Hills - Which gear?

[wmjinman]

Coming back from Reno today in my Swift, I took the old highway so I could go slower than the freeway. Another part of the reason is because there's a certain hill on that route I thought would be good to do some hill climbing testing. It's one of those hills where you may make it in top gear or not depending on which way the wind is blowing & whether you have the weight of a full tank of gas or not.

First I tried it at 55 mph in 5th. It wouldn't *quite* maintain 55, but did close.
Then I tried it at 50 in 5th. Same deal - the speed lagged, but I kept it floored.

After that, I did it in 4th at more speeds. Results follow:

5th - 55 - - 25.6 mpg
5th - 50 - - 25.7 mpg

4th - 50 - - 24.7 mpg
4th - 45 - - 26.0 mpg
4th - 40 - - 26.9 mpg
4th - 35 - - 27.4 mpg

During the research I was doing in conjunction with my engine replacement project, I came across the engine's performance specs that max torque was at 3000 rpm. So the next thing I did was to see what speed 3000 rpms came to in each gear. Results follow:

2nd gear - 3000 rpm is 27 mph
3rd gear - 3000 rpm is 40-41 mph
4th gear - 3000 rpm is 56 mph

I didn't check it for 5th this time, but seem to remember it's like 65-70 mph

So it seems my best mileage (so far) going up that hill is 4th gear and well below the torque peak. Since I was on "current" trip mode, I wasn't looking at the throttle position, but I wish I had been, now.

But if I'm ever heading up a mountain on a freeway and can't hold 5th, at least now I know that trying to hold 56 in 4th should be at it's torque peak.

[Varn]

Looking at your data, it looks like what I have done for years, (without really knowing) keep it in top gear and not at full throttle on a hill. Shifting down only when full throttle (or nearly) won't work.

[wmjinman]

Yup, same here. In my "old" Jimmy ('85 2.8 liter v-6 & 5-speed manual), I would do that too - 5th gear until it dropped, then a quick downshift to 4th & then tried to hold it at it's torque peak - which, if I remember correctly, was 2100 rpm. I also noticed that in 5th, 2100 rpm was right at 55 mph (the nationwide 55 mph speed limit was still in effect when it was built).

Hmmm - just thought of something; many speed limits nowadays are 65, and that's the speed the Gnat is going at max torque in 5th!!!! I wonder if they're designed that way? I'll bet they are!

[user removed]

One thing to consider is the time factor. Higher speed when climbing means less time at the much lower mileage than your average for the whole trip. While your mileage might not be it's highest climbing at higher speed, your time suffering through that lower mileage is less and therefore you will have an overall average higher by spending less TIME making the climb and MORE time at higher mileages.

I may be crazy but I'm fairly sure the time factor at much lower mpg is something to consider as well.

Maintain the best load in gear until the grade forces you to loose speed, then use the next lower gear to maintain the best load, but also make the total climb as short as practical. Not crazy different but err to the practical but shorter (time wise) climb.

This assumes the coast downhill will not force you to sacrifice your speed for staying legal as far as speed limits. Most of the hills here are not steep or sustained and when I can do so without aggravating others my speed will be lowest at the top of the hill, allowing me to reach my peak speed without risking a traffic ticket.

With lower grades, I try to keep my downhill speeds close to the limit in traffic and I can do this on the Interstate in 65 mph traffic, pulse drafting a big rig. I did this today going to the dentist and when I got there I was reading 61 mpg for 25 miles without causing any problems with other traffic.

regards
mech

[stiletto2]

Originally Posted by Old Mechanic

One thing to consider is the time factor. Higher speed when climbing means less time at the much lower mileage than your average for the whole trip. While your mileage might not be it's highest climbing at higher speed, your time suffering through that lower mileage is less and therefore you will have an overall average higher by spending less TIME making the climb and MORE time at higher mileages.

I may be crazy but I'm fairly sure the time factor at much lower mpg is something to consider as well.

Maintain the best load in gear until the grade forces you to loose speed, then use the next lower gear to maintain the best load, but also make the total climb as short as practical. Not crazy different but err to the practical but shorter (time wise) climb.

This assumes the coast downhill will not force you to sacrifice your speed for staying legal as far as speed limits. Most of the hills here are not steep or sustained and when I can do so without aggravating others my speed will be lowest at the top of the hill, allowing me to reach my peak speed without risking a traffic ticket.

With lower grades, I try to keep my downhill speeds close to the limit in traffic and I can do this on the Interstate in 65 mph traffic, pulse drafting a big rig. I did this today going to the dentist and when I got there I was reading 61 mpg for 25 miles without causing any problems with other traffic.

regards
mech

Yup, yer crazy. The fuel economy method ground vehicles use is in miles per gallon; how ever long that takes. Contrary to your thinking, one usually uses more fuel not less by going faster... to cover the same distance. "Suffer" with the lower speeds and save fuel or go faster and suffer poorer fuel economy to go a given distance... your choice. But the idea generally promoted around here is to use less fuel.

Gallons per hour is the method aircraft use to calculate economy because the time/speed in the air does not always correlate with the distance traveled over the ground.

Aircraft try to maximize the amount of TIME they can spend in the air on a given amount of fuel and ecomodder ground vehicles try to maximize the amount of MILES they can cover with a given amount of fuel.

Minimizing the time in either case uses more fuel... that means going faster. And the aircraft won't be able to spend as much time in the air and in the case of ground vehicle, it won't be able to cover as many miles with its allotment of fuel.
Speed Kills.