What is the best rpm range for highway

[m715]

I'm building a custom truck and planning on using a 12v cummins engine and was wondering what is the best RPM range for highway travel 55-65. This way I can plan my tire size and gearing to match the engine. From the little reading I've done it seems like 1600-1800rpms is a good range?



For those of you interested its a 1968 millitary Jeep M-715 pickup with a custom shorted utility bed and with be riding on 35-44" tires



Luke

'68 M715... under construction

 

[The patriot]

Welcome aboard!!

Mine usually runs between 2000 (about 63mph) to about 2500(roughly 75). I'd like it to be a little lower. 1600-1800 seems about right unless you plan on towing heavy loads while going uphill.

In that event, a few more RPM's would be nice.

Eric

 

[Dieselnerd]

To save your drivetrain, my understanding is that it should be at least 1800 to 2000. Some say 2000 as a minimum. Part of the reason for this is the Cummins power pulses tend to beat the transmission, rear end and U joints at lower RPM's. This is more important when towing or hauling heavy loads of course.

 

[DBaker]

I have a '98 3500, 5spd, 3:54 gears in Dana 80, 19. 5 inch Alcoa wheels and at 70 mph I'm turning 1800 RPM.

 

[DRR-24V]

With my 3. 54 Auto I run 1750 at 65 MPH. Excellent mileage. !

 

[rhoads]

RPM'S

The important question is what do you want your MPG'S too be??

the eng it self loves 2 grand.

You have to figure out how fast/slow you want at this point.

 

[Blakers]

Mine runs 1900 rpm at 73 mph. Checked with my gps. Runs along just nice and comfortable at that.



Blake

 

[Cooker]

Are you plsnning on running a stick or auto?



1700-1900 RPM is where you want to be.

 

[m715]

Originally posted by Cooker

Are you plsnning on running a stick or auto?

1700-1900 RPM is where you want to be.

Wow this forum is great... Thanks for all the info

Right now I am very open to ideas, I going to run a 12v and the NV4500 but axles/gears/tires are open for ideas at this point.

I'm putting together some excel spread sheets so I can see what gear/transmission/tire combonations will give me for rpms...



I've got a bunch of different ways I can jump, just depends on how much $$ I want to sink into this project:

- Stock axles with 4. 56 or 5. 87 gears & 35" tires

- Stock chevy with 4. 10 or 3. 54s & 35" tires

- or 2. 5 ton top loading Rockwells w/6. 72 gears & 40-44" tires

(except I would need a 2nd over drive to run them i think)



I guess I would like to run at 55-60mph and pull ~6-8,000 lb load



Luke

 

[Speedo]

m715 if you are planning on keeping the stock

axles and changing the gears did you know that

the Dana 70 rear axle in the M715 uses a different

gear set than most any 70 that you will find

today? It uses a . 500 offset pinion while most

everything newer uses a . 625 offset pinion.

 

[rhoads]

Gears

My gears are 4:10 w/235 michlens 5 spd and stroll past the big rig's up 4 mile grades at 7 % climbs like there setting on jacks and maintain your 55-60 road speed. and remember this is an overdrive transmission(5th) and backing in the load is easyer than a 3:54 ratio, But then you could increase your MPG's by about 1 mile per gal loaded an 4 running empty. . however not easy on the clutch going backwards in the loaded condition!!slip-slip-burn-burn.

 

[RJ2]

rpm range

Been tossing up doing something like that also. Big electric web site has been doing alot of transplants, mainly with the dodge M37's. They have the links to any parts you need .

 

[Bob Ellithorpe]

59 mph @ 1800 rpm 4:10's she's as happy as can be.

 

[m715]

Re: rpm range

Originally posted by RJ2

Been tossing up doing something like that also. Big electric web site has been doing alot of transplants, mainly with the dodge M37's. They have the links to any parts you need .

Do you have the address for Big electric web site?



Speedo - Your right, the M715 has an OLD style carrier, they will hold a 4. 56 gear set, there's several people on the m715zone.com running them...

Would that "Huntin' Buggy" of your's be an M715?



What kind of fuel miliage do people see running at 2000 rpms?



- Stock axles w/5. 87 gears will cost me $2. 3k for a US Gear OD

- Stock alxes w/4. 56 gears will cost me $650 for gears * bearings + setup costs

(I have a good set of 35" tires/rims for stock axles which a plus for the stock axles)

- 2. 5 ton rockwells will cost me $1k(axles) + ($2. 3K) US gear OD + tires & 20" rims ~$2k or $500 for surplus split rims/tires



Sorry for my rambing... I'm thinking out loud



Thanks for all the GREAT info, its gotten me on the right track...



Luke

 

[Turbo Thom]

For my part the 3. 55 rear,235 tires on a 3500 the best hwy rpm is 1800-2200 rpm. I do have a US Gear splitter and at 2000 rpm I'm running 80 mph in fifth over.

I love it like that.

Preston

 

[RJ2]

rpm range

The Big Electric site is http://www.bigelectric.com/index.shtml

 

[Blakers]

Ok, what's an M37 and an M715? You guys are talking in tongues again...



Blake

 

[m715]

Originally posted by Blakers

Ok, what's an M37 and an M715? You guys are talking in tongues again...

Blake

They are older Military trucks, just begging to have a cummins transplanted into them

the M-37 is a Dodge 3/4 ton from '50-68?

the M-715 is a Kaiser/Jeep 1 1/4 ton from '67-69



Here is a Dodge M-37

 

[m715]

Here is a Kaiser/Jeep M715

 

[Blakers]

Thanks guys for clearing this up for me. Yes I sure agree that a Cummins would be nice in one of them.



I have a little 4 cylinder Benz diesel engine I was always going to put in a flat-fendered Willys CJ. Never got around it though, but still have the engine and she runs good. I rebuilt it years ago and the car still has fuel in it from 1988; I start it up once a year or so to make sure it runs ok.



Blake