Thread: rpm reducing
View Single Post
Old 02-09-2016, 01:24 PM   #9 (permalink)
Master EcoModder
Ecky's Avatar
Join Date: Dec 2011
Location: Michigan
Posts: 4,479

Gaptooth (retired) - '00 Honda Insight
Team Honda
Gen-1 Insights
90 day: 54.26 mpg (US)

Such Fit - '07 Honda Fit Sport
90 day: 41.27 mpg (US)

K-sight - '00 Honda K-sight
Team Honda
90 day: 41.97 mpg (US)

Aerocivic - '92 Honda Civic CX
90 day: 64.49 mpg (US)
Thanks: 2,293
Thanked 2,064 Times in 1,285 Posts
Originally Posted by Andrei_ierdnA View Post
In my 2013 Honda Fit with manual 5-speed, cruising at 100 km/h (62mph) - which is the legal speed limit here - makes the little 1.5 L engine spin at 3,000 RPM. If I drive consistently at this speed, slow down a bit as climbing hills, and coast downhill, I can easily hit 40-45 mpg over an 1 hour drive.

My point is that RPM's aren't everything. If you have an instant fuel consumption display, it becomes very clear that fuel consumption depends on the load more than RPM's. I could be wrong, but I think engine load is affected by many factors such as grade/slope of road, air temperature, wind speed and direction, weight of car and all the stuff in your trunk, type of transmission, 4x4 drive, tires (type and pressure of tire), drag forces / aero of your car, type of engine oil, etc...and of course how many cheeseburgers you had that day.

Yes and no. Assuming a fixed power need to travel at a fixed speed, increasing RPM means you'll have lower percent load at the same speed (because the engine can make more power at higher RPM), and low load causes parasitic losses (mostly vacuum/pumping). Higher RPM also has exponentially increased friction = more parasitic losses. Sometimes this is partially offset by an engine being more efficient at higher RPM, but ideally you want to be geared so that you're at high load at the lowest RPM possible when cruising, to maximize efficiency.

EDIT: My previous car had a very, very short transmission. The 1.8L DOHC engine spun at ~4500RPM at 72mph, and at this speed, I usually got about 30mpg. If geared differently, let's say closer to 2500RPM, it's likely I'd have been closer to 45mpg, because of the tremendous decrease in pumping and frictional losses.
  Reply With Quote