My ex and I were debating (and of course I want to win since he's an ex...) when the best time is to down-shift when climbing a hill at 65+ speeds. I want to maintain the speed, and obviously cannot at certain points, decreasing to 45, and want an opinion on when to take the cut. He says before I start the climb, still at such high rpms, I say drop when I start slipping and just realize will take a sec to catch back up. Any idea which will give more power long term without damage or burning gas like crazy? Help!!

I'll side more with you. Let's say you're going 70 and starting up the hill. As the hill gets steeper, you notice you're giving it more gas to maintain speed. Sometime before you've actually got the gas pedal nailed to the floor, I'd make the cut and shift down a gear and go from there.

Might as well ride out the lower gear until you start leveling back out. No use upshifting, then having to downshift again before you get to the top. The automatics do the same thing, holding the lower gear and/or kicking out the torque converter lockup until there is less demand on the engine.

Best to keep the engine near the "sweet spot" the best you can. I've found in my automatic that while I have trouble holding 70 on a hill in 4th, I have no trouble at all holding 80 on the same hill in 4th. It's all because the engine is working in a stronger part of the power band at the higher RPM at 80. Horsepower peak on the non-supercharged engine is about 4800 RPM (per Nissan), and that's about where the torque curve starts to fall off. Best power/torque combo is about 4200 RPM. Here's a dyno run I did a couple of weeks ago:

Every time I see an Xterra dyno it makes me want to cry

Here's a dyno from a friend's car that weighs half as much:

[img]http://www.s2ki.com/forums/index.php?act=module&module=gallery&cmd=viewimage&img=270280&md=1[/img]

Depending on the hill, of course, I tend to downshift from 5th to 4th when I drop to around 2,000 rpm. That gives me plenty of power to bring my speed back up with no problem.

What size engine is that dyno from? I have teh 2.4L and i hope that isnt the 3.3 dyno

Not sure what is meant by maintaining power long term...but to save gas, you don't maintain speed, you let it lose some speed going up...flooring it going 45 in top gear is not saving fuel.

Offroad X's chart says it all.

What ever gives you the least gas flow at a given speed gets you the best MPG....so, adjusting the gearing to give you the least pedal pressure (You are giving it the least gas...) is the answer.

The sweet spot in the power band, where the engine can match the ground speed with minimal application of the gas pedal, is your target for mpg.

You might be going slower of course than you were when approaching the hill...if you want to waste some gas to maintain the same speed though, use the same principle, but at the new speed, find the gear that allows the least pedal to maintain.

"Drive the load, not the speedometer" is the expression used in long haul trucking for example.

So that being said about limiting your throttle position to save gas and MPG, you ideally want to establish what the lowest SAFE RPM that you could run on a straight road that would not lug the engine and give you the best MPG. I think the lowest that I go if I really want to get some nice MPG is 1,500 RPM in 5th, cruzing on a straight road around town with no real wind effect. What are some other opinions?

Yep... that's the "170hp" 3.3L :rolleyes:

Kaiser, that's power at the rear wheels on a chassis dyno, not at the flywheel on an engine dyno.
15-20% loss through the driveline is fairly typical, my numbers are closer to 20% off the rated flywheel hp, and my engine has over 100K, an exhaust leak, but is otherwise in decent shape.

oh yeah.. I tend to forget about that.

I wasn't knocking your truck specifically... I'm in exactly the same denim-blue boat.

Hmmmm running the motor at 4800 rpm. Can you say 8 mpg?

Yeah, for a mile or two tops, who cares?

Who cares? Apparently all those people in the millions of MPG threads cares.

Like TJ said, maintaining speed up a hill is the best way to kill MPG. Learn to drive by load - maintain a CONSTANT throttle up a hill.

On a 50 mph highway, I get up to 60 mph to gain some momentum before a hill and HOLD the throttle there. If speed drops below 45 or so (converter about to unlock) I turn off OD.

Combined with 35 psi tire pressures, I can get 22 to 23 mph on 50 mph hwys in the summer.

Thinks that's bad, my "250hp" Intrepid dynos 180 fwhp.

RJ, you do it your way, I'll do it mine, but the mileage thing is secondary to being able to live with myself and not backing up traffic behind me when I can keep pace by sucking down an extra pint of gas during the climb...

Higher RPM's and more power doesn't necessairly mean you're eating up more gas... it all depends on the gearing. Shift down so that your RPM's at 3.5k and you can fly up the hill without flooring it. That's the supercharger sweet spot.

I'll add my opinion too since I drive a lot of mountain passes in the Xterra and with a tractor/trailer combination. While it does depend somewhat on how steep the hill is and how long it is, what TJ said is pretty much the way I drive. Usually I'll let the engine speed drop a little in the top gear or two (or three or four...LOL!) until I get to the gear and speed that allows me to pull the hill without using full pedal pressure. In the Xterra this is usually about 60 MPH in 4th gear (manual tranny). If you hold the throttle down all the way in top gear and the vehicle just continues to lose speed then you are not only losing more speed than you need to, but you've also got your injectors and your intake wide open, and that sucks fuel. I don't know the exact scientific point where the fuel useage meets, but basically the idea is that you will use less fuel at say 4K RPM with 50% throttle application than you will at 1500 RPM with 100% throttle application.

It's a little bit hard to explain how to know when to shift and when not to, but for me it's not even something I think much about. It's just an automatic response. I know my method works though because I'm consistently a fuel mileage leader. We talk about this stuff in the driver's lounge too and most truck drivers who buy their own fuel use a variation of this method to achieve greater mileage.

The driving by load thing really doesn't apply anymore once we're talking about >25% throttle.

Obviously, throttle and rpm must be matched, or you'll not only waste fuel but damage the engine.

The place I live right now is about 2000' in elevation, and highways are 50 - 55 mph limited; this is very different than say Colorado elevation + 80 mph interstates. Most hills here can be climed at 25% throttle @ 2000 - 3000 rpm going about 50 mph (6% to 8% grades).

Anytime I have to use more than 25% throttle I always make sure the engine is at or above its peak torque (2800rpm).