economy better in the hills?

[mpg_numbers_guy]

As a relevant comparison, between today and yesterday's trips my Insight got 82.9 MPG driving on "normal" roads (flat + slight hills) and "only" 73.7 MPG through mountain passes. MPG was lower because I couldn't maintain lean burn for 80%-90% of the passes.

[litesong]

My 1979 Plymouth Champ never got higher MPG(50.1) than the 962 mile drive from Browning, Montana (4400 feet) to Superior, Wisconsin (630 feet). The drop must have did some good.... maybe the westerlies too. My 1988 Ford Festiva traveling the 380 mile North Cascade loop, over Washington Pass & Stevens Pass, got 50MPG, with 11,000+ feet of ups & downs. Another time however, I had a 57 MPG tank of gas, in the lowlands over 2 days & cool-downs!

[jamesqf]

Originally Posted by mpg_numbers_guy

As a relevant comparison, between today and yesterday's trips my Insight got 82.9 MPG driving on "normal" roads (flat + slight hills) and "only" 73.7 MPG through mountain passes. MPG was lower because I couldn't maintain lean burn for 80%-90% of the passes.

That's been my experience with the Insight, too. Used to regularly drive over Carson Pass (California 88) then Altamont Pass into San Jose, and would get mid-70s for the round trip. But a flatter* drive out US 50 &c to the Toiyabe Range would get in the low 80s.

And of course the one-way trips downhill from the top of Carson Pass would see me getting 150 mph (the highest the Insight's gauge will show) for 50-60 miles :-)

*I do realize that my idea of "hills" may differ from the norm :-)

[Ecky]

Originally Posted by mpg_numbers_guy

As a relevant comparison, between today and yesterday's trips my Insight got 82.9 MPG driving on "normal" roads (flat + slight hills) and "only" 73.7 MPG through mountain passes. MPG was lower because I couldn't maintain lean burn for 80%-90% of the passes.

I have to wonder if your mountains aren't smaller than ours.

When I was down in MA my fuel economy improved from ~58mpg to upper 80's. It was a little warmer but I expect most of that is from things being less mountainous. It's frequently necessary to be in 3rd gear WoT to maintain speed in my car on Vermont highways, forget about lean burn.

[skyking]

The more modern injected engines get the most benefit on the hilly sections due to Deceleration Fuel Cut-Off. Engines with carburetors still use ( waste ) a little fuel even with the throttle closed, but injected engines can shut that right off with DFCO.

[mpg_numbers_guy]

Originally Posted by jamesqf

And of course the one-way trips downhill from the top of Carson Pass would see me getting 150 mph (the highest the Insight's gauge will show) for 50-60 miles :-)

But what is it on the return trip?

Originally Posted by jamesqf

*I do realize that my idea of "hills" may differ from the norm :-)

Michigan doesn't have a lot of crazy hills, but those mountain passes in WV are insane. Definitely steeper than in Tennessee and out west. Can't compare to Vermont though. Closest I've been to is NY, and that was only the west part, and I had my Civic then.

Originally Posted by Ecky

I have to wonder if your mountains aren't smaller than ours.

When I was down in MA my fuel economy improved from ~58mpg to upper 80's. It was a little warmer but I expect most of that is from things being less mountainous. It's frequently necessary to be in 3rd gear WoT to maintain speed in my car on Vermont highways, forget about lean burn.

This was actually through the steep mountain passes in West Virginia. 3rd gear at 80%+ load getting 30-40 MPG was frequently necessary to climb the hills. On some hills I could maintain 45-55 MPG if I got enough momentum on the downgrade.

Through Michigan and elsewhere where there aren't any mountain passes I got 80+ MPG like you did in MA.

I also let my speed dip down to as low as 40 MPH on the inclines if there is no traffic. On the downhill my MPG is always above 100, and I sometimes reach incriminating speeds, which helps maintain momentum and get better MPG. Being able to bleed off 35 MPH from 75 to 40 allows me to use less throttle going uphill. I also downshift before the hill to keep the RPMs around 2000 (2000 RPM in 3rd gear is better MPG AND power than 1500 or so RPM in 4th gear going uphill, probably BSFC related voodoo ). That's probably the biggest reason why I was able to get the 73.7 MPG through the passes.

Most of the traffic on I-64 is truckers, who on mountain grades aren't going much faster than I am, so I don't mind going that slow near the top of the pass. If I tried to maintain 55 MPH going up the passes my fuel economy would definitely be lower, but if I did that, then I would have to use engine braking on the downhill to avoid reaching dangerous speeds, and DFCO's savings over neutral coasting doesn't make up for losing 5-15 MPG on the incline. 40 MPH uphill and 75 MPH downhill, if split evenly, is an average speed of 56.25 MPH, which is pretty much the same speed I averaged outside the mountains.

[Vman455]

Originally Posted by mpg_numbers_guy

40 MPH uphill and 75 MPH downhill, if split evenly, is an average speed of 56.25 MPH, which is pretty much the same speed I averaged outside the mountains.

Not quite! There was a similar problem on my physics homework this week.

If you traveled the same time at each speed, your average speed would be the arithmetic mean of 40 and 75, or 57.5 mph. But since we're assuming an equal distance at each speed--up the mountain and down the other side--your average speed is the harmonic mean:

2/((1/40)+(1/75)) = 52.1739... mph

Of course, that's assuming instant acceleration for the sake of simplicity, which means that estimate is a little high.

[mpg_numbers_guy]

Thanks for the correction! Looks like I plugged in 45 MPH and 75 MPH instead of 40 MPH and 75 MPH. 52.17 MPH is correct.

That's also assuming 75 MPH is the highest speed I reached on the downhills.

[Vman455]

Originally Posted by mpg_numbers_guy

That's also assuming 75 MPH is the highest speed I reached on the downhills.

I remember one trip to Oklahoma City in the Civic, coasting down the hills in Missouri with the engine off, zipping past all the pickups that had just passed me going uphill--allegedly.

[mpg_numbers_guy]

Originally Posted by Vman455

I remember one trip to Oklahoma City in the Civic, coasting down the hills in Missouri with the engine off, zipping past all the pickups that had just passed me going uphill--allegedly.

Yup! Although since I was pretty much going as slow as the truckers uphill, their weight gave them more momentum on the downhill, but they kept hitting their brakes for some reason, imagine that , so I passed 'em.

Also those people that pass you five times because they keep stopping for stuff while you just keep truckin' along.

Slower speed doesn't always mean a longer trip.