I don't think that my 3.2 gets better fuel mileage at 80 vs 60....gearing and torque band certainly has huge effects but, pushing that larger wall of air at 80 mph would likely consume more fuel per mile. The best fuel economy I can usually get is at 60-70 mph (and the economy is in the same ball-park as you) but when I really "give her the beans" (shifting through the gears, running up the rpm, ) the economy drops off a cliff...but oh so worth it! 12-15 mpg isn't difficult to "achieve". Kev, I hope your drive back from Sun Peaks was uneventful...Cheers.

[redridge]

Yes... Leaving the cat home will give you better gas mileage.

[stlrj]

I would be very careful about basing mileage calculations on the assumption that everytime I filled up my tank, the volumn inside the tank was the same. Due to the 911 gas tank design, the bump under the spare will never allow you fill consistent amounts of fuel depending on how much air is trapped inside the inverted bowl unless properly vented, which clearly, it is not.

All you can do is make assumptions which may not be accurrate.

If you need proof, fill up after running your tank less than half. Drive around the block and refill again. Suprisingly you may need to add more than 1 gal just to top off.

[Canada Kev]

Any disparity wasn't due to ethanol - everywhere I filled was marked as no ethanol. Leaving AB, I used Shell 91 octane, filled in BC with Chevron 94 and then again with that at the same station two days later, before I came home.

I was just curious why I didn't get substantially better mileage when cruising at a mostly steady 60 and 2800 rpm as opposed to often 35-5500 with foot on the floor. It wasn't ALL passing, but when coming home, I just wanted to get there, so my speeds were notably higher. The two driving styles were VERY different.

I realize that pushing more air at higher speeds should create poorer mileage. Maybe the car is more efficient at 4000 rpm. Or maybe, it is just because the tank isn't topped up the same amount from one fill to the next. I have no idea. I generally try to be consistent, but perhaps that's not enough. I just found the whole thing interesting.

I just find the overall fuel mileage from a performance vehicle driven the way I often do, to be unexpectedly good. I'm not trying for fuel economy and really don't care about it, but it is always a pleasant experience compared with my Jeep. Even on a recent tour with some other Pelicans for the 2012 CanAm Tour on the only full day I could be there, I still managed almost 24 US MPG . This was on some nice twisty routes and with a bunch of others who like to drive fast. And when ripping round town with a few short trips thrown in, my mileage goes all to hell with numbers in the 21-22 US MPG area.

[Canada Kev]

Originally Posted by redridge

Yes... Leaving the cat home will give you better gas mileage.

That was probably it, though the cat was in my buddy's car, so maybe not.

[ivangene]

I am getting WAY better mileage at the track then on the street.....I get about 10-11 on track and way the hell up in the 20's on the highway.......

someday I aspire to be in the 7-8 range but you have to be a really good driver to get those type of numbers

[Targalid]

Twenty five years ago when I had my 85 911, Montana had no speed limit and one could easily do these kinds of comparisons as long as one was "safe and prudent". I found my best mileage came between 90 and 105 mph. The engine had a different note above 90 mph. It sounded like it finally was happy and no longer struggling to breathe. Remember that these cars were designed for the Autobahn where speed is allowed. Traveling at 100 mph is fairly sedate on the highways where our cars were born. This is what our cars were designed to do. I don't know about higher speeds but cruising near 100 gives good mileage. Germans are quite thrifty and I am sure that they have run the numbers to find the optimum speed for best mileage versus time to destination. It certainly isn't at legal North American highway speeds.

[Ronnie's.930]

WOW, there sure is some misinformation and delusional thinking going on in this thread regarding high speed/high rpm = good fuel economy (unless, like ivangene, you define good economy as using a lot of fuel to go fast)!

Canada Kev - what your mileage findings indicate is that you need to re-think your fuel mileage monitoring/calculating techniques.

Targalid - were you being sarcastic or humorous when you stated that "cruising near 100 gives good mileage" (like, "good" compared to fuel mileage of a Cessna 152 at 100mph), or did you mean that the sensation of high speed equates to "good mileage"?

[GaryR]

If I average 90MPH at Limerock and run for 20-25 minutes I use 5 gallons of gas, so figure +/- 30 miles traveled so that's about 6 MPG..

[Canada Kev]

Gary, you're gonna make Ivangene jealous...

[Targalid]

Well, that's my story and I'm sticking to it. Those who race their cars are changing speeds, accelerating out of turns and so on. Cruising on the highway at a steady speed is another story. German highway speeds are higher than here. I noticed no drastically higher fuel consumption when cruising above 90 mph across the 460 miles of I-90 in Montana. I did notice a different engine note at those speeds, a throaty pleasant sound which is different than that which our cars make at our now normal highway speeds. I do not believe German drivers would accept 6 mpg from their cars while cruising. The last time I drove the Autobahn was in a Rover sedan and we traveled at 100 mph without doing much worse than 20 mpg. With the cost of fuel over there, believe me, we would have noticed. But then we were just driving along, not trying to get a fast time on a track. There is a difference.