Driving 55mph? (Is it the most efficient speed?)
 

Sorry if this is in many other threads... just was reading this article on BusinessWeek, and saw these comments:

http://www.businessweek.com/bwdaily/...n_id=rss_daily

# Gau Mar 11, 2008 3:16 PM GMT Actually no, 55 isn't the most efficient speed The best ratio for gas is just above minimum rpm in your highest gear. RPMs is what eats gas, not speed... t Generally older cars run from 45-60 depending on gears and ratio. However on a lot of newer higher end cars with 5 and 6 speed gears, those number goes up...

# Tobin Mar 11, 2008 3:09 PM GMT the difference between gas savings at 70 and 55 is negligible on today's vehicles, in fact some cars in overdrive have better MPG at 70 than at 40 or 55

Is this true?

The first one about the lowest you can be in top gear is generally considered a good gauge among ecodrivers, however, speed (and therefore aero drag) is a HUGE factor and that number is no where near where he has it. I think my most efficient measured speed was 35 in 5th...in my mom's cars it's probably somewhere closer to 30.

I'd agree with SVOboy. #1 is true as per SVO's statement. #2 is false. Aero losses kill your mileage at higher speeds.

My geo definitly likes to go as slow as is practical in 5th gear. It has pretty low gearing.

There is also the effect of the engine's efficiency curve. Aero load goes up with speed but the engine may also be going towards a more efficient rpm to help offset drag (depending on gearing). I believe my GMC does this on the highway (it has a 5spd manual). It has pretty tall gearing. The scangage seems to average similar FE going either 55 or 65 on the highway. I can only attribute this to having something to do with the engine's efficiency range. Of course anything over 65 causes a decrease in mpg and speeds under 55 return better FE.

It depends on the specific model. For some cars, the difference is negligible.
http://www.greencarcongress.com/2006...onsumptio.html
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My car has a fuel consumption curve that is flat from 30mph to 65mph, and drops only slightly at 70mph. My Cd is low enough that the slight increase in aero drag from 30 to 65mph is offset by the increase in ICE efficiency as I increase the load on it. It is actually easier for me to get good mileage at 65mph than at 30mph because at the lower ICE loadings (barely above idle), the throttle/mpg relationship becomes super sensitive. I have to use the hand throttle to consistently keep getting the better mileages at the lower speeds.

Driving 55
 

I'm also in the "it depends" camp.I think every vehicle has a sweet spot.There's no question that loads increase with speed,however,how the powerplant reacts to increased load may be counterintuitive,as in my case with the Dodge D-100.It's mileage is better above 55mph,so in it's particular case,it's brake specific fuel consumption hits the "sweet spot" at 65mph,not true for any of my other vehicles.Experimentation may be the only means to discover peak efficiency for each vehicle.

between 60 and 65MPH seems to be the most efficient for my car, however, it's also an automatic so if I slow down, it will downshift, I don't have a choice...However, I also noticed that at 65MPH I'm at 2100 but the minimal RPM for that gear is around 1500RPM but I can only go around 45MPH at that RPM....what gives? Maybe I'm just confused or something.

aerohead -

I think we need a "BSFC Topo Map Collection" sticky. It would be a cool way to compare engine behavior. There could be little "X marks the gold" spots on it for MPG, :D .

CarloSW2

My old diesel has a very flat BSFC curve from 800-2000 RPM. That plus its outrageous low-end torque allows me to think of truly radical gearing.

With an add-on overdrive I can cruise 70 MPH @ 1350 RPM and 60 MPH @ 1050 RPM. Aero mods will reduce the road load at these speeds and I'll have good roll-on capability even at ridiculously low RPM.