'96 Silverado vs 2009 Outback

[vteco]

I live in a world of hills. Going downhill idling, the Silverado picks up speed very quickly. The Subaru noticeably less so on the same routes. The Silverado bleeds off speed more slowly going back uphill.

The Subaru has all wheel drive. The Silverado has manual electrically engaged 4 wheel drive.

I'm guessing this is the explanation?

[vteco]

Couple other thoughts re coasting:

1.) At least with a wheelbarrow, big wheels push easier than little ones.

2.) Wheel inertia, disparaged under power, especially when needing to frequently brake, is not necessarily such a detriment when largely coasting up and down hills in our area.

Particularly if the wheels are storing energy by retarding speed which might be wasted in braking at curves, where speed buildup downhill otherwise would get excessive with small light wheels.

[vteco]

Third thought. Everybody has different conditions. If I lived in a desert, or a city, or had a long commute, every aspect of working towards reducing fuel usage might be different. I'm shooting towards optimizing a very particular niche set of conditions in a specific vehicle.

[cRiPpLe_rOoStEr]

Wheel inertia tends to be troublesome for overall efficiency, even if it could seem at a first glance to be beneficial while driving through hilly terrain. Fewer usage of the brake OTOH tends to happen once you're already driving in a more efficient way.

[vteco]

Well having read through the 100+ tips here I can say truthfully that I am already driving that way, nothing seemed new, and particularly I've been maximizing since the Subaru has instantaneous and trip average MPG readout which I've paid careful attention to for the last 6 months.

Nevertheless, the Subaru, just tested a half hour ago on a 10 mile round trip down the mountain and back, will slow down with the foot off the pedal on even a moderately steep downhill grade. I'm thinking this must have something to do with the "all-wheel drive" being computer apportioned somehow during coast.

The Silverado easily and rapidly accelerates on practically any downhill grade, pedal off. While it has 4 wheel drive, it's only when manually engaged.

This is a separate issue than wheel inertia.

I think it's not established that "wheel inertia tends to be troublesome for overall efficiency" in my own case since that overall efficiency is composed primarily of winding hill conditions, with little highway usage.

I'm going to guess that:
80% of my driving is up and down winding mountain roads at 35-45 mph speed limits
12% hilly town streets, 25 mph speed limit
5% hilly 55 mph highway speed limit
3% hilly 65 mph highway speed limit

These conditions probably have their own set of optimization scenarios. That's what I'm interested in. I think that will take an open mind to work out.

[aerohead]

Originally Posted by vteco

Well having read through the 100+ tips here I can say truthfully that I am already driving that way, nothing seemed new, and particularly I've been maximizing since the Subaru has instantaneous and trip average MPG readout which I've paid careful attention to for the last 6 months.

Nevertheless, the Subaru, just tested a half hour ago on a 10 mile round trip down the mountain and back, will slow down with the foot off the pedal on even a moderately steep downhill grade. I'm thinking this must have something to do with the "all-wheel drive" being computer apportioned somehow during coast.

The Silverado easily and rapidly accelerates on practically any downhill grade, pedal off. While it has 4 wheel drive, it's only when manually engaged.

This is a separate issue than wheel inertia.

I think it's not established that "wheel inertia tends to be troublesome for overall efficiency" in my own case since that overall efficiency is composed primarily of winding hill conditions, with little highway usage.

I'm going to guess that:
80% of my driving is up and down winding mountain roads at 35-45 mph speed limits
12% hilly town streets, 25 mph speed limit
5% hilly 55 mph highway speed limit
3% hilly 65 mph highway speed limit

These conditions probably have their own set of optimization scenarios. That's what I'm interested in. I think that will take an open mind to work out.

The wheel inertia is only experienced during acceleration, or deceleration.
On cruise control, at a constant velocity, the wheel/tires are just flywheels, storing kinetic energy.
On a hybrid or EV, a propensity to overspeed on a downhill during cruise control, up to 81.1% of the 'braking energy' lost to maintain speed would be recovered to the battery by regeneration.
On the Chevy, if you brake to maintain constant speed, you just lose all that energy to brake heat.
In my Morris Minor 100, Rambler American, 40-hp Karmann-Ghia, and 60-hp CRX, I would just let the car accelerate on the downhill, to assist the 'climb' on the next grade.
Chrysler Corp. used this in their 'momentum' driving ( now referred to as 'hypermiling' ) to win the annual Mobil Economy Run.

[freebeard]

What year and model was the Rambler? In college I had a 1959 station wagon.

[vteco]

aerohead, Luckily for me my winding downhill drive to town is really a set of up and downs plus curves which mean I rarely have to brake in the Silverado when coasting. The changes tend to keep it in range of the speed limit. This is all 35-45 mph posted.

The Subaru on the other hand has to be driven on the same route because it just has too much rolling resistance. I don't know why.

These winter days I'm getting 18 mpg average with the Silverado and 22.5 mpg with the Subaru.

When I bought the Silverado the seller said it got 15 mpg. Since then I've replaced many components, including wheel hubs, and am using lower visc synthetic oil. And my driving habits are much improved. Aero mods are coming. PCM programming is coming.

I'm going to try to catch the Subaru's mileage figure, as a first goal.

[cRiPpLe_rOoStEr]

Originally Posted by vteco

I'm thinking this must have something to do with the "all-wheel drive" being computer apportioned somehow during coast.

I don't know what's the manufacturer of the transmission in your Subaru, but my closest experiences with a Subaru were in an Impreza with manual transmission, which my father used to own. Maybe the electronic controls of the automatic transmission have a greater impact than the AWD, as some automatic transmissions were programmed to provide an enhanced engine braking. Well, at least that's what I remember from some Nissan ads back in 2006.

[vteco]

Enhanced engine braking.....! Heh, that's a new one, cr.

Lemonade from lemons sort of thing, I guess.