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transmission shifting question

R

Red2000

NAXJA Forum User
Location
Arlington, VA
I have the 4.0 HO with an AW4 and the tow package if it matters. I drive through mountains at least once a week. Is there any advantage for gas mileage by shifting into 3rd instead of D? I notice the RPM's jump around when i shift but have not figured out the pattern. I'm just curious if i can manage an extra MPG or two by shifting while driving in the mountains.
 
You can waste fuel by lugging an engine, and cause damage to the engine also.

The "charm" of a slush box is its supposed to "auto" shift depending on the load on the engine. Rather than having the transmission shift up/down repeatedly it is sometimes better to manually select a lower gear so you can stay within the power band consistently.

4.0L I6 MPI "Power Tech HO" (High Output) - 190 hp @ 4750 rpm, 220 ft lb @ 4,000 rpm - used in 91-95 and updated in 96-01 to 225 ft lb torque @ 3000 rpm (00-01 models use a distributor-less ignition system)
 
Depending on your RPMS, and if they stay constant while going up hill then downshifting may not be necessary. But, not a bad thing to downshift manually. GOing up hills you will use more fuel anyways to keep your speed or accelerate.
 
I'd try to keep it at around 2450-2500rpm personally.

Johnnie - for some reason I could swear the quote in your sig is from Fear and Loathing in Las Vegas, but I'm not sure. It's been over a year since I saw that movie and it's one of those "you only need to watch it once in a lifetime" ones as far as I'm concerned.
 
the quote is definately from fear and loathing, when his big lawyer buddy is trying to kill him. While that story was bizzarre the auther prortrayed some brilliant writting. Plus johnny's character covers the vegas to reno race. Watch it once more and you may appreciate it.

OP I ALWAYS manually shift when going through the mountains. While our trannies are stout there not smart enough to see the road ahead and sometimes get confused on how often to shift. I notice the tranny sometimes going between 3rd and 4th far more often than i'd like when going through the sierra's. To keep it from over heating i often keep it in 3rd the whole way up the summit.
 
Don't lug your engine on hills. Go ahead and downshift if you deem it necessary. You'll create less engine heat in both your engine and trana and you probably not use a teaspoon more of gas.
 
cruiser54 said:
Don't lug your engine on hills. Go ahead and downshift if you deem it necessary. You'll create less engine heat in both your engine and trana and you probably not use a teaspoon more of gas.
Click to expand...

You actually use less gas lugging your engine (giving it more throttle) than you would at higher RPM with less throttle. This is due to vacuum loss. At high RPM with an barely open throttle (butterfly) there is more load on the motor because as the pistons descend pulling in air, there is more vacuum with the throttle plate almost closed. At lower RPM and the throttle more open it easier for the motor to pull in air (less vacuum).

People think (I"m giving it more "gas" (aka Throttle) they are using more "gas" which is not the case.. its more complicated than that..
 
stephenspann27 said:
You actually use less gas lugging your engine (giving it more throttle) than you would at higher RPM with less throttle. This is due to vacuum loss. At high RPM with an barely open throttle (butterfly) there is more load on the motor because as the pistons descend pulling in air, there is more vacuum with the throttle plate almost closed. At lower RPM and the throttle more open it easier for the motor to pull in air (less vacuum).

People think (I"m giving it more "gas" (aka Throttle) they are using more "gas" which is not the case.. its more complicated than that..
Click to expand...

you got it backwards bub, the higher the vacuum the more efficiently the motor is running and the better fuel economy you get. always downshift to 3 in the hills
 
It is sometimes referred to as pumping loss. Read post #9: http://www.4btswaps.com/forum/showthread.php?11718-Relationship-between-HP-and-MPG

If it was true that higher mpg (less lugging) was more efficient, then engineers wouldn't program automatics to shift to high gears so early. My '95 Z-28 for instance had an economy "feature" where if you were lugging the engine in first and tried to shift to 2nd, it would lock you would of 2nd gear and force you into 4th gear instead its called "skip shift." Basically you'd be going 10mph, and to safe fuel, it would force you to shift to 4th gear and lug the engine (more open throttle plate).

The new camaro also has it: http://www.youtube.com/watch?v=aU1Tc8EsGMg

Basically when you think of using burning fuel, you need to think of RPM not throttle position. The term "giving it gas' is misleading. You give a car air, not gas (unless its a diesel). If you are running 1200 RPM and "floor it" you don't dump fuel in, you basically open the butterfly to provide more air if needed.
 
Last edited:
Excerpts from the above thread:

"A typical road going engine spends 99% of it's life at part throttle and idle. That is where the pumping losses put the gasoline engine at it's greatest disadvantage and give diesels a 40%+ advantage in fuel economy."
"The lack of a throttle plate can save whole kilowatts of pumping loss at part throttle and idle too.
But comparing petrols and diesels at their best operating points, the compression/expansion ratio is the biggest factor."
 
uhh... the biggest disadvantage a gas engine has to a diesel engine as far as fuel economy goes is the fact that diesel fuel has way more btu per gallon than gas, pumping losses are negligible at best

you will use more fuel lugging around at 1700 rpm at half throttle vs dropping a gear and doing 2500 rpm at 1/8 throttle. forget "pumping loss" the engine is plain working harder when it is lugging because it isn't running at the optimum rpm for the given load, why do you think big rigs have so many forward gears? its all about keeping the engine in the right rpm range for each situation. now if you decided to drop 2 gears and brought it to 4000 rpm you will use more fuel because its going to be outside of the optimum rpm range again
 
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