Weight, where is the loss? - Fuel Economy, Hypermiling, EcoModding News and Forum

brucey · 06-14-2010, 11:19 AM

I realize that this sounds silly, but hear me out:

My usual driving these days is a 320 mile high round trip across WV and Maryland. I've been pretty consistently getting around 35~ mpg round trip with this.

I usually get better mileage east than west, but I realized this is because the station I fill up there isn't level. There is a 500 foot elevation change from where I start to where I go but over 160 miles that's not an issue. When I get back it averages out so it's not that big of a deal.

Anyways, scangauge is usually correctly calibrated each tank fill. (Off by less than .1 gallon)

Recently though, my car was loaded up like this:

And I could really feel all that extra weight in my glides. I've driven this route so many times I know where to glide and where I can keep coasting down the next mountain, I know where to neutral coast versus in gear coast for DFCO, etc etc

But this changed it all. I could coast much much further than I could normally, and I could even DFCO on parts that before I couldn't maintain speed in neutral.

And to top it all, the engine didn't seem to notice, as at 55 or 60 (what I'm usually doing up the mountains) the engine still would maintain 16~20 mpg up. Then I could glide all the way back down. And then some.

Scangauge was actually reporting 38 mpg until I hit traffic near the city, it usually is reporting 36 until I hit traffic.

I understand that removing weight helps with the city, but how much does it really hurt on the highway, through the mountains?

I might do a mini experiment and throw some weights in the car and see how much farther it will coast.

RobertSmalls · 06-14-2010, 11:46 AM

Rolling resistance increases linearly with weight, so it takes more energy to move the car down the road, even with no braking or accelerating. Check out the Calculator to find out exactly how much.

By P&G'ing, you've increased your engine efficiency, and perhaps by enough to compensate for the extra rolling resistance.

If you had to engine brake down the hills, though, then the extra mass would be a huge disadvantage.

cfg83 · 06-14-2010, 11:59 AM

brucey -

I've always imagined an uphill/downhill commute where I had jugs of water on the downhill run. The water or ballast or whatever would be "delivered" at the bottom of the hill. On the uphill run the car would be empty. That would make for a net MPG gain.

The trick is having a situation where the cargo is "useful", aka doing something important like making $ for me or ???

CarloSW2

Piwoslaw · 06-14-2010, 04:56 PM

I was also surprised that extra weight didn't kill my fe on last year's trip to Romania. The car had 5 people and a bunch of cargo, filled to its weight limit (over 2 tons). The coasts were long, but the acceleration was noticibly slower. Lots of steep climbs with multiple switchbacks and I still managed 4.2 l/100km, much less than I got in everyday driving.

bgd73 · 06-15-2010, 03:09 AM

I am a sube nut..

the boxer likes the weight.

keep good rear and front bearings, change oem out yet?
the federal mogul replacemnts..nice.

I am awaiting the ring nut tool subes use to change one that is just starting to have a wiggle in the cold.(163 foot pounds and 23 years.. I best get the real tool to remove it)

with the weight in the back, take a peek at the exhaust pipes distance from the body while loaded, and then while empty...

I found something sube fans hate me mentioning...
stiffening what I found was another amazing thing to do for these physics.
subys have a squishy spot...they stay elusive for years and years..

I am at 150+ hours in weld and fabrucation saving a ten geared sube..to learn the new ones did something even squishier. The car can even do better than it is now...it is a nice engine.

texanidiot25 · 06-15-2010, 03:45 AM

Quote:

Originally Posted by bgd73

I found something sube fans hate me mentioning...
stiffening what I found was another amazing thing to do for these physics.
subys have a squishy spot...they stay elusive for years and years..

I am at 150+ hours in weld and fabrucation saving a ten geared sube..to learn the new ones did something even squishier. The car can even do better than it is now...it is a nice engine.

What?

autoteach · 06-15-2010, 10:24 AM

might as well be pops and clicks... I dont get what he is saying either.

brucey · 06-15-2010, 12:10 PM

As for going into DFCO going downhill, I only do that if my speed is picking up above the speed limit, since otherwise I'm going to have to brake anyway.

Even in DFCO I will still have to brake sometimes on some of the steeper parts. I'll turn the headlights on for the extra drag (minimal) and the A/C compressor even (you can actually feel the drag when it comes on) but sometimes doing that will kick the car out of DFCO. Silly thing. But Once I'm nearing the bottom of a pass I'll knock it back into neutral and hold my speed till a few miles under the speed limit.

As I said, climbing it will get 16~20 mpg IF I can keep the torque converter locked (35 TPS on scangauge) and if not, it will hit 11's or 12's, so I try my best keep it locked.

Anywho, on the stretches of the journey where I normally would have it in neutral, I was in DFCO to keep my speed at the limit. At the bottom of the grades where I'd normally have to start gassing again, I was still coasting and not even losing speed. For having an extra 400 lbs in the car, the engine really didn't seem bothered by it.

I'll do a test this weekend, I might be able to squeeze my entire weekend travel from one tank (hoping)