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Old 01-17-2007, 12:02 PM   #1
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Driving slower uphill, and faster downhill

I've read somewhere that losing speed uphill, and gaining speed downhill, can
be a good way to get economy. I have found a graph that seems to show this:



It is for the American Ford Escort.

Edit : at one point, the 'grade 4' uphill has very similiar MPG to the level, at about the same speed - this would be the perfect speed for codfishing.

Also, note on the very steep uphill - after one of the gearchanges, the economy actually gets worse, as the
engine is being laboured too much!
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Old 01-17-2007, 06:46 PM   #2
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I'd say that whoever did the driving in this graph didn't upshift early enough on the uphills. There's a big jump in FE right after each shift, which supports the practice of using plenty of throttle at low rpms.

Do you have any other info for this graph? It would be interesting to see engine rpm. I'm also curious why FE took a dive on the grade6 line at high speed. Did the driver downshift at that point? Is this an auto tranny?
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Old 01-18-2007, 10:06 AM   #3
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Quote:
Originally Posted by DRW View Post
I'd say that whoever did the driving in this graph didn't upshift early enough on the uphills. There's a big jump in FE right after each shift, which supports the practice of using plenty of throttle at low rpms.

Do you have any other info for this graph? It would be interesting to see engine rpm. I'm also curious why FE took a dive on the grade6 line at high speed. Did the driver downshift at that point? Is this an auto tranny?
Auto trannies co$t forever, in both gas and efficiency...as well as price!! They're great for hitting Northbound bulls in the South end, however!

Let's use a physics book for graph analysis...."What goes up must come down." Deviating from a steady-state (level, cruise) condition, we encounter a steep, simple hill. We can either increase our throttle opening... to maintain steady-state velocity( and greatly increase the fuel consumption rate)... or we can maintain our throttle opening and our velocity will slowly decrease (we are "coasting" uphill). If our throttle foot is smart(steady) and the hill ramp is brief the fuel consumption rate won't change that much. The torque remains steady (fuel burned) within a narrow RPM range.
We crest the hill ; our throttle becomes a bit too much for the steady-state velocity we were going before we got to the hill, so we back off a bit. Our fuel consumption rate drops! Assuming the same distance down (to level) as up, the net ... over-all ... gas consumption is down. Note: Shifting gears complicates our picture!
Providing the up & down ramps are equal in slope & length, the road is straight, and gear changes do not occur...we can save a bit of gas, maybe. Of course, this is a textbook hypothetical scenario. Variations drink gas!
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Old 01-18-2007, 12:01 PM   #4
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The interesting things are, however:


Going up a 4% gradient is practically equal to going on a level when both are done at 40km/h. This would give good scope to coast down the hill

Also, the steeper the downhill, the faster you have to go to find the peak fuel economy - useful to note if doing highway driving!.

It is for the 'USA ford escort' - that is all I know. It was posted on a 200SX web forum!
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Old 01-18-2007, 12:01 PM   #5
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Yup none are coasting down the hill they are in gear all the time and as the RPM increases even on the downhill the fuel consumption increases - also remember that the 6 grade is about a 3 degree hill and not very steep.
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Old 01-20-2007, 07:58 AM   #6
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That's a cool graph landspeed. best mpg at 30mph(ish) on flatland looks painful But my average speed is like 25, so if there were a way to let drivers go a constant 30 it would be an improvement in mpg and time.

I tend to freewheel downhill as long as I'm somewhere around the speed limit.
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