gear ratios

[opus]

Running a 1995 8.3 Cummins, mechanical. 2000 rpm gives me 60 MPH. 4:10 rear gears. What should the perfect rpm be cruising on the highway?  I really have no need to go faster than 65. Trying get the best economy of course. Rehearing might be worth it.

[pvcces]

Without considering startability, I get 3:10 gears as pretty much an optimum for fuel economy. If you could add a gear in the transmission instead, that gave you a 500 RPM drop at 60 mph, that would get you to the same place, except your startability would be as good as it is now.

At 50 mph, you would be at 1260 RPM and you would top out at 2100, which would be 83 1/3 mph. Cruising at 60 mph would mean you would be turning 1512 RPM.

Is that what you wanted to know?

I don't know what might be available if you wanted to change your gearing, but I doubt that you could improve very much with a different ratio. You wouldn't want RPM much lower or much higher at cruising speed. Your current gears don't make optimum use of your engine, as far as I can see.

Tom Caffrey

[luvrbus]

That's a 2600 rpm engine I don't think dropping the rpm on the engine by gearing is going to help much with fuel economy then you lose power something they don't have a abundance of 

good luck

[pvcces]

Hi, Cliff.

You got me, there. I just looked up the 8.3 ISC and it shows maximum horsepower as anywhere from 240 at 2200 to 330 at 2600. I guess that I would have to do some digging to see what might work.

Our coach only uses about 100 HP when cruising, but it only weighs about 27,000. That leaves us enough power to peg the speedometer, but probably not much more.

I wouldn't be surprised if the best ratio was some compromise in the range of 3:00 to 4:00. I gather that some of this would depend on the power the computer was set for, if it is not an MUI.

For what it's worth.

Tom Caffrey

[TomC]

You're really not that far off from what Cummins wants to have you at the magical 1800rpm for cruise speed.  The 8.3 would be geared much like a 92 series.  With your present setup, you could just drop down to 55 mph and be at 1833rpm.  Or to get to what Cummins suggest, is 1800rpm at 65mph, your gearing would be 3.42.  I don't think you'll ever see the savings in fuel compared to the cost of changing the rear end gear set.  When you have time, just slow down to 55mph-that will be your best fuel savings overall.  Good Luck, TomC

[opus]

Just got home, 9mpg for the 1000 mile trip.  We try to do 60-62mph tops.  The engine is 250hp @2200rpm.  I was surprised to find out it was a 2600rpm engine.  I am used to big Cummins that were 2100 tops and even lower.  Another transmission gear aint happening.  The transmission I have is quite nice....non-computer too.  I am not sure where the peak torque is.  For some reason I am thinking 1600 or 1800.  I havent found anything online yet for an all mechanical 8.3.

This thing surely doesnt lack for power at all.  It eats up mountain passes.

55mph across the prairies of MT is pretty painful!  So I will assume the change in rear gears wouldnt be worth the fuel savings.  We probably do 10,000 miles a year.

[bevans6]

You might find that it eats up mountain passes because it's in the sweet spot for the engine for climbing now.  Changing the gear might hurt that more than you want.

Brian

[luvrbus]

That is really a 504 Case engine peak torque is 1500 rpm forgot the torque somewhere around 623 lbs + 


good luck

[opus]

You might find that it eats up mountain passes because it's in the sweet spot for the engine for climbing now.  Changing the gear might hurt that more than you want.

Brian

Exactly, thats what I am trying to figure out.....BEFORE I do something. 

[TedsBUSted]

Two speed diff?

Per TomC, payback would be slow.

Ted