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RPMs to fast?

I

InIdaho

Seasoned Expediter
A MBE900 engine specs: 300 HP @ 2200 RPM/ 860 LBFT @ 1200 RPM

If a class D expediter truck cruises at 65 mph and is turning 2100 rpm is it geared correctly?

For fuel economy, would it be better if the cruising rpms were closer to the slower max torque rpms. Would lower cruise rpms mean a longer engine life as well?
 
C

chetjester

Veteran Expediter
Driver
That doesn't sound too healthy to me. Or fuel efficient, either.

I have an MBE900-330 with a 6 speed Allison automatic transmission. I cruise at 65 mph with 1550 rpms after the overdrive kicks in.

Something seems wrong with the gear ratios.
 
greg334

greg334

Veteran Expediter
InIdaho said:
A MBE900 engine specs: 300 HP @ 2200 RPM/ 860 LBFT @ 1200 RPM

If a class D expediter truck cruises at 65 mph and is turning 2100 rpm is it geared correctly?

For fuel economy, would it be better if the cruising rpms were closer to the slower max torque rpms. Would lower cruise rpms mean a longer engine life as well?
Click to expand...

Simple answers -

1 - no it is not correct

2 - yes it would be better.
 
R

RichM

Veteran Expediter
Charter Member
You may have a chassis powertrain set up that was designed for a dump or a tow truck or city tractor useage.
 
Jefferson3000

Jefferson3000

Expert Expediter
DaveKC explained in another thread recently that dealers usually buy class 7 trucks in bulk in order to get the best price, and then outfit them accordingly for specific use. You may have a truck outfitted for local use or as RichM said.

I am finding that a lot of the trucks we buy for expediting were not really spec'd for it from the ground up.
 
LDB

LDB

Veteran Expediter
Retired Expediter
If you are talking about a class 7 Sterling with that powertrain and especially if it is a used truck there's a chance it's not going into overdrive. My first one would always go to run in 1-5 when first started. There's a button at the top left of the shift panel labeled option or something and pushing it would switch it to 6 speed mode. I have no idea why it was programmed that way but that made a big difference in cruising rpm of course. I believe in the 300/860 configuration it was about 1650 rpm at 65mph in 6th and much higher in 5th. That option button (whatever it's called) can be set for a number of functions. It does one thing and is an on/off switch. If you start the truck and put it in drive and it shows 5 in the digital display press it and see if it goes to 6.

To answer your original question, that is definitely way wrong gearing and will kill your mpg and kill your engine much sooner. I wouldn't buy that truck without a gearing swap to cruise about 1600 at 65 in overdrive. Good luck.
 
I

InIdaho

Seasoned Expediter
Thank you all for your input.

This all came up after talking to a seller. Said its a MD2106 with a 6 speed synchro (MDT?) The rear ratio is 4 something I think it is 4.1.

I started this thread because owner claimed it averages 10 + mpg and says cruising @ 2100 is about right since the max hp kicks in about there.


Any ideas how costly a gear swap should cost?
 
R

Refer Hauler

Expert Expediter
You maximum horse power will probably be achieved at the high RPM however torque is what gets you moving and keeps you moving. Truck should be able to cruise at an acceptable highway speed say 65 mph at the upper end of the torque band which is probably in the 1450 to 1650 range depending on engine. For example my 12.7 ltr Detroit is rated for 430 Hp at 2100 RPM but max torque 1450ft/lbs is at 1475 RPM.
I would look at your build sheet and see what rear end you have.
To look at the cruising RPM difference between rear end ratios Google in "Road Speed Calculator" and see the difference betwen a 4.11 and a 3.36 rear end. I used the generic calculator but Eaton Road Ranger has one also. a 295-75 X 22.5 low pro tire is 40.6 inches high on average. Getting you RPM's down by 3 to 5 hundred will pay for a rear end swap very quickly. I've been thinking about changing the rears in my tractor presently 2.79's this thing will fly but it don't like long hills
 
F

FIS53

Veteran Expediter
I had a salesman at a KW dealer tell me that the PX-6 engine should be run at over 2200 rpm when cruising. I was a little surprised at that as the max rpm is not much more. So I kinda left there soon afterwards. I know smaller diesels run at that speed on the highway and the 1900-2000 range for the power stroke and gm 6.5L motors but for a px-6???? Not so sure about that speed myself. Thought it was too high. As some said maybe for a dump or such but not for a freighter going from town to town.

So went elsewhere to look at trucks...

Rob
 
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