Ideal engine speed for highway FE?

[SoobieOut]

Millions of dollars were spent developing the double nickle (55 MPH) signs in the seventies.

From what I have read it is the magic number for FE. I am sure there are different gear ratios to place the number a bit higher or lower.

[zonker]

nothing the government ever does is purely scientific. throw a few special interest lobbyists in the mix and then you have the real reason on the 55mph selection.

i'm sure the best vehicle speed has to do when the aerodynamic drag starts to contribute substantially to the speed equation.

as for engine speed, i'm going to guess it's on the idle speed side of the peak torque curve, so in my engine's case, it produces peak tq numbers at 2500 rpm, so somewhere between 1000 rpm and 2500 rpm would be ideal.

[Frank Lee]

55 is generally the point at which aero loads overtake other resistances. Due to the nature of aero loads- that they increase exponentially rather than linearly- 55 seems a sensible place to assign max velocity in order to get maximum benefit.

[cfg83]

zonker -

Originally Posted by zonker

nothing the government ever does is purely scientific. throw a few special interest lobbyists in the mix and then you have the real reason on the 55mph selection.

i'm sure the best vehicle speed has to do when the aerodynamic drag starts to contribute substantially to the speed equation.

as for engine speed, i'm going to guess it's on the idle speed side of the peak torque curve, so in my engine's case, it produces peak tq numbers at 2500 rpm, so somewhere between 1000 rpm and 2500 rpm would be ideal.

I think basjoos likes to maintain a higher average MPH than most of us because of his aeromods. For the majority of cars, however, this is the way things play out :



In all cases the trend is downward, and it was going down *before* 55 MPH. The only special car in all of this is the Mercedes C 180 K. It "bucks the trend", but it is still going down.

CarloSW2

[zonker]

thank you for the chart!

from what info i have been picking up your chart and from other websites, it seems 37-62 mph is "the zone" you don't want to leave, and where it falls after that depends more on the car's own engine/trans/aero specs.

so... i know the car will shift into it's tallest gear (0.75:1 with TQ lockup) at about 46mph (1700 rpm approx).

so i'm going to guess for me it's somewhere between 46 and 57mph. I'd say 62mph but my car is NOT aerodynamic

i think i need to take my lil 90 mile loop and slow my speed from 65-70mph to 50-55mph and see what happens.

p.s. i did donate for a mpguino so hopefully i'll have that in my measuring arsenal real soon.

[t vago]

Engine tune has a great deal to do with optimal engine speed on the highway. This includes volumetric efficiency of the engine, as considered to be an air pump. If low end on a given engine is questionable to begin with because that engine can't really breathe at low RPMs, then odds are it will get worse FE on the highway with taller gearing, than with lower gearing.

On the other board I sometimes visit, there was at least one example of somebody swapping in a transmission computer to get an extra overdrive gear for their electronically controlled transmission, and they actually ended up hurting their FE as a result. When I looked at the other engine mods this guy had, I could see why.

[zonker]

a couple other good graphs...

[zonker]

well besides the 90 mile trip at 50-55mph, i have another thought... there is a substantial uphill grade for approx 1 mile on the I-15 near my home. I will see how slow i can lug the car before it wants to shift out of TC lockup, so basically i'll try it at 50mph (1850 rpm) and then try at subsequently higher speeds until the trans will not unlock the TC. That should give me an idea about the motor's torque efficiency at lower rpms.

UPDATE: did the I-15 hill climb. Set the cruise control for 50mph. The white rag would lose speed and downshift from TC lockup. At 55mph, she climbed without a speed drop and kept in TC lockup. So... I think my magical hwy speed rpm is greater than 1850 and less than 2050 rpm.

[zonker]

Originally Posted by t vago

Engine tune has a great deal to do with optimal engine speed on the highway. This includes volumetric efficiency of the engine, as considered to be an air pump. If low end on a given engine is questionable to begin with because that engine can't really breathe at low RPMs, then odds are it will get worse FE on the highway with taller gearing, than with lower gearing.

On the other board I sometimes visit, there was at least one example of somebody swapping in a transmission computer to get an extra overdrive gear for their electronically controlled transmission, and they actually ended up hurting their FE as a result. When I looked at the other engine mods this guy had, I could see why.

+1!

I understand that it's the sum of the whole. I've already made air/fuel/ignition modifications to the motor and thru testing have found more mpg thru smaller injectors, a smaller diameter intake tube (creates higher air velocity thru the maf for increased ignition timing/fuel to offset smaller injectors), and a full time warm air intake by removing the flap valve that directed only cold air into the tube at operating temps.

That and some under-driving of the accessory speeds have gotten me a personal best of 31 mpg so far (it was 26 when all was stock, same as the epa hwy specs). That was at speeds between 65-70 mph constant. Now with the new info, I'm going to attempt the same trip, but 50-55mph speeds and see if I pick up any fuel economy.

[Frank Lee]

http://ecomodder.com/forum/showthrea...peed-1477.html

There seems to be a point of lower rpms at which engines don't get more efficient.