6.4 hemi not shifting into 8

[Matrix]

I bought a 2500 hemi with 3.73 gears and was surprised it reved about 2100 on the highway and fuel mileage wasn’t great. I started wondering if it was in 8th gear and after shifting it to 7 the rpm stayed the same.after playing with the gears I got it to shift into 8 and the mileage was a lot better and rpm’s went down to about 1600. The problem is it didn’t last and went back to only running in 7 on the highway. Any advice?

 

[Grateful Dad]

Mine doesn’t shift into 8th until 69 mph, occasionally 68 but rarely. It will hold 8th below those speeds but pretty much gotta hit 70 before I’ll get 8th.

 

[Matrix]

I’ll pay attention to that. Thanks

 

[Rama_Lama]

Mine doesn’t shift into 8th until 69 mph, occasionally 68 but rarely. It will hold 8th below those speeds but pretty much gotta hit 70 before I’ll get 8th.

That’s what I have noticed as well, which I’m glad it doesn’t do the Tacoma thing where it upshift too soon just to downshift 5 seconds later

 

[JMacoubrie1]

I bought a 2500 hemi with 3.73 gears and was surprised it reved about 2100 on the highway and fuel mileage wasn’t great. I started wondering if it was in 8th gear and after shifting it to 7 the rpm stayed the same.after playing with the gears I got it to shift into 8 and the mileage was a lot better and rpm’s went down to about 1600. The problem is it didn’t last and went back to only running in 7 on the highway. Any advice?

Obviously no one has any answers. Mine does the same thing. Why would you put an 8 speed transmission in a vehicle, when it will never be in 8th gear.

 

[Wanderlust073]

Obviously no one has any answers. Mine does the same thing. Why would you put an 8 speed transmission in a vehicle, when it will never be in 8th gear.

Drive faster. Goes in to 8th just fine.

 

[JMacoubrie1]

Drive faster. Goes in to 8th just fine.

So your answer is to break the posted 60 mph speed limit?

 

[flan]

Reprehensible...

 

[pizzatots]

Can 100% confirm 69mph with the 3.73s. There's another thread with the RPM charts out there. I set my cruise at 69mph.

 

[JMacoubrie1]

Can 100% confirm 69mph with the 3.73s. There's another thread with the RPM charts out there. I set my cruise at 69mph.

Setting my cruise at 69 will result in a ticket, since the speed limit is 60. So this doesn't solve the problem.

 

[Rama_Lama]

Setting my cruise at 69 will result in a ticket, since the speed limit is 60. So this doesn't solve the problem.

I don’t believe it’s a problem.
If you’re maintaining 60 mph 7th is the golden gear in this truck if it’s equipped with a 6.4 and 3.73 gears

If want to catch 8th gear, get up to 69-70 mph and coast back down to 60. The truck will hold 8th until you hit a hill or heavy wind.

I believe the engineers tuned the transmission properly by holding gears instead of needlessly up shifting in an attempt to save mpg.
If want that, get a 3rd gen Tacoma. Those love to up shift too soon, just to downshift at the slightest incline, wind, expansion joint etc…..

 

[AH64ID]

Setting my cruise at 69 will result in a ticket, since the speed limit is 60. So this doesn't solve the problem.

There isn't a problem, they don't want the rpms that low...

 

[Rama_Lama]

There isn't a problem, they don't want the rpms that low...

Indeed! It’s much better to give up 1-2 mpg instead of lugging the engine.

 

[JMacoubrie1]

I don’t believe it’s a problem.
If you’re maintaining 60 mph 7th is the golden gear in this truck if it’s equipped with a 6.4 and 3.73 gears

If want to catch 8th gear, get up to 69-70 mph and coast back down to 60. The truck will hold 8th until you hit a hill or heavy wind.

I believe the engineers tuned the transmission properly by holding gears instead of needlessly up shifting in an attempt to save mpg.
If want that, get a 3rd gen Tacoma. Those love to up shift too soon, just to downshift at the slightest incline, wind, expansion joint etc…..

Nothing like trying to insult me, when all I want is some answers to this obvious problem. Common sense tells me, the lower the rpm, the LESS fuel it will use. With the he 6.4, I can't believe that it wouldn't have enough power to maintain a speed of 60 mph, at 1600 rpm. If I bought one with a manual six speed, I would expect to be able to get it into sixth, and not drive all the time in 5th, until I reached 68 mph. We aren't talking about a Delorian trying to get back to the future.

 

[AH64ID]

Nothing like trying to insult me, when all I want is some answers to this obvious problem. Common sense tells me, the lower the rpm, the LESS fuel it will use. With the he 6.4, I can't believe that it wouldn't have enough power to maintain a speed of 60 mph, at 1600 rpm. If I bought one with a manual six speed, I would expect to be able to get it into sixth, and not drive all the time in 5th, until I reached 68 mph. We aren't talking about a Delorian trying to get back to the future.

You have been given the answers, and also been told it’s not a problem.

Lower rpms doesn’t always mean more efficient.

There are also things to consider other than just fuel burn, such as engine longevity.

7th is the basically the equivalent of 6th in your manual comparison, so you’re already there. I’m not aware of any manual transmissions used in these trucks that had a top gear as tall as 8 in the 8HP75.

 

[Wanderlust073]

From a thread last year:

8th gear - highway speed?

New truck. 6.4 with 3.73s. Noticing at 65mph the truck stays in 7th gear. If I bump up to 70 it will fall down to 8th gear. What speed does this truck like to go in to 8th? Anyone notice?

hdrams.com

Also it’s comical to ask ‘why give a truck an 8th gear it will never use’ when your specific situation of a 60mph speed limit is hardly the scenario the transmission was designed for…

 

[JMacoubrie1]

You have been given the answers, and also been told it’s not a problem.

Lower rpms doesn’t always mean more efficient.

There are also things to consider other than just fuel burn, such as engine longevity.

7th is the equivalent of 6th in your manual comparison, so you’re already there.

I don't know where you got your engineering degree, but you need a refund. I have had several trucks, with 6 speed automatics, All different models, and have NEVER had one not go into high gear at around 55mph. Longevity of the engine, also depends on a lower rpm. Don't believe this, the why do diesel trucks operate at around 1600 rpm? It makes absolutely ZERO sense, to build a truck that will spend 95 percent of it's life, NOT in high gear! Not everyone drives on interstate highways only. In Missouri, the speed limits are 60 mph or less. Unless you are on a four lane road.

 

[AH64ID]

Now you’re comparing gas engine to diesel engines, no wonder this discussion is difficult for you.

 

[dieselscout80]

It’s not an issue with 4.10 gears.

 

[JMacoubrie1]

Now you’re comparing gas engine to diesel engines, no wonder this discussion is difficult for you.

Well, I'm so glad that you have all the world's problems solved. But you don't know your ass from a hole in the ground. If you can't come up with an actual, reasonable answer to a problem, that obviously exists, then you are definitely wasting my time. Obviously you can't, or you would have.