An Engineer's Ultimate Guide To 3.21 VS 3.92 Axle Ratio

[bwsRam19]

Take two otherwise identical trucks, one with 3.21 and the other with 3.92 gears running side-by-side at a steady speed on the highway. Both engines will be producing exactly the same horsepower, but the engine in the one with the 3.21 gears will be turning ~20% more slowly. Lower revs mean lower friction losses in the engine. It also means the slower-turning engine will be running with higher manifold pressure, which means lower pumping losses. Is the difference "significant"? Hard to say, but it definitely exists, and I bet it would be measurable if we actually performed this experiment as described.

Your wrong, the engines aren't producing the same horsepower if they are at different rpm's

 

[BowDown]

Your wrong, the engines aren't producing the same horsepower if they are at different rpm's

Exactly how much power difference do you think is occurring at 1500 vs 2000 rpm?
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[bwsRam19]

doesn't matter if its 1 or 100, it still not the same

 

[BowDown]

Means its inconsequential but you go on thinking 3.92 don't get any worse gas mileage than 3.21s despite the fact that there's a forum full of people stating exactly that

 

[silver billet]

Gas engines need to operate at much higher rpm's to get into their peak power band and so taller gears work against the engine. When rock crawling or towing a heavy trailer the engine provides more usable torque and horsepower with lower gears. The idea that 3.21 gears are going to make a significant difference in fuel economy is ludicrous to the extreme.

The engine at low vehicle speeds is working to move the mass of the truck and get its wheels spinning. At freeway speeds 90% of the horsepower is used to overcome air drag and trucks have the aerodynamics of a brick and generate a great deal more air drag than any passenger car made in the last 50 years. If someone wants to get more miles per gallon with their pickup truck then they need to drive slower as the air drag increases exponentially with speed so there is 4x as much air drag at 80 mph as there is at 40 mph.

If you never tow and never take your truck off the road then accept the factory default 3.21 gears and don't worry about it.

3.21 gives you an extra 400 rpms lower than 3.92 for the same speed on the freeway. Of course it's going to use much less gas. It would be the same effect as adding an overdrive extra gear to your 3.92. I don't know how you get from "gas engines need to operate high in rpms to make power" to "low rpms don't use less gas", but that's some serious mental gymnastics and is completely contradictory. You can't have one without the other.

The mass and aerodynamics of the truck is irrelevant for this particular discussion, because it has the same effect on two otherwise identical trucks with the different gears.

I do tow with my truck, 3.21, and yes I "don't worry about it".

 

[Rossum]

Your wrong, the engines aren't producing the same horsepower if they are at different rpm's

No, I'm not wrong. You don't understand basic high school physics. It take a certain amount of horsepower to push the truck down the road at a given speed. If both trucks are otherwise identical except for the axle ratio, then their engines are producing exactly the same power to keep them moving at that speed.

 

[SpeedyV]

No, I'm not wrong. You don't understand basic high school physics. It take a certain amount of horsepower to push the truck down the road at a given speed. If both trucks are otherwise identical except for the axle ratio, then their engines are producing exactly the same power to keep them moving at that speed.

YES. They MUST be producing the same power, all other things being equal. I was sitting on my hands with this one

 

[bwsRam19]

No, I'm not wrong. You don't understand basic high school physics. It take a certain amount of horsepower to push the truck down the road at a given speed. If both trucks are otherwise identical except for the axle ratio, then their engines are producing exactly the same power to keep them moving at that speed.

Yes you are wrong, you don't understand gearing, you can make a 10hp motor push the truck down the road with the right gearing, I'm done here.

 

[bwsRam19]

So what your saying is, the 5.7l hemi engine is making the same power at 1500 rpm and 2000 rpm......your wrong

 

[bwsRam19]

Means its inconsequential but you go on thinking 3.92 don't get any worse gas mileage than 3.21s despite the fact that there's a forum full of people stating exactly that

Don't recall ever saying that

 

[BowDown]

So what your saying is, the 5.7l hemi engine is making the same power at 1500 rpm and 2000 rpm......your wrong

The power at 1500 rpm will push it down the road, thats obvious. The truck turning 2000 rpm to go the same speed is simply using more RPM and fuel.
This is a simple rpm x gearing algebra, ;ess rpm = better fuel economy. You simply dont get it

 

[BowDown]

Yes you are wrong, you don't understand gearing, you can make a 10hp motor push the truck down the road with the right gearing, I'm done here.

Sure can but at what speed and what effort to maintain said speed? You should be done, you're 100% wrong on every count

 

[bwsRam19]

The power at 1500 rpm will push it down the road, thats obvious. The truck turning 2000 rpm to go the same speed is simply using more RPM and fuel.
This is a simple rpm x gearing algebra, ;ess rpm = better fuel economy. You simply dont get it

more rpm and more fuel=more horspower

 

[bwsRam19]

The power at 1500 rpm will push it down the road, thats obvious. The truck turning 2000 rpm to go the same speed is simply using more RPM and fuel.
This is a simple rpm x gearing algebra, ;ess rpm = better fuel economy. You simply dont get it

i know less rpm is better fuel economy, as well as more rpm is more horspower

 

[Rossum]

Yes you are wrong, you don't understand gearing, you can make a 10hp motor push the truck down the road with the right gearing, I'm done here.

I most certainly do understand gearing. Can you make a 10 hp motor push the the truck down the road with the right gearing? Yes, but only rather slowly; it will never reach highway speeds, because 10 hp is simply not enough to overcome the drag one of these trucks has to fight at, say, 70 mph.

 

[bwsRam19]

if im running 70 in my 3.92 gears at 2000 rpm, this means i'm running more in the power band and I'm able to maintain that speed better because the engine is producing more horsepower and torque. At 70 mph in the 3.21 gears at 1500 rpm you are less in the power band getting better fuel mileage, but aren't able to maintain the speed quite as well making less horspower and torque, which is why it will downshift more.

 

[Jeepdinger]

Anyone else find it strange that those dyno charts were used for arguments sake, but all start at 3000rpm? When we're debating the benefits of 1500-2200rpm power ranges? Anyway, if there is a dyno chart for hemi out there that shows power at the relevant rev range I would be interested in seeing it.

Rule of thumb: Power = gas. Combustion process converts chemical energy of gas to mechanical rotation of engine (usable power). Each squirt of injector uses gas to create power, and will reduce mpg accordingly. This is why cruising rpm is so important. Let us compare 1600rpm vs 2000rpm cruising speeds. A 400rpm (2 rev = 1 combustion cycle) difference in cruising speed is equivalent to 1600 (8 cylinders X 200 combustion cycles) additional squirts from fuel injector. That's 1600 additional fuel injector squirts per minute while cruising down highway. Even though the throttle may be at a lower position for the engine at 2000rpm, the percent decrease in throttle position will most likely not be able to compensate for the increased rpm. I think it's entirely possible that under ideal circumstances you may be able to achieve same or better fuel economy at higher rpm, but it will be brief and not sustainable over a tank of fuel. Over a tank of fuel, 400+ miles depending on tank size, the overall averages will drive fuel economy. The engine cruising at lower rpm will have better fuel economy over long haul (highway driving). City driving will be more dependent on driver style since there is a lot more acceleration cycles and not as much constant speed driving. Comparing city vs highway is an apples to oranges comparison. Better fuel economy will be seen by those with high percentage highway or steady speed driving.

Higher cruising speed = more air resistance = more power required to overcome air resistance = more fuel used. So, be careful comparing notes when one driver cruises at 65mph while another may be at 80mph. Totally different scenarios and usages. Explanation of aero has been made multiple times in this thread so I won't go back into it.

In general, this thread has been fascinating. It does get entertaining when details get dragged into it because then you get apples to oranges comparisons in discussion. I look forward to seeing what else you add to this thread. Cheers.

 

[BowDown]

if im running 70 in my 3.92 gears at 2000 rpm, this means i'm running more in the power band and I'm able to maintain that speed better because the engine is producing more horsepower and torque. At 70 mph in the 3.21 gears at 1500 rpm you are less in the power band getting better fuel mileage, but aren't able to maintain the speed quite as well making less horspower and torque, which is why it will downshift more.

No it doesn't, there's probably 7 hp difference between 1500 and 2000 rpm and the HP made at either RPM isn't necessary to keep the truck moving at that speed. Throw in some hills then yes but both are downshifting at that point.

 

[BowDown]

i know less rpm is better fuel economy, as well as more rpm is more horspower

Tought you were done?

 

[BowDown]

Anyone else find it strange that those dyno charts were used for arguments sake, but all start at 3000rpm? When we're debating the benefits of 1500-2200rpm power ranges? Anyway, if there is a dyno chart for hemi out there that shows power at the relevant rev range I would be interested in seeing it.

Rule of thumb: Power = gas. Combustion process converts chemical energy of gas to mechanical rotation of engine (usable power). Each squirt of injector uses gas to create power, and will reduce mpg accordingly. This is why cruising rpm is so important. Let us compare 1600rpm vs 2000rpm cruising speeds. A 400rpm (2 rev = 1 combustion cycle) difference in cruising speed is equivalent to 1600 (8 cylinders X 200 combustion cycles) additional squirts from fuel injector. That's 1600 additional fuel injector squirts per minute while cruising down highway. Even though the throttle may be at a lower position for the engine at 2000rpm, the percent decrease in throttle position will most likely not be able to compensate for the increased rpm. I think it's entirely possible that under ideal circumstances you may be able to achieve same or better fuel economy at higher rpm, but it will be brief and not sustainable over a tank of fuel. Over a tank of fuel, 400+ miles depending on tank size, the overall averages will drive fuel economy. The engine cruising at lower rpm will have better fuel economy over long haul (highway driving). City driving will be more dependent on driver style since there is a lot more acceleration cycles and not as much constant speed driving. Comparing city vs highway is an apples to oranges comparison. Better fuel economy will be seen by those with high percentage highway or steady speed driving.

Higher cruising speed = more air resistance = more power required to overcome air resistance = more fuel used. So, be careful comparing notes when one driver cruises at 65mph while another may be at 80mph. Totally different scenarios and usages. Explanation of aero has been made multiple times in this thread so I won't go back into it.

In general, this thread has been fascinating. It does get entertaining when details get dragged into it because then you get apples to oranges comparisons in discussion. I look forward to seeing what else you add to this thread. Cheers.

Most dynos cant cleanly make a pull from 2000 rpm so most operators don't start the run at that low of an RPM, you can also get erratic reading from a dyno that low in the rpm scale, that chart was to compare power differences at various rpms.

Heres a dyno graph of my car, notice how erratic the scale is down low