M725 gear ratio

MJCougler · Jul 22, 2010

[FONT=&quot]Hello, I’m new and interested in purchasing an M725 ambulance. Everything I’ve read on e-bay or other sites says that the truck is geared down with a top speed of about 50 MPH. I did several searches but could not find out any information about how hard is it to swap out the gearing to at least be able to go 65 on the interstate without being near redline. Thanks.[/FONT]

LOWGEAR · Jul 22, 2010

The m725 has 586 gears in it from the factory. You can change your gearing to match your needs. Check out The m715 Zone to find all you need to know about your Kaiser. [thumbzup]

bcowanwheels · Jul 27, 2010

4:56 is all thats available and there pricy. the truck isnt designed to be run & stop at those speeds so if you are wanting to run at modern speeds at mid level RPM,S swap out axles..... and dont forget to also swap the t-case.....

Wolf.Dose · Jul 30, 2010

bcowanwheels is right, the truck is NOT designed for "racing". The max speed governed is 60 mph in the original setting. And a good useable speed is between 50 and 55 miles. You have to accept that or quit. The other gearing might help to get the rpm down, but not realy solve the problem of the truck. It was designed for the late 60's and for Army speed up to 45 mph. So the reading on the passenger door in Germany was: City 30 , Hightway 50, Others 45. And that was ment as the maximun not as an average!
If you stay in this range, the truck is ok with every day useability.
I know what I'm talking about, I drive a M715 since October 1982.
Wolf

Elwenil · Jul 30, 2010

Keep in mind that only the later models had the speed governor. Earlier models did not have it though the same top speeds still apply.

bcowanwheels · Aug 2, 2010

45MPH is considered wide open. yes they will go faster but if you want the engine to live.45

Wintermute · Aug 3, 2010

But you should be able to change the axle gearing from a 5.87 to a 4.56, add selectros to the front hubs, put in an NP205 instead of the NP200, upgrade the brakes (remote booster will help), change to a civvy carb, change to electronic points and then get highway speeds (60-65 mph) without killing the motor. It's an expensive package overall, but it can be done.

--Wintermute

Wintermute · Aug 5, 2010

Okay, I decided to run some calculations:

The high gear for the NP200 is the same as the NP205, however, there is a difference in noise (NP200=straight cut gears, NP205 = helical gears) and in heat/RPM tolerance due to the difference in gear cut and bearings between the two. Here's some calculations to look at:

Stock configuration transfer case, transmission and diff gears:
Tire diameter = 35.4 (Power King Super Traction NR-50 tires)
Transmission final gear ratio = 1 (T98 stock transmission)
Transfer case high gear ratio = 1
Axle ratio = 5.87
RPMs @ 65mph = ~3621 (motor not happy)
RPMs @ 75mph = ~4178 (motor blown up)

Stock configuration transfer case and transmission but switched to 4.56 diff gears:
Tire diameter = 35.4 (Power King Super Traction NR-50 tires)
Transmission final gear ratio = 1 (T98 stock transmission)
Transfer case high gear ratio = 1
Axle ratio = 4.56
RPMs @ 65mph = ~2813 (motor alright)
RPMs @ 75mph = ~3246 (sort of doable)

Transmission changed out to an NV4500 (again, still don't know if this is possible) but with the stock 5.87 diff gears:
Tire diameter = 35.4 (Power King Super Traction NR-50 tires)
Transmission final gear ratio = .73 (NV4500 5 speed transmission)
Transfer case high gear ratio = 1
Axle ratio = 5.87
RPMs @ 65mph = ~2643 (motor alright)
RPMs @ 75mph = ~3050 (should be okay?)

Transmission changed out to an NV4500 (still trying to figure out if this is possible) and with 4.56 diff gears:
Tire diameter = 35.4 (Power King Super Traction NR-50 tires)
Transmission final gear ratio = .73 (NV4500 5 speed transmission)
Transfer case high gear ratio = 1
Axle ratio = 4.56
RPMs @ 65mph = ~2053 (motor alright)
RPMs @ 75mph = ~2369 (motor alright)

Hope that helps you out. I'm thinking that the NV4500 is a great idea if it's possible to do. I'm already going to get the diff gears swapped out, but that will have to wait for the money to do it. I'm going to do all of the research I can to find out if I can get an NV4500 to mate up to the stock motor. It would be nice to be able to cruise at 75mph if necessary in my truck. At minimum, the diff gears will get me 65mph and with an NP205 and selectros, the truck should be happy at that speed. If there is a way to mate an NV4500 transmission to the stock engine, it looks like I'll have no problems hitting 75mph.

--Wintermute

91W350 · Aug 5, 2010

The M715 had no power to spare in stock form, it was a load for the original engine. I think dropping the engine rpm by raising the gear ratio will make the load more than the engine can pull. It is pretty optimistic to think the little six is going to be able to pull a M725 75 mph. I am betting the sail it is trying to pull will turn into a concrete wall someplace before 75 mph. Sure, you can save a little power by lockout hubs reducing the rotating mass, maybe save a little by swapping the case, but I think you will find yourself woefully short on go power.

I spent quite a bit of time with the M715 and did lots of modifications. Even set up properly with a V8, 55 is fast enough in one. If you are going to can everything that is stock, steering, brakes, engine, transmission, axles, transfer case... You might as well go find a heavy duty chassis and adapt the M725 body to it.

Nothing about this truck was designed to run over 50 mph, sure, it can be done, but I think the costs outweigh the benefits. Glen

Wintermute · Aug 5, 2010

I don't know about that. I've had no problems hitting 60mph in my M715 with a stock motor, trans, diff gears, hubs, etc... I know I wouldn't want to drive at 60 for any kind of distance, but in my case, the transfer case didn't even get really hot like most peoples did. Now, there may be some substantial differences between the M715 and the M725 due to the additional weight and the even less aerodynamic body of the M725, however, I don't think it would be impossible to get it to 65mph on the stock engine.

At 60mph right now with the tires I have on my truck, I'm around 3342-3350 RPMs. This definitely isn't the best thing in the world to do to the engine, but it does do it and it's below the max for the engine. I definitely wouldn't see the truck doing this up a heck of a hill though. I would think that the gearing ratio changes when on a relatively flat surface would actually have the truck moving pretty well at around 65mph (although it wouldn't get there really fast, and without upgraded brakes, it's definitely not going to stop from that speed really quickly).

--Wintermute

91W350 · Aug 5, 2010

I had four M715s, two of which would run 60 if you stayed in the carburetor long enough. Two of which would not go over 45 without a tow. One of those was an ex Forestry truck with less than 7,000 miles on it. It was just flat gutless. Anyway, we are asking an awful lot of an engine with a connecting rod as long as a man's forearm and as big around as his thumb. It was a very poor design as far as rpm goes.... even worse than a 6.2 diesel.

Most rural fire trucks around here went with Chevrolet sixes or small block V8s. They could not keep the original engines alive at the speeds and loads they were trying to run.

They are a neat truck and can do a tremendous amount of work and my favorite as far as appearance goes among military trucks. Speed is just not their game.

Wintermute · Aug 5, 2010

I don't disagree that they weren't built for speed to begin with

I just think they can get there with the right modifications.

On the other hand, I got really lucky with my engine, it's got pretty much perfect compression on all cylinders (140-155 across all six of them). After tuning everything up correctly (ignition, timing, rocker clearance, etc... etc... etc...), the engine is running like a champ and it does quite well.

I think with at minimum, changing out the differential gears, carburetor to a 2bbl Holley 230, electronic points in the distributor, NP200 swap to NP205 and putting selectros on the front hubs, I should be able to hit 65 on the highway without a problem on the stock T98 transmission.

It also helps that I don't pay anyone else to do any work, so I save quite a bit on labor costs

--Wintermute

popacom · Aug 9, 2010

The problem I always encountered with the stock motor was ..............................................

..............IMEO "IT'S SCRAPMETAL".........................

I have owned no less than 25 M-715 series truck everyting in the series and all of them had
one thing in common "TO MUCHROTATING MASS IN THE CRANKSHAFT " for the amount of
bearing surface,not to mention a weak oiling configuration!!!! Do a little research and you will find what I am saying is true. Kaiser Jeep knew that motor was substandard (Note; they used it in their civy Gladiators a couple of years.....MANY ISSUES ) then the Govt. put out
a requirement for a Modified COTS replacement for the M-37. Jeep,Chevy were the top contenders and Jeep only won out because the Jeep was half as expensive! Not long after Jeep started supplying M-715's did the Govt. figure out that Jeep had knowledge of these issues when they dumped them in the trucks for the Govt contract. This eventually cost
Kaiser not only the contract but they had to sell out to AMC under financial trouble that was
a direct result of their dealings on the 715's.
Out of all the M-715's I have had over the years I only left a couple running the original engine
and that was just because they were very low mileage trucks near mint condition and I was
not going to be driving them. If you are going to keep your truck and want to use it as a daily driver I would suggest you look at re-powering it with a more modern engine/trany/transfer.
The best stock gas engine I ever put in one was a 300 cid Ford inline six with a Ford
bulldog tranny,that combo worked exceptionally well. I also would re-gear to the 4:56's
and put some taller tires on as well. I think the 11.00-16 LAV Michelen's would be my choice
even though some say they will not run correctly on the stock rims I never had any issues with them. And if you want to keep the NDT military look ,try a set of 11.00-18 Gama Goat tires on adapters(WHICH ARE AVAILABLE) all this said in the end game you will find that what I have told you is factual and you will eventually admit defeat trying to make the stock engine live a long life at 65 MPH> That's my two pennies worth and I will yield the floor to
the Senator from Tenn. who will undoubtedly dispute all I have said here....LOL :jumpin:

popacom /BILL in Ky.

Wintermute · Aug 9, 2010

so very very angry rofl

popacom · Aug 9, 2010

NOT ANGRY.....................JUST EDUCATED>>>School of Blown Up M-715's

"LOVE THE TRUCKS"
~HATE~
"THE ENGINES......"
popacom / BILL in KY. :driver:

91W350 · Aug 9, 2010

Wintermute said:
Okay, I decided to run some calculations:

The high gear for the NP200 is the same as the NP205, however, there is a difference in noise (NP200=straight cut gears, NP205 = helical gears) and in heat/RPM tolerance due to the difference in gear cut and bearings between the two. Here's some calculations to look at:

Stock configuration transfer case, transmission and diff gears:
Tire diameter = 35.4 (Power King Super Traction NR-50 tires)
Transmission final gear ratio = 1 (T98 stock transmission)
Transfer case high gear ratio = 1
Axle ratio = 5.87
RPMs @ 65mph = ~3621 (motor not happy)
RPMs @ 75mph = ~4178 (motor blown up)

Stock configuration transfer case and transmission but switched to 4.56 diff gears:
Tire diameter = 35.4 (Power King Super Traction NR-50 tires)
Transmission final gear ratio = 1 (T98 stock transmission)
Transfer case high gear ratio = 1
Axle ratio = 4.56
RPMs @ 65mph = ~2813 (motor alright)
RPMs @ 75mph = ~3246 (sort of doable)

Transmission changed out to an NV4500 (again, still don't know if this is possible) but with the stock 5.87 diff gears:
Tire diameter = 35.4 (Power King Super Traction NR-50 tires)
Transmission final gear ratio = .73 (NV4500 5 speed transmission)
Transfer case high gear ratio = 1
Axle ratio = 5.87
RPMs @ 65mph = ~2643 (motor alright)
RPMs @ 75mph = ~3050 (should be okay?)

Transmission changed out to an NV4500 (still trying to figure out if this is possible) and with 4.56 diff gears:
Tire diameter = 35.4 (Power King Super Traction NR-50 tires)
Transmission final gear ratio = .73 (NV4500 5 speed transmission)
Transfer case high gear ratio = 1
Axle ratio = 4.56
RPMs @ 65mph = ~2053 (motor alright)
RPMs @ 75mph = ~2369 (motor alright)

Hope that helps you out. I'm thinking that the NV4500 is a great idea if it's possible to do. I'm already going to get the diff gears swapped out, but that will have to wait for the money to do it. I'm going to do all of the research I can to find out if I can get an NV4500 to mate up to the stock motor. It would be nice to be able to cruise at 75mph if necessary in my truck. At minimum, the diff gears will get me 65mph and with an NP205 and selectros, the truck should be happy at that speed. If there is a way to mate an NV4500 transmission to the stock engine, it looks like I'll have no problems hitting 75mph.

--Wintermute

You spent a lot of time there, but all of your rpms are higher than the stock setup with 35.4 inch tires at 45 mph, which is about all they should run and that is really too hard for extended distances. I get 2507 at 45 mph for the stock setup. Those who are really in the know recognize that is about as hard as the engine should be run for an extended period. Most of the trucks I owned or were around has speed limit 45 painted on the inside of the driver's door. I suspect after the motor pool was swarmed with blown engines.

The stroke is long and the bore is small, they had a fragile rod that was prone to distortion and once they distorted, the bearings failed quickly.

Most people recognize the small block Chevy as a 3,000 cruising engine, which brings the ground speed up to 55 mph. Stepping up to a 38" tire, which is not as easy as it sounds, still runs you over 3,000 at 60.

Do not get me wrong, I love the trucks, I just hate to see people misled into thinking one can be easily adapted to cruise at 65-70, or even 60 for that matter. The military found out they would not survive, fire departments and forestry units across the country found out they would not survive, yet we are here talking speed.

Along this line, has anybody actually been able to buy a higher gear set for the stock axles? They may be a Dana 60 and Dana 70, but they have different dimensions than everything else. Just try buy about anything attached to that axle, seals, brake cylinders, brake drums, even brake shoes. I had to take my old shoes in and have them relined as there are none to be bought. Glen

Wintermute · Aug 9, 2010

Educated here as well...school of building a heck of a lot of old engines up from blown to blueprint and fast. But...figured the all red blurb with the giant caps meant angry. Giant colored lettering and all caps just isn't my thing. Specifications, measurements, noting items like it having four main bearings instead of 7 (which the IKA Torina revision of it has) would be more useful. Clarifying that the rocker arm retaining system isn't the best in the world would also help, as would noting that the military model did have some refinements to the top end oiling system which promoted better oiling of the rockers and cam over the prior civvy models.

Just saying, simple text discussing the details and reasoning is better off than giant colored letters which distract from communicating the statement and it's supporting data.

--Wintermute

Wintermute · Aug 9, 2010

91W350 said:
You spent a lot of time there, but all of your rpms are higher than the stock setup with 35.4 inch tires at 45 mph, which is about all they should run and that is really too hard for extended distances.

If the NV4500 is possible (or another alternate transmission with OD), then the RPM's for 65mph would be quite happy for the engine at only around 2053rpm.

91W350 said:
Do not get me wrong, I love the trucks, I just hate to see people misled into thinking one can be easily adapted to cruise at 65-70, or even 60 for that matter. The military found out they would not survive, fire departments and forestry units across the country found out they would not survive, yet we are here talking speed.

I didn't say anywhere in there it would be cheap...I hope I didn't imply it. It would cost to get this motor geared to that level (the diff gearing changes are somewhere around $800 a piece, and just getting the NV4500 would be in the couple of grand range from what I've seen...that doesn't even begin to account for installing it). The weak connecting rods are something that could be addressed (while at the same time helping the rotating mass problem) by replacing them with a custom made H-beam set and possibly lighter weight pistons while you're at it. Yet again, that's expensive unless you machine your own. The low number of main bearings is something you really aren't going to be able to address without replacing the engine or replacing the block with the updated block from the Argentinian Torino which is an engine swap no matter how you look at it. With four main bearings, more rigid and lighter connecting rods and lighter pistons, you can probably keep the mains alive.

But that will all be very expensive unless you are machining your own components.

91W350 said:
Along this line, has anybody actually been able to buy a higher gear set for the stock axles? They may be a Dana 60 and Dana 70, but they have different dimensions than everything else. Just try buy about anything attached to that axle, seals, brake cylinders, brake drums, even brake shoes. I had to take my old shoes in and have them relined as there are none to be bought. Glen

You can get replacements, people have done it...price shown above. They are expensive, and that doesn't count labor on getting them installed if you aren't doing it yourself.

--Wintermute

91W350 · Aug 9, 2010

The last one I converted to a V8 ran great, the best running M715 I had ever driven. The clutch bearing went out and the clutch was shot. After searching for a while, I got frustrated and went V8.

They really work nice with a 305/327/350 Chevy, they are geared so deep, you really do not need a lot of power. My last one was a 305.

I have read about the hot transfer case issue. I ran Mobil One or Amz Oil in mine. The cases were warm, but it was lugging a lot of weight down the road.

Neat trucks, one day I will wish I had restored one and set it back. Glen

popacom · Aug 9, 2010

All I am saying in small print since you prefer it .............you can polish a Turd.........but when you are done ,you still have a Turd. Repower it and you will enjoy the truck more . This is not based on Motor science it is based on my personal experiences with a Fine truck with a less than desirable
engine. I was not trying to offend you,just share my opinions if that is un wanted OK .
Thank you for your time and consideration,and good luck whichever direction you take YOUR truck
cause it is YOUR decision and right to have it the way it suites YOU. :mrgreen:

popacom / BILL in Ky.

M725 gear ratio

New member

Member

Member

Active member

New member

Member

New member

New member

Well-known member

New member

Well-known member

New member

Member

New member

Member

Well-known member

New member

New member

Well-known member

Member