Weight Vs. Economy

[CommodAnt]

Hi Everyone,

Over many years (and several different vehicles - automatic and manual, sedan and 4WD), I have consistently found that my fuel economy is better on longer highway trips when the car is loaded up to the max with weight. Conventional wisdom says that this should not be the case with lower weight resulting in better fuel economy.

My thoughts are that this is due to either 1. the weight lowering the car (so less air goes underneath, hence less air resistance), or 2. The extra weight means you can glide longer down inclines, with the engine operating more efficiently (harder) in pulling the vehicle up the hills.

Anyone else have any real world data or thoughts on what is going on?

[freebeard]

I thought this would be [more] about payload vs vehicle weight. I posted about the Xbus in another thread. I've since learned that it weighs 992lb, and has a payload of 1/2 ton and a tow capacity of 2000lb.

As to your question -- it sounds reasonable. A well designed vehicle will accommodate loads and grades and {possibly] appreciate the opportunity to 'stretch it's legs'. Optimal BSFC (in pounds per hour) is prolly at 75% throttle were BSHP lies.

[oil pan 4]

The real world data says to pick up a few mpg you have to strip the car to the point where it's undriveable.
And putting on lighter parts is so expensive it will never pay for its self in fuel savings.

[CommodAnt]

Thanks. But is this real world data stop/go, or continuous speed? Does anyone here have any data from their experimentation?

[cRiPpLe_rOoStEr]

I'm sure results may vary according to road condition, so it's not a one-size-fits-all. Plus a heavier vehicle would have more inertia to overcome while accelerating, so if you're able to keep driving for a longer stretch of road at a steady pace without any sudden change to the cruise speed it may seem more economical.

[rmay635703]

Off topic but related to this discussion

https://priuschat.com/threads/epa-te.../#post-3182709

Weight and efficiency is in there somewhere

[SDMCF]

Originally Posted by CommodAnt

I have consistently found that my fuel economy is better on longer highway trips when the car is loaded up to the max with weight.

Have you been able to compare that with economy on similar trips without the extra weight? If so, how much improvement in economy do you think the weight makes? If not, what leads you to conclude that it is the weight rather than trip length or highway driving that makes the difference?

[Isaac Zachary]

If you're going up and down hills I find something similar in my Avalon and other vehicles I have driven. I can get similar or even better fuel mileage going over mountain passes than on flat highways even though that means I'm at a much higher throttle going up the mountains and then releasing a lot of it back into the atmosphere as heat as I engine and friction brake going down the other side.

PS. Regen braking in the Avalon does help some, but on most passes the battery fills up very quickly leaving me with normal engine and friction braking the rest of the slope. I also have noticed similar results in non-hybrids.

I think what happens is that it's similar to pulse and gliding. Engines are generally much more powerful than needed and so tend to run in a less efficient load on flat ground. But going to a slope puts the load at a higher, more efficient range.

In your caae you may not have the 5-8% grade passes that go on for miles and miles. But by adding more weight you get more of the effect, especially if the downhill portions aren't enough to merit braking letting you use the built up kinetic energy.

[aerohead]

Originally Posted by CommodAnt

Hi Everyone,

Over many years (and several different vehicles - automatic and manual, sedan and 4WD), I have consistently found that my fuel economy is better on longer highway trips when the car is loaded up to the max with weight. Conventional wisdom says that this should not be the case with lower weight resulting in better fuel economy.

My thoughts are that this is due to either 1. the weight lowering the car (so less air goes underneath, hence less air resistance), or 2. The extra weight means you can glide longer down inclines, with the engine operating more efficiently (harder) in pulling the vehicle up the hills.

Anyone else have any real world data or thoughts on what is going on?

1) the answer is hiding in plain view.
2) the vehicle's inefficiency is during transient loads.
3) the longer highway drives keep throttling to a minimum, and the gearing keeps the engine close to it's brake-specific-fuel-consumption optimum.
4) if you could drive continuously at 35-40-mph around town, never stopping until you arrived at your destination, you'd see your highest-possible mpg.
5) AeroStealth's 2014 F-150 will do 22-mpg @ 62-mph (100-km/h ).
6) Same truck @ a constant 35-mph = 32-mpg.

[CommodAnt]

Originally Posted by SDMCF

Have you been able to compare that with economy on similar trips without the extra weight? If so, how much improvement in economy do you think the weight makes? If not, what leads you to conclude that it is the weight rather than trip length or highway driving that makes the difference?

Yes and no. I've done a lot of trips that are further than a single tank of fuel over a couple of decades and a bunch of vehicles. Fuel improvement seems to be about 10% better with a very full load. The full load does not happen often, but I consistently get my best milage on these long trips when packed full to the ceiling.