Optimal Cruising speed vs. Optimal Gas Mileage

[wbcooksey]

Does anyone know if there is info out there on optimal speed for optimal gas mileage for the R18 engine. I did a brief thread search and found lots of people reporting their own gas mileage, but couldn’t find anything much else.

We know that that slowing down improves gas mileage, but is there a *reasonable* cruising speed (other than zero!) that maximizes fuel usage? Are there any charts/graphs/data out there they says something like “best gas mileage is at 59 mph on flat terrain”, or something similar?

If not, then it sounds like an interesting project…graph gas mileage at various speeds and see what the curve does.

William
07 Civic EX + 03 Pilot

[xxhaimbondxx]

I actually started this experiment since my last fill up. I try to keep 60 MPH and no hard accelerations. So far so good.

[vtak07]

I've tried doing this with my car. My usual cruising speed on the highway (90% of my driving) are at speeds of 75+, maybe more. Last fill up I went no more than 60 mph and used cruise control almost all of the time. Got the best tank in my Si ever at 350 miles

[wbcooksey]

Sweet. Thanks.

Keep us updated. Would love to see a graph plotted for these experiments.

William

[jthoske]

@ 70mph cruising I get a steady 30.3mpg.

[OrdnanceMarine]

Originally Posted by wbcooksey

Does anyone know if there is info out there on optimal speed for optimal gas mileage for the R18 engine. I did a brief thread search and found lots of people reporting their own gas mileage, but couldn’t find anything much else.

We know that that slowing down improves gas mileage, but is there a *reasonable* cruising speed (other than zero!) that maximizes fuel usage? Are there any charts/graphs/data out there they says something like “best gas mileage is at 59 mph on flat terrain”, or something similar?

If not, then it sounds like an interesting project…graph gas mileage at various speeds and see what the curve does.

William
07 Civic EX + 03 Pilot

While I can't definitively tell you this on the R18 since I've got the K20, I would put money that you'd see the best MPG at whatever minimum speed you can drive in your lowest gear, probably about 35-45 MPH. I don't ever get more than a mile or two where I get to cruise at those speeds but when I do I usually see 42-48 MPG in my Si. In my '05 CR-V the best MPG on a flat is achieved just over 40 MPH when the automatic transmission shifts into 5th and locks up the torque converter. In a higher gear you lose more efficiency to friction and pumping losses, and at a higher speed you spend more energy overcoming air resistance. Again, I'd bet you see these two curves cross between 35 and 45 mph.

[JKinATL]

Originally Posted by OrdnanceMarine

While I can't definitively tell you this on the R18 since I've got the K20, I would put money that you'd see the best MPG at whatever minimum speed you can drive in your lowest gear, probably about 35-45 MPH. I don't ever get more than a mile or two where I get to cruise at those speeds but when I do I usually see 42-48 MPG in my Si. In my '05 CR-V the best MPG on a flat is achieved just over 40 MPH when the automatic transmission shifts into 5th and locks up the torque converter. In a higher gear you lose more efficiency to friction and pumping losses, and at a higher speed you spend more energy overcoming air resistance. Again, I'd bet you see these two curves cross between 35 and 45 mph.

Your most efficient cruising speed is, in fact, the speed that you maintain at the lowest RPMs in the highest gear. This is prob between 45 and 50 for us with the R18, from my experience, on flat terrain. Of course, unless you're in the plains or desert, no terrain is completely flat, so it's most efficient to employ this technique on hills:

At the top of a decline, slowly apply more gas to get us to good bit of speed for the bottom of the hill, and then, as you go up the subsequent incline, slowly let off the gas. What will happen is that, of course, your speed will be highest at the bottom of the hill and slowest at the top, but you can maximize the use of gravity as your friend by building up, and then using, your bank of momentum raised on your way down. After said hill, get back up to cruising speed with reasonable acceleration, and continue!

This is all ideal, though.
I still can't bring myself to cruise so slowly on the interstate!

[aki]

Ideal cruising speed is contingent upon two factors:

1) Engine friction/drag--At lower speeds engine friction is what drives down fuel efficiency; at higher speeds drag becomes the key factor. The amount of drag doesn't increase linearly with speed, and your MPG quickly goes down the faster you go. Usually the balancing point where engine friction and drag are optimal is 50mph.

2) Torque per RPM. As for engine performance, the best gear is determined by the highest torque per RPM. It depends on your engine characteristics. The K20Z3 for example has a good amount of low-end torque, so you can stay around 2k RPM and get great gas mileage. Not sure what the R18 low-end is like.