Help needed on going to smaller tires

[grimmjeeper]

On the JK 5-speed, I would agree, but with the 8-speed, there is certainly an argument to be made to keep the RPMs below 2,000 to maximize fuel efficiency, which OP stated is one of the reasons he is considering doing this in the first place.

The argument being made, which I agree with in this case, is that if 8th gear is running at 2,500 RPM on the freeway, then you are losing out on potential fuel economy.

If 8th gear is running at 1,800 RPM, you will benefit from improved fuel economy under low load conditions (i.e. flat or slightly downhill). If the load increases, the tranny will downshift for more torque as needed. Therefore, there is no loss in towing performance.

I would even take it a step further and say that if 8th is running at 2,500 RPMs on the freeway that the tranny has gears that are wasted. The entire benefit of having 8 speeds is to have closer ratios in the shifts. Downshifting 1 gear in an 8-speed might only raise the RPMs by 200 whereas downshifting in a 4-speed might be closer to 800, which might be WAY more than you need. That is why people tend to put lower gears in the older Jeeps so their RPMs aren't jumping crazy high just to maintain speed on a slight incline.

Hope that makes sense.

When you operate an engine at too low of an RPM you lose efficiency. For the 3.6, that's around 2,000 RPM. While you can get away with it on the EPA test loop that operates under very strict conditions that don't match the real world, you don't get any real world benefit when you operate an engine on the wrong end of the BSFC (Brake Specific Fuel Consumption) curve.

For every engine, there is an RPM where you make the most power with the least fuel. Sure, up above 3,000 RPM you're losing fuel economy. But with the 3.6, you pass the efficiency point before you drop below 2,000 RPM. The best efficiency point is different for every engine. It depends on the compression ratio, the head design, and valve timing (along with other details). To get the high end horsepower, the 3.6 sacrifices it's low end efficiency and power.

Hope that makes sense

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[Zandcwhite]

When you operate an engine at too low of an RPM you lose efficiency. For the 3.6, that's around 2,000 RPM. While you can get away with it on the EPA test loop that operates under very strict conditions that don't match the real world, you don't get any real world benefit when you operate an engine on the wrong end of the BSFC (Brake Specific Fuel Consumption) curve.

For every engine, there is an RPM where you make the most power with the least fuel. Sure, up above 3,000 RPM you're losing fuel economy. But with the 3.6, you pass the efficiency point before you drop below 2,000 RPM. The best efficiency point is different for every engine. It depends on the compression ratio, the head design, and valve timing (along with other details). To get the high end horsepower, the 3.6 sacrifices it's low end efficiency and power.

Hope that makes sense

I get the theory, but I can't find any real world evidence that shows that you pass the efficiency point at 2k rpms? Further the most efficient point for horsepower per rpm is irrelevant when you're going downhill where you need 0 hp. At that point the lowest possible rpm should burn the least fuel. And that is a real world, everyday condition that can't be mimicked on the dynos that the epa is testing on. Obviously their testing won't represent exact real world conditions as it's dyno runs in a lab but the idea that undergearing beyond the efficient rpms is going to make some magic gain on the various city, combined, and highway dyno cycles just makes 0 sense logically. If more rpms were more efficient at low loads on the street they'd be more efficient on the dyno too.

 

[grimmjeeper]

I get the theory, but I can't find any real world evidence that shows that you pass the efficiency point at 2k rpms? Further the most efficient point for horsepower per rpm is irrelevant when you're going downhill where you need 0 hp. At that point the lowest possible rpm should burn the least fuel. And that is a real world, everyday condition that can't be mimicked on the dynos that the epa is testing on. Obviously their testing won't represent exact real world conditions as it's dyno runs in a lab but the idea that undergearing beyond the efficient rpms is going to make some magic gain on the various city, combined, and highway dyno cycles just makes 0 sense logically. If more rpms were more efficient at low loads on the street they'd be more efficient on the dyno too.

I have emperical evidence from a few people running 3.6 engines with manual transmissions.

I ran my old 3.6 manual JK (the JL engine isn't fundamentally different) in a ~100 mile test loop with stock 3.21 gears and stock sport tires. This loop included both back roads and interstate. I ran it in 6th gear and measured fuel consumed (filled to all the way full at the filler neck each time). Then did the loop in 5th gear.

6th had me at under 2,100 at 75 and just under 1,800 at 65. 5th put me up at 2,600 at 75 and just under 2,300 at 65.

Got within a couple of tenths of a gallon of fuel consumed in both runs. And running the Jeep in 5th meant I never lost speed going up and down the mild Iowa hills (where I lived at the time). Every single hill in 6th had me losing speed after just a couple of seconds. And these are Iowa hills so nothing really significant.

Everyone I talk to who does a similar test in a stock non-Rubicon (JL or JK) gets the same results. Every single one.

By that measure, the lowest fuel consumption point has to be up in the 2,000s at the very least.

Another data point. I was one time driving into a significant headwind one afternoon and couldn't hold speed at all when I was in 6th. I had to downshift just to maintain speed on a flat road at low elevation. (This is what prompted me to do the test in the first place.)

And it also tells me that overdrive putting you below 2,000 will downshift any time you experience any moderate grade, not to mention any time you have any load on the engine like trying to tow.

And if you are always downshifting out of top overdrive, you're geared too tall. Furthermore, if your transmission holds top gear at those RPMs, you're not going to get great mileage on any hill or with any load because your engine works a lot harder (i.e. uses more fuel) to make the power it needs.

Finally, watch your RPMs when you're holding 8th up a hill. If they rise while you're holding constant speed it means the computer unlocked the torque converter, letting the engine get up in the power band it needs to be in to make the power to hold speed. And that's effectively cheating, letting the torque converter compensate for gears that are too tall.

 

[c20040215]

In this case she's asking which will make her look thinner while you know that both will do the opposite.

When you operate an engine at too low of an RPM you lose efficiency. For the 3.6, that's around 2,000 RPM. While you can get away with it on the EPA test loop that operates under very strict conditions that don't match the real world, you don't get any real world benefit when you operate an engine on the wrong end of the BSFC (Brake Specific Fuel Consumption) curve.

For every engine, there is an RPM where you make the most power with the least fuel. Sure, up above 3,000 RPM you're losing fuel economy. But with the 3.6, you pass the efficiency point before you drop below 2,000 RPM. The best efficiency point is different for every engine. It depends on the compression ratio, the head design, and valve timing (along with other details). To get the high end horsepower, the 3.6 sacrifices it's low end efficiency and power.

Hope that makes sense

In my personal experience, I feel like the 3.6 under 2,000 rpm is lugging in 4th, 5th, and 6th gear (MT). Before I regeared, I see no difference in fuel economy in 5th or 6th gear at around 75 mph. If I remember correctly, 37s tires and 4.11 gear doing 75 mph is at about 2050 rpm. 5th is at about 2300 rpm. Slightly higher rpm but lower engine load and it felt much better. At the same speed, 4.88 gear puts me at 2500 rpm in 6th gear. The fuel economy stays the same. Every engine has different rpm range for the best engine efficiency. If I have to guess, 3.6 sweet spot is around 2200 to 2700 for highway cruising.

 

[Zandcwhite]

I have emperical evidence from a few people running 3.6 engines with manual transmissions.

I ran my old 3.6 manual JK (the JL engine isn't fundamentally different) in a ~100 mile test loop with stock 3.21 gears and stock sport tires. This loop included both back roads and interstate. I ran it in 6th gear and measured fuel consumed (filled to all the way full at the filler neck each time). Then did the loop in 5th gear.

6th had me at under 2,100 at 75 and just under 1,800 at 65. 5th put me up at 2,600 at 75 and just under 2,300 at 65.

Got within a couple of tenths of a gallon of fuel consumed in both runs. And running the Jeep in 5th meant I never lost speed going up and down the mild Iowa hills (where I lived at the time). Every single hill in 6th had me losing speed after just a couple of seconds. And these are Iowa hills so nothing really significant.

Everyone I talk to who does a similar test in a stock non-Rubicon (JL or JK) gets the same results. Every single one.

By that measure, the lowest fuel consumption point has to be up in the 2,000s at the very least.

Another data point. I was one time driving into a significant headwind one afternoon and couldn't hold speed at all when I was in 6th. I had to downshift just to maintain speed on a flat road at low elevation. (This is what prompted me to do the test in the first place.)

And it also tells me that overdrive putting you below 2,000 will downshift any time you experience any moderate grade, not to mention any time you have any load on the engine like trying to tow.

And if you are always downshifting out of top overdrive, you're geared too tall. Furthermore, if your transmission holds top gear at those RPMs, you're not going to get great mileage on any hill or with any load because your engine works a lot harder (i.e. uses more fuel) to make the power it needs.

Finally, watch your RPMs when you're holding 8th up a hill. If they rise while you're holding constant speed it means the computer unlocked the torque converter, letting the engine get up in the power band it needs to be in to make the power to hold speed. And that's effectively cheating, letting the torque converter compensate for gears that are too tall.

Again all of your examples of getting better economy at higher rpms are increased load situations, where you should downshift (or the auto will automatically). If you need more power you need more rpm and you get there by gearing. That doesn’t remotely prove that gearing so low that 8th effectively is 7th will increase efficiency. If that was the case the trans would just be a 7 speed and we'd all get better mileage. Sure if you want to drive a manual like it's an auto you're better off gearing extra low, but that just proves you're doing it wrong. It's losing speed because it needs more power in that situation, not because it needs more power all the time. We might as well drive around at 6k rpm so we have all the power possible if the load increases? Or just downshift? Let the auto downshift automatically?

 

[GabeBoyTheGreat]

When you operate an engine at too low of an RPM you lose efficiency. For the 3.6, that's around 2,000 RPM. While you can get away with it on the EPA test loop that operates under very strict conditions that don't match the real world, you don't get any real world benefit when you operate an engine on the wrong end of the BSFC (Brake Specific Fuel Consumption) curve.

For every engine, there is an RPM where you make the most power with the least fuel. Sure, up above 3,000 RPM you're losing fuel economy. But with the 3.6, you pass the efficiency point before you drop below 2,000 RPM. The best efficiency point is different for every engine. It depends on the compression ratio, the head design, and valve timing (along with other details). To get the high end horsepower, the 3.6 sacrifices it's low end efficiency and power.

Hope that makes sense

This has not been my experience with either of my JLs. As much as I hated the 3.45 gears that came with my 2018, it was surprisingly efficient on the highway (over 25 MPG). And both JLs I have tested from time to time 5th vs 6th gear, and 6th is generally 1 MPG better than 5th on the highway (albeit, this is according to the "real-time" computer calculations).

I understand what you are saying in theory, but for the theory to be valid, your torque output would have to increase EXPONENTIALLY with RPM increase. The 3.6l torque curve flattens out right around 2,000 RPM, so if anything, the "sweet spot" would be 2,000. However, that doesn't take into account that combustion engines are LESS efficient under LIGHT loads, which is why you want lower RPMs to increase load on the engine, thus increasing efficiency.

But I digress as I feel we have gotten too far from the topic at hand.

 

[c20040215]

This has not been my experience with either of my JLs. As much as I hated the 3.45 gears that came with my 2018, it was surprisingly efficient on the highway (over 25 MPG). And both JLs I have tested from time to time 5th vs 6th gear, and 6th is generally 1 MPG better than 5th on the highway (albeit, this is according to the "real-time" computer calculations).

I understand what you are saying in theory, but for the theory to be valid, your torque output would have to increase EXPONENTIALLY with RPM increase. The 3.6l torque curve flattens out right around 2,000 RPM, so if anything, the "sweet spot" would be 2,000. However, that doesn't take into account that combustion engines are LESS efficient under load, which is why you want lower RPMs to increase load on the engine, thus increasing efficiency.

But I digress as I feel we have gotten too far from the topic at hand.

The sweet spot is a moving target depends on many factors, not just the engine output curve.
You might find a sweet spot at 2000 rpm for a 2 door Jeep with no lift and 33" tires. For a 4 door, 3.5 lift, 37" tires, 2000 rpm at cruising speed might not be enough to hold speed, or more efficient. Even for the same engine, same speed, same rpm, the tire rolling resistant is different, drag is different, and the vehicle weight is different. These all affect the optimal cruising rpm.

 

[grimmjeeper]

This has not been my experience with either of my JLs. As much as I hated the 3.45 gears that came with my 2018, it was surprisingly efficient on the highway (over 25 MPG). And both JLs I have tested from time to time 5th vs 6th gear, and 6th is generally 1 MPG better than 5th on the highway (albeit, this is according to the "real-time" computer calculations).

I understand what you are saying in theory, but for the theory to be valid, your torque output would have to increase EXPONENTIALLY with RPM increase. The 3.6l torque curve flattens out right around 2,000 RPM, so if anything, the "sweet spot" would be 2,000. However, that doesn't take into account that combustion engines are LESS efficient under LIGHT loads, which is why you want lower RPMs to increase load on the engine, thus increasing efficiency.

But I digress as I feel we have gotten too far from the topic at hand.

I'm not talking about the torque curve. I'm talking about the BSFC curve, which generally isn't published.

 

[Zandcwhite]

The sweet spot is a moving target depends on many factors, not just the engine output curve.
You might find a sweet spot at 2000 rpm for a 2 door Jeep with no lift and 33" tires. For a 4 door, 3.5 lift, 37" tires, 2000 rpm at cruising speed might not be enough to hold speed, or more efficient. Even for the same engine, same speed, same rpm, the tire rolling resistant is different, drag is different, and the vehicle weight is different. These all affect the optimal cruising rpm.

And even still, the auto will adjust automatically. I've never had a JL losing speed because it's trying to cruise at too low an rpm for the load. I've driven in to heavy headwind in the desert where it would cruise in 7th or even 6th up slight grades. None of that is evidence that cruising at more rpms than you need for the given load is more efficient. For every one of those examples where more rpm is needed there's the opposite situation. Driving across the desert with a heavy tailwind 2k rpm is plenty. Down hill? Clearly more rpms isn't going to be more efficient there. Nobody is saying tow a heavy trailer on 3.45s and 35s and lock it in 8th gear. But 4.56s and 31s with this auto is a complete waste.

 

[GabeBoyTheGreat]

I'm not talking about the torque curve. I'm talking about the BSFC curve, which generally isn't published.

Here is an example BSFC curve. I know this is not for the Pentastar, but most curves follow a similar pattern.

From the graph, you can see that there are very few scenarios where increasing RPM and decreasing engine load result in better efficiency, and these scenarios ONLY occur in 80%+ engine load situations. You can't just move across the graph into a more efficient zone; you have to move down the graph as well. Higher RPM = lower gearing = less engine torque. That is why there are few if any scenarios where increasing RPM/decreasing engine load result in better efficiency.

However, the graph proves what I stated, which is that combustion engines are LESS efficient under LIGHT loads. Throughout the entire RPM range, it is clear that efficiency improves when engine load increases (to a certain extent).

So it becomes a balance of making sure that the engine is under sufficient load that it is operating efficiency but not extremely high loads where the BSFC curve becomes non-linear.

Edit: Added a second figure that better displays what I am talking about. This figure actually graphs horsepower, so now you can see for a given horsepower requirement, in almost ALL scenarios (except for the high horsepower scenario), lower RPM is more efficient.

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