RPM

[PWalls]

I've got the HO engine and 3.92 gears in my 2025 1500 Limited. What I have noticed on the way home, which is an easy 5 miles of country driving, the truck will stay in 7th gear between 55-60MPH and around 2000RPM. It will not shift into 8th gear and drop down to 1600RPM unless I lay on it hard in a short span to get it to 65MPH or more and then ease off the gas. It will then drop into 8th gear and lower the RPM. Also, Aerodynamic ride height comes on about that time also.
Question, is the spurt to try and get it to shift into 8th gear worth the effort or just let it ride in 7th? I am wondering if the 8th gear, lower RPM and aero ride height is helping gas mileage in that short of a time or should I just not worry about it.

 

[silver billet]

I've got the HO engine and 3.92 gears in my 2025 1500 Limited. What I have noticed on the way home, which is an easy 5 miles of country driving, the truck will stay in 7th gear between 55-60MPH and around 2000RPM. It will not shift into 8th gear and drop down to 1600RPM unless I lay on it hard in a short span to get it to 65MPH or more and then ease off the gas. It will then drop into 8th gear and lower the RPM. Also, Aerodynamic ride height comes on about that time also.
Question, is the spurt to try and get it to shift into 8th gear worth the effort or just let it ride in 7th? I am wondering if the 8th gear, lower RPM and aero ride height is helping gas mileage in that short of a time or should I just not worry about it.

The hemi with 3.21 is basically around 1200 RPMs at that speed. You'll never get that low with the 3.92 but it should definitely not be hanging onto 2000 rpms when you have another gear left. Talk to the dealer? Sounds like a poor tune to me.

 

[mikeru82]

You're comparing a naturally aspirated engine with turbocharged engines. There are some differences that many people don't know. I honestly don't know for sure what the behavior of the SST engines is supposed to be, but it would make sense that the rpm's would be held higher. One of the worst things you can do with a turbocharged engine is go to WOT at low rpm's (lugging the engine). Lugging a turbocharged engine puts much higher stress on the engine. This is explained very well in this Engineering Explained video, specifically at the 2:22 mark. It also touches on this being harmful to catalytic converters, which may help explain why so many people have had issues with melted cats. I suspect the engine designers spec'd it this way because of this but I have no way of confirming that.

Here's a link to the video I mentioned aboue...

 

[silver billet]

You're comparing a naturally aspirated engine with turbocharged engines. There are some differences that many people don't know. I honestly don't know for sure what the behavior of the SST engines is supposed to be, but it would make sense that the rpm's would be held higher. One of the worst things you can do with a turbocharged engine is go to WOT at low rpm's (lugging the engine). Lugging a turbocharged engine puts much higher stress on the engine. This is explained very well in this Engineering Explained video, specifically at the 2:22 mark. It also touches on this being harmful to catalytic converters, which may help explain why so many people have had issues with melted cats. I suspect the engine designers spec'd it this way because of this but I have no way of confirming that.

Here's a link to the video I mentioned aboue...

Lugging is hard on any engine, turbos or NA makes no difference. High load at low rpms are bearing killers. This is why I tow above 2000 RPMs on the freeway even though my truck would often upshift another gear if I let it.

But if the hurricane can't handle 1600 rpms at 55 mph and feathering the pedal then it shouldn't be put in a truck. I know for a fact that the GM turbo (little 4 banger) in the silverado did not rev high at all, in fact it seems quite happy to upshift ASAP.

 

[1791]

my 5.7 also shifts and holds 7th like that. i just let it do it's thing.