aussie77

1500 miles and counting, and I was beginning to get a little frustrated. My tanks to date had been as follows:

17.2 mpg
16.1 mpg
15.8 mpg
16.8 mpg

Every time I would fill up on saturday morning before heading out to tennis, which was usually a fairly decent highway drive away. I'd feel like I was getting decent mileage on the highway, but come monday I'm back idling in stop and go traffic taking 20 minutes for my 3 mile commute to work. Sitting there I could literally watch the needle on my gas meter slowly dropping one notch at a time.

So this past weekend I knew I had a fair bit of highway driving to do. I filled her up saturday morning as usual. Got on the freeway, set cruise control to 73 mph and went. Used the same home, and then the same again taking a trip to visit my brother. 157 miles. 7.776 gallons. Almost all highway driving, but I broke 20 mpg for the first time

Note: I used cruise control at 73 mph because that works out around 3600 rpm, just below the second port opening. I figured that's going to give me my best speed/mileage ratio. Could probably get better at lower speed, but I'm not old yet!

RX-GR8

was this the first time you had done this much highway driving? do you think its more because the engine is breaking in?

aussie77

To be honest I am not entirely sure. I had never filled up after almost 100% highway driving before. Being tuesday I'm back to my stop-go drive to work again so I might fill up later this week just to see what it is like for comparison.

I have definitely noticed a correlation between time spent idling and gas usage though. Idle seems to just swallow gas like lemonade.

Greg

mmm lemonade man im thirsty. Hey I get pretty decent mileage and i only use gears 1,2, and 6. Last tank was 260 mixed commuting and traffic miles.

LL7

When I first got my car (about 400 miles on the odo) I took a drive that was mostly highway and got over 20 MPG. Since then I drove mostly in the city and was getting about 16-17MPG. Last weekend I again took a drive that was mostly highway and got about 20.5. The last trip was 230 miles of 45 to 65 rural higway with no fast acceleration (looking a leaves with the rest of the north east) and about 30 miles of over 80MPH driving. My car now has over 200 miles and I have not seen any change in milage. Around town it gets about 16-7 and on the highway it gets about 20. It also does not seem to matter how hard you drive either. On the tansk where my wife drives most of the miles the car is driven very easily and she shifts around 5K. I drive it harder and shift frequently at 9K. We get the same milage (+/- 1 MPG each tank).

mikeb

mpg isn't consistant on my 8
I went from 13 to 19 in the first 5k
its about 18 right now , so I happy

rxeightr

aussie77,
Welcome to the 20+ mpg Club.

I have been averaging 21-22 mpg.

mikeb

dang
you guys started a club

I'm just not there yet

vibec

I don't think I will ever join the 20+MPG club

I'm getting about 15MPG but it is about 90% city, very little highway

unless the fuel economy drastically improves at 600 miles +

mikeb

It should improve at 2k miles

Shocka

same here..

Coqui

Miileage seemed to be improving for me, but now it's been going down again. I'm at 2500 miles. I actually just got my worst mileage ever - 15.3mpg, but there was a bit more city driving than usual... Maybe that check engine light that just turned on has something to do with it?

mikeb

I'm in
7.2 gallons and 154 miles l

XUrotaryrocket

I get, consistently, 270 - 290 miles per tank. This is roughly 20-22 mpg. However, I do an obscene amount of highway driving. My car is only 2.5 months old and I already have 8300 miles on it.

My best tank was 330 miles late in the break-in period when I was going very easy on the engine.

aussie77

An interesting update. I got my worst and then my best tanks yet back to back this past weekend!

On a fresh tank I ended up doing 100% city driving for well over half the tank. I knew it was going to be ugly when I hit the half-way marker and had only driven 110 miles.

So I did the math and it came out as 13.2 mpg.

Well crap. So I filled up this weekend, and my wife and I went up to North GA to go hiking on our one year anniversary. I was going to cruise at 73 again just to try and maximise fuel efficiency, but ended up averaing closer to 80 for most of the trip. Then we got up there and the roads were empty and curvy and I couldn't help but let her rip some, with a fair amount of acceleration and braking. Did the same thing on the way home, but definitely cruised close to 80 most of the trip home, with some bursts of speed here and there.

Stopped this morning to fill up on my way to work, and lo and behold: 21.45 mpg. Best ever.

So it would seem things are still capable of improving on highway driving. I don't quite know what was up with the tank before that, but I do have an idea. The weather turned much colder for about two weeks there, which coincided with the time I was doing my 100% city driving with lots of idling. I am beginning to suspect that with the colder weather the engine seals weren't working so well, which combined with the naturally terrible gas consumption at idle led to crazy bad milage. Funnily enough, this past weekend was quite warm, and bam, best mileage yet.

Just a thought, but kinda curious either way.

Ole Spiff

I'd be curious to know if you use the same brand of gas each time you fill up, and what octane. I switched to 87 Octane regular and in particular, Shell Regular and I'm seeing high 19's in gas mileage with my normal "spirited" driving style with a mix of 50/50 street/freeway driving. Before when I was using 91 Octane I was getting high 17's.

I don't baby it or try to get good mileage; I drive it to have fun. I have just over 6,000 miles on it. 6spd MT with Grand Tour pkg.

Ole Spiff

Ron

I am not sure that everyone is aware that %city mileage is extremely important and that city and higway percents don't average directly.

Suppose that you drive 100 miles in the city and use 10 gallons of gasoline. This would be 10 MPG. Suppose that you drive an additional 100 miles on the highway and use 5 gallons of gasoline. This would be 20 MPG. You might think that the tank average would be (10+20)/2 or 15 MPG. This is incorrect. You have gone 200 miles and used 15 gallons or 200/15 = 13.33 MPG. This says that tank average is biased by periods of low MPG.

Hope that this helps in the MPG discussion and identifies one reason why some people are getting low tank average MPG.

aussie77

No I am completely aware of the effect that the % of highway vs city has on your mpg. That was what makes the contrast even more startling. I *know* the one tank of gas was 100% city driving from the moment I filled it up to the moment I filled it up again. Then from that 100% city driving tank I went to what was probably 90-95% highway driving, and the driving I did off the highway was either a short hop to the highway itself, or on back country roads with no stop lights or traffic.

In both cases I calculated the mpg correctly, dividing the miles driven on that tank by the number of gallons required to fill the tank.

rabinabo

That is an excellent point Ron. The point is that to compare the city and highway driving by amount of gas used, not number of miles driven. In his example, 50% of the miles were on highway and 50% in the city, but the mileage wasn't halfway between the highway and city mileage. Rather, 2/3 of the gallons were used in the city and 1/3 used in the highway, so average mpg = (2/3)*city mpg + (1/3)*highway mpg.

On one tank, with combined city/highway, you only know average mpg and maybe the percentage of miles driven in city & highway. I guess you can use the stock mpg numbers to get an estimate of gallons used city vs. highway. However there is still only one equation and two unknowns, so you can't solve for the city and highway mpg from the average.

So aussie, the only way to reliably figure city & highway mpg is to do 100% one or the other. Those 5-10% of total miles used in the the city in the second tank equate to something more like 7%-14% of total gallons used in the city (using the stock mpg numbers to get relative city/highway mileage). For 25% city miles, it's 36% city gallons, and for 50% city miles, it's 80% city gallons.

Sorry, I just got my PhD in math, so I get carried away every once in a while.

aussie77

Well, I'm being somewhat generious with the 5-10% of 'city' driving. Most of the 'city' driving wasn't really city driving, but on back roads with no lights and no cars - hence no 'stop and go'. It was a little over 200 miles there and back, and approximately 10 miles of it at most were true city driving, and that's a generous estimate.

At any rate, the entire point of the post was the contrast between the two. To go from 13 mpg to 21.5 mpg (roughly) is a big turnaround however you look at it - obviously something weird is going on.

Reeko

I am starting to wonder if temp has a significant impact on MPG.
Here in Portland Oregon, we were having warm weather 2 weeks ago (70s to 80s). Last week a cold front came through, we are now having highs in the 40s and lows at about 30 F.

Anyway, up until the cold front, I was consistently getting about 17 MPG. Typically I was going about 230 Miles on a tank and filling 13.5 gallons.

This week, I had to fill at 195 Miles (again 13.5 gallons). This is about 14.4 MPG. On the current tank, I can tell it is also poor, since I have hit 1/2 at less than 100 Miles (Typically I hit 1/2 at about 110 to 120).

So, I am wondering if there is some correltation between Low Temps and Low MPG?

Greg

I'm in Savannah, GA. and its been consistently between 60-80 and I see no correlation at all. I have good tanks with highway driving and bad tanks with city. 16-18 average.

Reeko

I understand.
But, we had a 40 degree drop in temps here. AND, my last tank was the worst ever, even though I was driving pretty much the same routes and speeds as all of my other tanks.

I will see if this was just a one or two tank Fluke.
Probably there is no correlation.

Also, since my wife gets to drive the car 1 day a week (part of the agreement), maybe she is racing behind my back

Sidenote: Any one else have problems switching back to otehr cars? I drive my RX 6 days a week, and a 3/4 ton Pickup the other day. Seems like I wait too long to brake, go into turns too fast, etc when in the truck. Takes me a while to tone down the driving...

labrat

On a 50/50 mix of highway (weekends) and weekday outer urban driving - no traffic jams, relatively free-flowing traffic; we routinely get 10.5L/100km or 22 miles per US gallon. Fuel is 95 RON, daytime temperatures 25-30 deg. C. Car is 6MT, "hi-power" model.

Greg

Let me conjecture that the 40 degree drop in temperature caused your e.c.u. to have to "re-learn" a bit in the beginning and that is perhaps what caused the difference. however, I think that if the weather stayed consistent you would see it even out on the 2nd and 3rd tanks. Cold air and hot air look alot different to the e.c.u. and could cause it to add more fuel. Just a thought.