Adding overdrive, cost vs. benefits

[Derrek]

Sent: 3/13/2003 12:42 PM

There have been several discussions on this board about changing gear ratios to improve mileage. And many other discussions on mileage in general and other ways to improve mileage. I have looked at several internet sites that offer aftermarket overdrive units that cost about $3000.00 to $3500.00 installed. I looked at an 88 Winnebago Super Chief on a lot the other day that already had a Gear Vendor system installed. While looking through a stack of reciepts in this unit I found one showing that this system had been installed by a truck repair shop in Indiana for $1700.00. Much more realistic than $3500.00. I would like to know if anyone has ever used one of these systems or knows someone who has. I am curious how much of a difference it makes in fuel mileage and overall performance.

Derrek 

[neil]

Sent: 3/13/2003 1:49 PM

You may be able to get an idea by calculating what the rpm reduction would be at cruising speed on level surface.  Driving in traffic would probably be the same.

[Boat Nut]

Sent: 3/13/2003 3:45 PM

Hi Derrek,
I do not have any idea whether an overdrive will help our mileage, but I just ran some numbers. If it were to give you an improvement of 2 miles per gallon. Think about how long it will take to recover your investment. My calculations say 40,000 miles to recover the $1,700, or 80,000 miles for the $3,500. Do you intend to drive it that far? I don't.

Chuck

[Derrek]

Sent: 3/13/2003 6:47 PM

You bring up a very good point Chuck.....You would have a very hard time recouping your money in just the fuel mileage alone. I believe that it was installed on the unit I was looking at for towing purposes. In a case such as that, if it were to keep you from burning up a tranny you would be a little closer to justifying the expense. Thanks for the responses. That is why I love this site....lots of good advice and opinions and many different viewpoints.

[MKoontz]

Sent: 2/18/2004 9:46 PM

I have a Dual-Range [extra] "underdrive" on my '85 Elandan (34').  But it's for towing or mountains. So the standard tranny and drive is the "highway" gear.

Oh BTW, on a good day I get 10 MPG. 

PS- I wouldn't buy it again. The extra weight, vibration, and oil leak isn't worth it. But then again, I don't tow.

[UluzYarx]

Sent: 2/20/2004 11:13 AM

Your Achilles heel is your torque convertor.

Your solution is either a post 92? torque convertor and box (with overdrive) or a complete post 92? lump and rad.

Or is originality important?

[tymhoff]

Sent: 2/23/2004 11:02 AM

You have to think of fuel mileage improvement in more than just monetary terms. 2 miles per gallon, times 60 gallons(or however much your perticular rig carries), will get you 120 miles further. That may be enough to get you where you're going without having to spend that extra half hour filling up on the way there. We all have noticed that the single gas station in that little town before the big one costs a lot more per gallon!

On a trip to Superstition OHV near El Centro, CA, I got 8-9 miles per gallon at an average of 62 mph through a mixture of hills and flat land coming from Chula Vista, CA. I have no overdrive or headers, just a good tune up and a recent carb rebuild. 22 foot Indian with a 440 V8.

[77surveyor]

Sent: 4/20/2004 11:29 PM

so this is what i was told from an old timer , find an old late 60s early 70s internationial loadstar 1600 or 1800 series and that they have a seperate unit that has an overdrive an underdrive and a direct drive position after fabercating mounting brackets it would frame mount inline with the driveshaft and after the transmission hooked up with a cable shifter . when going down the highway you would put the mh trans in nuetral and use the overdrive portion of the unit its supposed to ad another 5 to 8 miles to the gallon anybody have any input on this?   Richard

[77surveyor]

Sent: 4/26/2004 11:04 PM

well im going to give it a try when i called the local truck salvage yard he knew what i was useing it for before i even told him ... i have a m500 chassis so i have plenty of room to play with  he wants $500 for the setup so i guess its pay the money and take the chance ill keep it posted 

richard

[discoverer]

Sent: 7/12/2004 3:25 PM

BEWARE......putting in a OD unit or OD trans in a vehicle that the motor is weak - it will actually COST you mpg. ALSO depends on your driving conditions.......if most of your driving is on-the-flat - the OD will help mpg BUT if you do alot of driving in the mountains - your costs vs savings probably would be very small.

[Jlogue88]

Sent: 7/12/2004 9:01 PM

Has any one out there done it yet? if so how is it. I am not all that concerned about gas mileage but more about the differnce in engine noise.

I am about to swap the 727 for a 518 witch is the same thing only with a overdrive

[HeavyHaulTrucker]

Sent: 7/20/2004 2:24 AM

You know, I completely spaced this when I first read this discussion.

Why couldn't you simply change the rear-end to a 2-speed unit with a lower ratio for the second speed -- something like a 4.11/4.88 ratio?  Since all of our Winnies were built on truck chassis', there was a option for a 2-speed rear end.  That would seem to be the most cost effective method here -- a good take-out 2-speed rear would only cost you a couple of hundred, and you can change it out yourself with a couple of jackstands to support the frame.  Even if you have a shop do it, there would be no special fabrication needed -- it would be a simple bolt-up swap.  If you get a newer rear end, then the control would be electric -- just run one control cable to the cabin, and the switch can mount wherever convenient.  You shift it by simply decreasing throttle and flipping the switch while in high gear.

I agree that a weak or smaller engine would not give you the desired results.  In fact, I think (IMHO) that this would only be really cost effective for the 413, 440 & 454 engines -- but if you can get your engine speed down to 2000-2200 rpm at cruising speed on flat land with no headwind, imagine the increase in fuel efficiency!

John

[HeavyHaulTrucker]

Sent: 7/20/2004 2:43 AM

77surveyor wrote:

so this is what i was told from an old timer , find an old late 60s early 70s internationial loadstar 1600 or 1800 series and that they have a seperate unit that has an overdrive an underdrive and a direct drive position after fabercating mounting brackets it would frame mount inline with the would put the mh trans in nuetral and use the overdrive portion of the unit its supposed to ad another 5 to 8 miles to the gallon anybody have any input on this?

This is called an auxillary transmission; they usually come in 2 or 3-speed models.  They have a under drive, a direct drive, and (in the 3-speed unit) an overdrive gear.  They used to be used extensively on "big trucks", and are still mandatory if you gross over 150000 lbs GCW.  They are a pain to install; you have to fabricate mounting brackets, have two new driveshafts built & balanced, and install the shift linkage (which requires a degree in engineering to design   ).  They are used by leaving the MH transmission in "drive" and simply shifting the auxillary to the range you want to use; the MH trans shifts as normal, no matter what range you select -- you basically have 3 overlapping sets of gears to use.

I know a guy who is leased to Allied Van Lines; he ordered out his last truck with an Allison HD automatic transmission and a 3-speed aux box behind it.  He uses the underdrive range on the aux for city driving, and the direct & overdrive ranges for highway cruising; he get almost 10 mpg out on the open road -- and this is with a 475 hp Cat diesel for power!  My Kenworth had a 475, but the best mileage I EVER got was 6.2 mpg.

John

[denisondc]

Sent: 7/20/2004 6:44 AM

I dont think it will matter much how slow you get the driveshaft to turn if you still have the A727 tranny; because you will increase loss in the torque converter. With my driveshaft turning about 2600 rpm - about 57 mph- my engine is turning about 2800 rpm on level ground at a steady speed. With the RV -and driveshaft- motionless I can easily get the engine to turn 1500 rpm without having the gas pedal very far down. I would consider a manual transmission necessary if I were going to try an under-drive auxilliary.
I look under most every motorhome I can if its on a vintage Dodge chassis, looking for a two speed differential axle or an auxilliary hanging off the end of the transmission. Especially in salvage yards. But I havent seen one yet.
I enjoy manual transmissions, until I get into the Dallas Ft worth traffic!!!

And I dont plan to change my driveline, or to change my wheels or tires -- being happy with the whole design as it is.

[discoverer]

Sent: 7/20/2004 11:45 AM

Putting in the MHs - those two speed rear ends - like are in the BIG dump-trucks ?...... NAY - I say !! .... because their gearing is TERRIBLE ! .....something like 7 something - on the low side and low 5 or high 4-something on the HIGH side.

[HeavyHaulTrucker]

Sent: 7/21/2004 10:16 AM

No, the rear ends for the smaller trucks (10-14000 GVW rating) are no bigger than the one you have now; they just have a larger differential housing, and the high side ratios are still in the 4's.  Anyway, it was just a thought.

John

[Jlogue88]

Sent: 7/21/2004 9:20 PM

So to ansewer my own question, nobody here has done it.  But for poeple who have never done an overdrive you sure like to talk about it. to put my opinion out here I belive that with the engine running 30% solwer that means 30% quiter and a 30% increase in fuel economy and also 30% less ware on the engine.

Also some of you that atempted to figure out the time nessicery for a return on your money. You forgot to factor in that gas is only going to get more expensive. So that figure of 40,000 to recover will turn to 20 or 15 quiker than you think.

p.s. those with helpful messages - thank you!

[discoverer]

Sent: 7/22/2004 2:26 PM

those 30% numbers sound impressive BUT you can only get those #'s IF your motor is healthy. IF you put an OD of any # behind a weak motor - it will actually COST you MPG since you will always be "into" the accelerator-pump of the carb.

[Jlogue88]

Sent: 7/26/2004 10:05 PM

When towing or going up hills you can turn the overdrive off

[Moondisc1]

Sent: 7/29/2004 9:35 PM

I'm going to put a Gear Vendors OD in mine as soon as I find a bargin (just missed one for $375)
Don't give a hoot about MPG. All I know is 3200rpm's at 55mph is way too high for a 440!
Mopar OD trannys aren't the strongest, so I'm not even considering that route.
Never thought about a 2 speed rear.
That would be an option if you could find the right width & gear ratio.

[discoverer]

Sent: 8/1/2004 7:24 PM

Hay Moon..... DON'T give a hoot about mpg? ...what is your mpg now?...14?  3200 @55mph?.....what is your combo to get that? Mopar OD trannies not strong?......the ones behind a Cummins diesel? 2 speed rear end?..... WHAT choice ratios ARE available?

[Starbuck267491]

Sent: 8/2/2004 4:36 AM

Hi I have read with interest all comments of this subject of over drive units. I have been concidering changing the rear end ratio in my 1979 Dodge Minnie as the 440cu runs at 3000rpm @ 60mph. here in the UK. Running petrol 12pmg on LPG 10mpg Uk Gallons. The diff ratio of 4.56-1 in my 20ft unit is the same as used in the larger RV's even in a 36ft unit. as my doge is quite a light weight I feel it is under geared.
I do find when overtaking or on hills the ratio is so low geared i can pass most things with ease but at 80 mph is crazy.
It would appear I have a Dual wheel Spicer HD 60, 10bolt cover, full floating 3.3/8" axle.
so if anybody has a buck shee pinion & gear set to make my diff ratio somthing like 3.8-1 or 3.5-1 please contact this (at UK Petrol Prices) poor english man   

Kind regards Joe.

[Moondisc1]

Sent: 8/2/2004 9:29 AM

"Hay Moon..... DON'T give a hoot about mpg? ...what is your mpg now?...14?"

Guess i worded that wrong. I do care about mileage. But I'm more concered with wear on a 440 turning that kind of R's.
Got 7mpg when I bought it with the Thermo Quad on it, getting 9 with the Performer intake & carb.

"3200 @55mph?.....what is your combo to get that?"

Well, the owners manual says it's got 4:13 gears in it, but with 17.5 tires I think they're more like 4:88 or 5:13's.

"Mopar OD trannies not strong?......the ones behind a Cummins diesel?"

Exactly my point.
Everyone I know with a Cummins that tows with it had trannys go at 25,000-40,000 miles.
One landscaper I know got a new truck under the NY lemon law because he had so much tranny trouble.
Even when they don't tow, I've heard of trannys taking a dump between 50,000-70,000.
Since I've got a 10000# motorhome, and at times will be towing 6000# worth of trailer, I'm not about to take that chance.

"2 speed rear end?..... WHAT choice ratios ARE available?"

Don't have a clue. But I'm going to check it out. 

[Jupp318]

Sent: 8/2/2004 12:00 PM

Guys,

  If we are thinking of changing ratio's could you clear one thing up for me, is the Spicer HD 60 rear-end also known as a Dana 60?.

  If so changing ratio's is easy, just pop along to the Richmond gears section at Summit racing and get yourself a new set of gears along with the installation kit.

Assuming you are adept with a Dial Test Indicator (DTI) gauge and can make yourself the required "Spreader Tool" to open up the housing, the rest is a breeze.

Cheers Ian...

[enigma960080]

Sent: 8/4/2004 8:14 PM

someone wrote

A friend of mine  bought a 93 Dodge 1500 from  my brother (GRHS)  it  had  about 80K miles on it and  it went out.   Chrysler overdrive trannys, much like life,  is a crapshoot....

you  figure out the odds

Phil