MPG vs "Level" Speed

[GasSavers_maximilian]

Today I calibrated my ScanGauge II (+4%) and since it was a calm day wind wise I headed over to what passes for a level stretch around here (you can see the difference depending on direction). I used the current trip feature to get an average for the whole stretch, resetting it once up to speed. I wanted to go all the way from 35 mph to 75 mph (all in top gear), but the short test area made getting up to speed quickly enough difficult, once I got up to 70. Someone more comfortable in high speed turns no doubt could do better, entering the straight area with more momentum. Unfortunately, it started raining soon after I began the second set of runs, but here's what I have so far. Have to go back again sometime, but since I have modifications in the works I guess it'll most likely mean starting over. Surprisingly high economy at 35 mph. Makes me suspicious.

[GasSavers_maximilian]

Based on these results I've been making an effort to drive (even) slower when on a level and it does seem to help fuel economy over the 50-55 I was doing before. I've changed a couple of the routes I take to lower speed limit ones to make this easier, although other cars still limit it a lot.

[GasSavers_maximilian]

I just went to the local hardware store and I realized I was doing the same sorts of speeds my moped did (30-35)! Seems to work well. Didn't have podcasts on my moped, though. Big difference.

[Snax]

Just looking at your plots, I suspect the dip you see around 40 mph is the result of timing retard. Most likely it is mapped to provide maximum resistance to detonation at that rpm against that kind of loading, whereas just the drop to 35 mph reduces the drag loading substantially, allowing less timing retard to essentially just put along.

In other words, you might actually get better fuel economy in your next lowest gear at 40 mph. Quirky, but worth investigating IMO.

[GasSavers_maximilian]

I'll give it a shot, thanks for the tip.

[GasSavers_maximilian]

I don't suppose anyone else would be willing to generate a plot like the one I did to provide some comparison? The mpg vs speed graphs in my books don't show 35 mph being optimal, that's for sure. Maybe it's related to the mediocre coasting performance I've noticed. I'm waiting on my mirror mods to rerun it myself.

[GasSavers_maximilian]

Today I went on a trip to the nearest Lowes (about 66 miles away) and I used the limited speed option on my TomTom to get a route on which to try out the lower speed techniques. Took 2 hrs at an average speed of 35 mph, but I managed 50 mpg (on the long parts of the trip, bumming around shopping brought it to 49.5 for the day). I brought plenty of podcasts to pass the time. Kind of dull, but it does seem to work.

[FrugalFloyd]

I'll be doing some experimenting next week, and will try to generate charts like yours. I started slowing down when I discovered my mileage was linearly inversely related to my speed. I got 26 mpg at 72 mph, 32 mpg at 62 mph, and 38 mpg at 55 mph. I know my mileage rises to the mid 40s at 40 mph, but forgot the specifics. Pulsing and gliding raises my highway mpg to something like 43 mpg with a 50 mpg average. This is all in my Sentra.

[GasSavers_maximilian]

It'll be awesome to see another car's chart for comparison. I was totally surprised that the lower speeds continued to be better. What car computer are you using? I've never measured pulse and glide, since I'm too lazy to do it in practice.

[FrugalFloyd]

I've gone through some earlier notes, and my older data look pretty linear, peaking at 35 mph, just like you. The data are a mixture of tank fill-ups and Scangauge readings, but they look something like this:

72 mph = 26 mpg
62 mph = 32 mpg
55 mph = 38 mpg
40 mph = 45.6 mpg
35 mph = 50 mpg
high speed pulse and glide (avg 55 mph) = 43.5 mpg
low speed p&g (avg 28 mph) = 62 mpg

I'm using the Scangauge II in my Sentra SE-R. I get similar results with my Scion xBbut about 5-8 mpg higher overall.