Cd or weight?

[Ecky]

When I throw my car in 5th or 6th and idle along at 20mph, I get around 65-70mpg with my 2.4L engine.

[Charlie Cheap]

I guess I misunderstood your post. Looking at a very small engine pulling a heavy vehicle, it looks as if gearing is a major factor, coupled with the number of gears in the tranny, plus the height of the driving tires. Like I said, you must have an almost perfect combination. Overcoming weight is the single biggest factor in stop-and-go city driving as it relates to economy. My work is in building a vehicle for ALL TYPES of driving under normal conditions. If one simply adapts driving habits MPG can be improved, but the vehicle must be built for the task it is assigned. I am an old car builder who puts the overall LOOK of the vehicle near the top of my objectives, with efficiency at the very top, which causes compromises in all areas. A butt-ugly car that gets great MPG probably won't sell, and one that is what we call a Trailer-Queen Show Car, that gets very poor MPG, probably won't sell either. I think my reading of your post was me not understanding your parameters...MY BAD.

[oldtamiyaphile]

When I throw my car in 5th or 6th and idle along at 20mph, I get around 65-70mpg with my 2.4L engine.

Racecar gearing is not a brag on this forum though

Your engine capacity to weight ratio is about the same as my Jeep (4.0), but even that can't pull 6th until ~40mph.

[aerohead]

The premise of the thread contains 50% Hwy driving, which for example, in Texas, today, would include up to 85-mph, legally, between Austin and San Antonio.

[Ecky]

Originally Posted by oldtamiyaphile

Racecar gearing is not a brag on this forum though

Your engine capacity to weight ratio is about the same as my Jeep (4.0), but even that can't pull 6th until ~40mph.

I'm getting off into the woods a bit here, but I had, at considerable expense, a custom final drive cut a full 40% taller than what came in the transmission, and installed a 6th gear a further 14% taller, then upsized my tires a bit. I'm turning 650rpm @ 20mph, and 2100 @ 65mph, in a high revving engine (revs out to near 8000).

My fuel economy vs speed curve is approximately:




The displacement to weight may be similar to your Jeep's, but the power to weight is closer to that of a base C7 Corvette. It's less efficient than it was with the stock 1L engine, but I'd hardly call it "racecar" gearing.

[kach22i]

Originally Posted by aerohead

......average mpg upwards in spite of the inertial penalty. So I voted for aero........

Interesting list of experience, but all from a single person, a person that is interested in getting maximum fuel mileage.

This means that you probably accelerate with purposeful intent of higher mpg (slow and steady retiree on a Sunday) verses a more typical American that is trying to get there yesterday and their hair is on fire.

Meaning weight penalties for quick acceleration will be far more costly based on driving style.

[aardvarcus]

Acceleration is an investment in kinetic energy, one that would be best made based on results from an engine specific bsfc map. For my engines, those bsfc minima useful for charging my "inertia battery" are at moderate to high load. That coupled with my manual transmission means I have a bit of "jackrabbit starts" which is counterintuitive but works better for me. Others may have vastly different bsfc maps requiring different techniques.

The difference i have found lies in drawing down my "inertia battery" as much as possible via coasting to minimize the conversion of its energy to heat via the brakes. Thus I roll out of the throttle much sooner than most when approaching a stop. I leave additional following distance to have reaction time. Time the redlights. Know the speed you can comfortably take a turn, and the landmark that defines canceling the cruise on approach to that turn. Adjust behaviors based on weather or significant (2000lb+) loads being carried.

Yes there are times you have to slam on the brakes. Deer, other drivers, debris, etcetera. But under normal situations brake use is the enemy of fuel economy in a heavy vehicle.

[kach22i]

What amazes me is what I call "straight line" drivers.

Might be the software doing the actual throttle in todays cars, but I notice an acceleration slower than what I do, but where I ease off to do the speed limit they quickly catch up and pass me like I'm standing still. They have to be doing 10-15 over the speed limit.

And an irony, they come to almost a complete stop to turn, like a G-force allergy.

I mean seeing a $75,000 BMW turn at the same speed as a 1970's pickup truck overloaded with scrap metal is mind shattering.

Compared to myself people fly up on stops and brake at last second. No patience for coasting I guess.

Driving style is part of the math here, no doubt.

[aardvarcus]

Also "interesting" is when the straight line drivers tailgate me into a curve, my junk makes the turn no problem while they last second stomp the brakes in their luxury cars ending up out of control and in other lanes... they don't seem to tailgate me after that...

Unless someone is coming the other way and I slow way down for turns so the streight line tailgater doesn't kill them...

[freebeard]

I lured a 4WD pickup into a corner once with my Notchback. It went up on three wheels.

OTOH, a red Audi lured me onto a freeway onramp that had a solid sheet of ice one December night. I had to fight to keep the title clear on that one.