A little background...
While I was away from the blogs, I needed to make a semi long trip, about 2 hours each way. As I was on my way I found (via call) I would be arriving way too early and would need to wait maybe a half hour for the person I was going to visit. What I have done in the past on this trip is about half way I switch from highway to a parallel road that is four lanes and maybe 5 traffic lights for 40 miles, then I get back onto the super busy expressway for the last 10 miles to her house. I do this because the other road is not travelled by many people and it makes for a relaxing ride.
Anyway, I had a choice. Stop somewhere for maybe an hour, or drive slowly to get there when I needed. I went that route (the parallel road) and drove maybe 45 MPH instead of 55 to 60 on this stretch of my trip. It worked out right and I arrived after she did. On the way back I did the same, taking that road and enjoying the relaxing portion of the drive only going 45 MPG.
So when I stopped and got gas I found that my MPG had increased to 40 MPG for the last tankfull. This surprised me and got me thinking.... How much was my MPG for just driving a very relaxed, no stop light 45 MPH. Using some general guess-timates, here is what I came up with.
(Boring math analysis alert)
Start with my normal MPG. This tankful had lots of highway driving, and that will normally come in at 36 up to 38 MPG. I will go with 37 MPG for what it normally would have been. The overall trip on the tank of gas was 420 miles, but lets round down to 400 miles. My "slow" portion of the tank was 2 times 40, or 80 miles. So that means only 20% of the trip was at the higher MPG rate. That would mean that whatever increase I see in my overall MPG, I would need to multiply by 5 to get the increase for just the 20% of the trip. My increase was 3 (40 - 37) MPG, so for the time I was doing 45MPH, I was getting an increase of 3 times 5 or 15 more MPG. Normally I get 37, so this means I was getting 52 MPG for that portion of the trip.
OK that was a lot of assumptions, but I don't think it will be that far from the truth. Think of it, I can up my MPG to over 50 by driving really smoothly at a leasurely pace.
I dunno, but this really surprised me when I worked the numbers out. Maybe it will give you pause when you think smooth slow driving doesn't help the fuel efficiency of your car.
So the MPG increased that much?
November 23rd, 2006 at 03:53 am
November 23rd, 2006 at 05:01 am 1164258095
It's amazing how some things really can make a difference, like properly inflating the tires, not using the heater/AC, and not accelerating too quickly.
However, as you have also observed, the speed in itself (below 65) doesn't seem to tax the vehicle that much. It's not until I am driving over 65 I have experienced noticeable decreases in MPG.
While I am reasonably confident of these conclusions, I admit that this is just subjective speculation on my part.
November 23rd, 2006 at 03:08 pm 1164294511
I have been thinking I should drive slower - hehe. I generally drive closer to 70, but even 65 makes a big difference, though I probably couldn't drive any slower around here without getting ran over.
November 23rd, 2006 at 04:32 pm 1164299549
November 23rd, 2006 at 04:57 pm 1164301077
My old pickup was another story. I couldn't do more than 55 with it. When I tried it would shake so back I thought it was going to fly apart, so I never worried about speeding with it.
As for AC, with my stick shift I keep the AC off when I am pulling out of an intersection, going uphill, accelerating, or any other time I am pushing the engine. When it is flat and a moderate speed, going downhill, or slowing down to a stop, I engage the AC. The car is usually cooled just fine with AC on part of the time. Doing this I have found I lose at most 1 MPG in the summer. Thats like a 3% drop, something I can easily live with.