GEAR RATIO AND TRANSMISSION IN 1995 U300 AND U280 GV

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GEAR RATIO AND TRANSMISSION IN 1995 U300 AND U280 GV

February 16, 2013, 11:15:32 am

I have been doing extensive research on the GV coaches and have my eye on 3 units so being my background in the mechanical side of things I want to know all I can about these coaches, The most I am finding in the forum discussions on towing mpg and power and capacities etc

I haven't read hardly anything on the rear axle gear ratios and the model of the rear drive axle. So my point here is that I found a 1995 U280 GV which has a cummins 8.3 coupled to a MD3060R allison. Which if you dig farther has a 5.13 ratio, Ok now it has a GVW of 28,000 which is interesting because it doesn't say anything about what brand axle ( rockwell or eaton) and the size ( as in load capacity rating)

ok that all being said I decided to look up a 1995 U300 GV, which says it has a 3178 cat and a HD4060 allison with a drive ratio of 3.91 and a gvw of 30,000, The cat has a torque curve from1700 to 2200 rpm which would explain the higher axle ratio versus the cummins is 2200 to 2800 torque range. Also the cat is rated at 375 hp versus the cummins at 300. So I went digging farther on the input hp and torque ratings on the 4060 versus the 3060. The maximum rating on the 3060 is right at 375 hp and 1100 ft of torque and the 4060 is 525 hp and 1600 torque, the ratios in the gears between the 2 trannys is relatively close though there is a noticeable gap on the governed speed which stands to reason , I was surprised that the 3060R comes standard with a retarder where the 4060 isn't or I should properly state U280 versus U300 for that particular year,

I guess what I really want to find out now is what rear drive axle is on these 2 coach models as I really want to know the carrying capability better and why they are rated this way. The reason is I don't think that 2000 lb gvw between to similar coaches is (and I say this carefully, accurate) lets just say my inquiring mind wants to know, that being said tire rated load capacity is pretty much what you would want to maintain anyways for safety reasons, ok enough rambling I have my big boy pants on so hit me with whatever it is you all wanna say, There is years of experience and many thousand of documented miles on this forum,

Adam

Re: GEAR RATIO AND TRANSMISSION IN 1995 U300 AND U280 GV

Reply #1 – February 16, 2013, 11:43:34 am

Adam,

Welcome.

I am not an engineer, and definitely not smart. SO here is my 2 cents, and it isn't e en worth that...

I recall that best efficiency for my 450 (ISL) Cummings is at Max torque, which is at the lower end of the rpm range (my recollection it is 1500 RPM, but may be 1300 RPM. In any event I suspect that most of these coaches are geared so that they run 60 MPH on level ground when at max torque (best efficiency) rpm. Maybe someone who knows more than me (most everyone on the forum!) can weigh in.

My point is that weigh is probably not driving gearing as much as desire for maximum fuel mileage at typical highway speeds.

BTW, check out this link

FMCA Presentation - Cummins Engines - and then open the .pdf presentation by clicking on the "View FMCA presentation" in red :

Great graph that shows the power requirements to move the coach - net - Every 10 MPH over 60 MPH is a 1 MPG reduction due to Aero drag - Not surprising

Or check out this link - it is about Heavy Truck MPG, but speaks to the different factors effecting MPG including gearing, and how to select final gear ratios for Over the road applications.

http://cumminsengines.com/uploads/docs/cummins_secrets_of_better_fuel_economy.pdf

You will love any Foretravel you get, enjoy the search.

Re: GEAR RATIO AND TRANSMISSION IN 1995 U300 AND U280 GV

Reply #2 – February 16, 2013, 12:00:46 pm

I don't know if this helps to make sense or not but a chassis engineer at Country Coach told me they select a rear end ratio such that the maximum speed is about 85 mph. I don't know if Foretravel does the same though. I've noticed the bigger the engine the lower the maximum RPM - generally.

Re: GEAR RATIO AND TRANSMISSION IN 1995 U300 AND U280 GV

Reply #3 – February 16, 2013, 12:05:22 pm

The 3176 would be an instant quick choice if it were going to be mine. Plus it can have a jake brake added if not already on it.

The 8.3 will not be as fun in the mountains.

No replacement for more power. Used to turn up the 8.3's to add much needed power in the hills.

The cat will bring a grin on your face when you drive it. Move quick as the cat is rare and desirable.

Re: GEAR RATIO AND TRANSMISSION IN 1995 U300 AND U280 GV

Reply #4 – February 16, 2013, 12:21:59 pm

I would most likely put a different fuel plate and adjust the ar valve and the exhaust resonator if I end up with the 8.3. I cant do any huge gains as not sure how much the 3060 will handle without problems. I know from experience that I could get 450 hp out of it without spending a lot, the inline fuel pump on this eng is the same on the 5.9 and a fuel plate is 150 bucks , If this is done a pyro install is a must also. I would prefer the cat but they seem very scarce. I am finding all this very interesting and my GF says I have way to much time on my hands, my blue fermentation barrels only require me to run my water purification device every 4 days,

Adam

Re: GEAR RATIO AND TRANSMISSION IN 1995 U300 AND U280 GV

Reply #5 – February 16, 2013, 02:21:03 pm

Quote from: Caflashbob – February 16, 2013, 12:05:22 pm

The cat will bring a grin on your face when you drive it.

Unless you have driven a U300 with the CAT 3176b you'll never know what you are missing.

My 3176b is 350 hp, 1350 ft lbs., 10-11 mpg fully loaded and towing 3800#

1995 U300SE 30,000 GVW
CAT 3176b with full jake brake
Allison HD4060
Rockwell rear axle # RS 21 145 NFCF254
3:91 ratio

Re: GEAR RATIO AND TRANSMISSION IN 1995 U300 AND U280 GV

Reply #6 – February 16, 2013, 02:42:01 pm

I'm with you on the U300 Rick, awesome power my setup is 3176B with HD4060R retarder surely gives confidence when slowing down. I drove out from California to South Carolina 3 weeks ago on "I20 " towing loaded trailer 5K lbs and averaged 9.2 mpg, pretty much stayed at 62-66 whole trip.

Quote from: Rick – February 16, 2013, 02:21:03 pm

Unless you have driven a U300 with the CAT 3176b you'll never know what you are missing.
My 3176b is 350 hp, 1350 ft lbs., 10-11 mpg fully loaded and towing 3800#

1995 U300SE 30,000 GVW
CAT 3176b with full jake brake
Allison HD4060
Rockwell rear axle # RS 21 145 NFCF254
3:91 ratio

Re: GEAR RATIO AND TRANSMISSION IN 1995 U300 AND U280 GV

Reply #7 – February 16, 2013, 03:08:54 pm

Does anybody know what the average milage is on the 8.3 ? sounds like the cat averages real close comparable as far as coach with a cat to another same equipped, Also does any body know what the 95 U280 has for a rear axle? I want to express my appreciation for all the input and ideas and advice. A lot of forums I have post on usualy has involved into something I dont want to be associated with. Thank you to all you here.Adam

Re: GEAR RATIO AND TRANSMISSION IN 1995 U300 AND U280 GV

Reply #8 – February 16, 2013, 06:55:06 pm

Mine averages right at about 8mpg not towing anything and running 70 to72mph. My axle is a rockwell but don't have the manual in front of me so can't tell you the gear ratio.

Re: GEAR RATIO AND TRANSMISSION IN 1995 U300 AND U280 GV

Reply #9 – February 16, 2013, 07:20:27 pm

Most Rockwell axles have the ratio stamped on the tag on the axle itself.

Re: GEAR RATIO AND TRANSMISSION IN 1995 U300 AND U280 GV

Reply #10 – February 16, 2013, 07:37:11 pm

Your 'sweet spot' is about 63mph. With my 36 foot U280 I could get 9.5 to 10 mpg reliably pulling a 4300 lb toad (TDI wagon on a Mastertow dolly). This was an MT647 4 speed. No real difference with the 6 speed as they changed the axle ratio to accommodate the double overdrive.

Re: GEAR RATIO AND TRANSMISSION IN 1995 U300 AND U280 GV

Reply #11 – February 16, 2013, 10:11:28 pm

Quote from: kreagsman – February 16, 2013, 11:15:32 am

I guess what I really want to find out now is what rear drive axle is on these 2 coach models as I really want to know the carrying capability better and why they are rated this way. The reason is I don't think that 2000 lb gvw between to similar coaches is (and I say this carefully, accurate) lets just say my inquiring mind wants to know, that being said tire rated load capacity is pretty much what you would want to maintain anyways for safety reasons, ok enough rambling I have my big boy pants on so hit me with whatever it is you all wanna say, There is years of experience and many thousand of documented miles on this forum, Adam

Adam,
Here is a Rockwell spec sheet on the RS21145 Rick has in his 95 300SE :

http://www.todaystrucking.com/images/9441lh4%281%29-1.pdf

You will see in truck service the RS21 145 is rated at 21,000 lbs GVWR on it's back and 55,000 Lbs GCWR train (pulling) capacity so it is not the week link. (the last page in the pdf)

Cheers
Ted

Re: GEAR RATIO AND TRANSMISSION IN 1995 U300 AND U280 GV

Reply #12 – February 17, 2013, 01:58:03 am

Quote from: kreagsman – February 16, 2013, 12:21:59 pm

I would most likely put a different fuel plate and adjust the ar valve and the exhaust resonator if I end up with the 8.3. I cant do any huge gains as not sure how much the 3060 will handle without problems. I know from experience that I could get 450 hp out of it without spending a lot, the inline fuel pump on this eng is the same on the 5.9 and a fuel plate is 150 bucks , If this is done a pyro install is a must also. I would prefer the cat but they seem very scarce. I am finding all this very interesting and my GF says I have way to much time on my hands, my blue fermentation barrels only require me to run my water purification device every 4 days, Adam

The jake on the cat used to be able to operate on the cruise control on marquis. I would not want to be without my retarder after an emergency stop with it was amazing.

Seen and driven a 409hp 8.3 long ago. 60 up 6 percent grades. The cat will go up 6 percent grades at 85 or more.

Foretravel turned up the cats if I remember correctly. Unofficially

Re: GEAR RATIO AND TRANSMISSION IN 1995 U300 AND U280 GV

Reply #13 – February 17, 2013, 12:19:40 pm

Quote from: Caflashbob – February 17, 2013, 01:58:03 am

The cat will go up 6 percent grades at 85 or more.

However the forum does not recommend you drive so recklessly (and illegally), whatever engine you may have.

Re: GEAR RATIO AND TRANSMISSION IN 1995 U300 AND U280 GV

Reply #14 – February 17, 2013, 01:01:30 pm

Despite all the bluster, it all comes down to weight vs torque & HP.
Any med weight RV witha medium engine will always run better than a heavier RV with the same engine.
Try a 30,000 lb RV with 1550 ft lb & 500 HP.
Some talk all about and some actually do it.
Maybe why I laugh so much.
Dave M

Re: GEAR RATIO AND TRANSMISSION IN 1995 U300 AND U280 GV

Reply #15 – February 17, 2013, 01:30:49 pm

Is that what you are driving Dave? We could do a 1/4 mile I might have some competition in my 600hp dodge lol. I like power and wouldnt be happy to struggle up hills, kinda always been that left lane type,

Adam

Re: GEAR RATIO AND TRANSMISSION IN 1995 U300 AND U280 GV

Reply #16 – February 17, 2013, 01:55:30 pm

As I recall, the rear axle ratio is a 3.07 on my '93 U300. Gives a true top end of 82 mph at 2100 rpm. Detroit with a 747 Allison. Like Brett said, there will be a little tag under one of the third member nuts with the ratio on it.

Pierce

Re: GEAR RATIO AND TRANSMISSION IN 1995 U300 AND U280 GV

Reply #17 – February 17, 2013, 02:04:28 pm

You have to remember, Foretravels are engineered in Texas whereof speed is important!

Re: GEAR RATIO AND TRANSMISSION IN 1995 U300 AND U280 GV

Reply #18 – February 17, 2013, 04:43:37 pm

Quote from: PatC – February 17, 2013, 02:04:28 pm

You have to remember, Foretravels are engineered in Texas whereof speed is important!

Just excactly what does Texas and speed have anything to do with each other and pertaining to this post? I have lived in Texas for years till 13 months ago and only then was forced to move north because a family obligation and am quite curious as your comment has no bearing to my original post here. Adam

Re: GEAR RATIO AND TRANSMISSION IN 1995 U300 AND U280 GV

Reply #19 – February 17, 2013, 05:06:25 pm

Hi Adam, As forthe U300 with the 6V-92, everyone I have seen had the 3.08 rear when using the Allisn 4 speed, some had the Built in retarder, a very desirable feature over the Jake setup.
Yes, I have the Cummins ISM450 but was able to talk the Cummins factory shop to make it into the ISM500, so without the EGR, it is a very nice setup, besides, the shop manager said it would be a verfy strong 500 because Cummins never had the 2001 year model as a 500, when they went the 500, it uses a different ECM, a VG Turbo and wiring harness, did have the EGR, so mine is made up with a bypass turbo and a wastegate controller mounted on the intake to sense the boost psi, and it is wired into the ECM with existing wiring harness, then they installed the 500 HP / 1550 ft lb program. This setup euipment wise is from a truck engine, then they installed the 500/1550 program. Yup, it sure made a huge improvement.
I have been a gear head my entire life, so this just had to happen. The good part, it has Cummins blessing and I have the part numbers and the ECM program number thinking maybe one day someone might like to make the change, seems the drawback is the $3,300.00 parts & labor at Cummins.
Agree on going up is where it is, going down I am much slower.
Anyway, glad to see you here with good questions.
Thanks
Dave M

Re: GEAR RATIO AND TRANSMISSION IN 1995 U300 AND U280 GV

Reply #20 – February 17, 2013, 05:16:28 pm

Quote from: Dave M – February 17, 2013, 01:01:30 pm

Despite all the bluster, it all comes down to weight vs torque & HP.
Any med weight RV witha medium engine will always run better than a heavier RV with the same engine.
Try a 30,000 lb RV with 1550 ft lb & 500 HP.
Some talk all about and some actually do it.
Maybe why I laugh so much.
Dave M

Dave I have read a big percentage of your post and always have a lot of respect for a person that has a responce about facts based on real life experience . I think some misunderstand why I posted this post. I am buying a FT that fits into a certain category that I am seeking in a coach for a peticular niche I want to persue in my life. Boondocking, my mobile shop or man cave, transportation for the lady that completes me, I will not ever be dependent on the socialist society that we live in. How fast I get there is irrelevant if I cant do it safely,economically , and have fun doing it. I ask the questions I ask to further my knowledge, hp, torque , gear ratio , weaght to ratio all work together as I am sure most here know. I have now desire to turn a fuel pump up and blow black unburned fuel out the pipe just to say I can. I do know from many different situations that these engines are restricted and if you open there breathing up and give them the proper fuel to air ratio you have a beautiful thing. In my dodge I increased my mileage 8 mpg and my power 225 hp. So that all being said can a person take a 8.3 and pull 12,000 lbs behind and get 10 mpg and maintain 64 mph 80% of the time? Just asking, Adam

Re: GEAR RATIO AND TRANSMISSION IN 1995 U300 AND U280 GV

Reply #21 – February 17, 2013, 05:35:13 pm

Adam, I am certain the 8.3 can handle the 12K tow weight, my first concern would be the hitch on the frame, both weight of tongue plus weight on trailer wheels. I am sure the 8.3 can be turned up, I am not familiar which would be the better method, I would lean toward better turbo first, then add fuel.
Total HP gained etc, unknown by me, I have litle experience with the 8.3 away from generator engines.
Does sound interesting and wish you well, as you have questions, just sing out, some one will try.
Cheers
Dave M

Re: GEAR RATIO AND TRANSMISSION IN 1995 U300 AND U280 GV

Reply #22 – February 17, 2013, 05:35:52 pm

I am actively searching for thr rockwell axle specs on a 95 GV280, The U300 I was looking at turned out to be a U280 when the seller sent me pics of vin plate today. I am now changing my sights on this coach though I think its overpriced, Used 1995 Foretravel M-4000 U280 SE for Sale | RVRegistry.com
I think it will do what I need and has a lot of new parts, I would need to add solar which I would rather do myself then fix a addon that didnt work properly. The owners have health issues so have been talking to there soninlaw and am trying to find out if its the RS 21 145 REAR AXLE, any help would be appreciated. Thanks Adam

Re: GEAR RATIO AND TRANSMISSION IN 1995 U300 AND U280 GV

Reply #23 – February 17, 2013, 05:44:35 pm

OK. Just located the info for the 95 280, 8.3 @ 300 hp 3060R transmission 6 seed, 2 O.D. ratios, axle is 5.13, the should be a real find trailer puller ratio.
Dave

Re: GEAR RATIO AND TRANSMISSION IN 1995 U300 AND U280 GV

Reply #24 – February 17, 2013, 05:51:28 pm

Thanks Dave, the ratio should work good, do you know what gvrw is on it? The U300 uses the RS121 axle which is rated for 21000. Adam

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