Changing Rear-End Gears

[bigray]

Hello All,

Wanted to check to see if anyone had any luck changing the rear-end gears in their clipper. My brother said that I can change the gears to get a little better gas milage.

I just bought my 1979 party model and I get about 6 miles/gal.

Please let me know!!

[uncleleroy]

BigRay,  I'm in the middle of doing the ring/pinion change on my '78.  Gears are available at salvage yards, or can be bought new from various suppliers.  Your rig probably has 4.10 gears in it.  I'm planning to go to 3.54, but would not recommend going that high if I planned to do any towing.  I'm assuming you have a Dodge chassis with a Dana 70 rear axle.  Used gears cost from $100 to 200 and new ones about 2x that.  Check the tech forum for more opinions on this idea.   uncleleroy

[bigray]

thanks uncleroy,

I don't plan on towing anything. I have a chevy chasis with 400 engine. I spent the weekend changing the passenger side manafold. Talking about tight quarters.

I'm trying to improve my gas milage. Please let me know how your new gears work out.

BigRay

[uncleleroy]

BigRay - I just got back from Portland where I had 3.54 gears and new bearings installed.  The job cost $400.00 including all parts and labor. (These were used gears I had purchased earlier at at a salvage yard.)
  Performance is very similar to what it was with the old gears (4.10), but engine rpm at 60 mph dropped from 3,000 to about 2,400.  I'll let you know how this translates into fuel mileage improvement, if indeed it does at all.  It did seem more relaxing driving with engine speed at a more reasonable level.  later, uncleleroy

[Plante]

uncleleroy - we'll be at Ft. Stevens the weekend of the 23rd, 24th, 25th if you want to meet and swap Clipper stories.

[uncleleroy]

Gerry & Donna -
    That would be great!  We're planning a little trip to Brookings early in the week, but will be back by then.  We'll come out and look you up.  uncleleroy

[mollerus]

I've always been skeptical about changing the differential gear ratio to improve mileage or speed without increasing horsepower.  Lowering the rpm is certainly desirable for engine wear but 3000 rpm at 60 mph. is not mechanically excessive and is probably in the better torque curve region of the 440 engines.  A shorter stroke Chevrolet 400 may be another thing. Despite the torque converters on our automatic transmissions, I think a driver would have some loss of start up and passing acceleration (yes, I have passed VW's!) and would have to shift down the transmission sooner, resulting in slowing down more on steep grades.  In the Northwest this could be a frequent to a near constant problem.  Frequent loading up the engine is not good.  You run a richer fuel/air mix resulting in a higher heat output which may cause overheating and can accelerate engine wear, particularly burning exhaust valves.  A better although a far more expensive solution might be to install a gear splitter or a Mitchell overdrive behind your transmission.  Then you can run in overdrive on the flat land and easily go back to your 4:10 gears on the hills. I can't imagine at best more than a 5-10% improvement in gas economy with changing the gear ratio at considerable expense and I can buy a lot of gas for that.  All this said, I hope uncleleroy will give this forum some follow up on how his conversion works out and if time proves it to be a good modification.

[bigray]

Hi folks,

I drove my Clipper on our first trip from Houston Texas to Atlanta. I had a few mechanical problems along the way but I'm still impressed with my Clipper.

When I left Atlanta the rig sounded like it was running out of gas in 3rd gear and on hills. I replaced the fuel pump when I got to my brother's in Alabama.

Unfortunately my waterpump went out about 4 hours from home. I purchase Good Sam Roadside before we left but the repair bill was $400.00 (ouchh).

At any rate, my brother and I started to change the gears by dismantling everything. On the way to pickup the new gears, we stopped to talk to one of his buddies who owns a speed shop. He advised us not to change the gears as it would only cause the engine to load up just as Bob stated. He said that we should go with a overdrive transmission.

I don't have the money to do that right now, especially after my waterpump repairs.

Bob, I heard about a gear splitter like Gear Vendor but have not heard anything about the Mitchell overdrive. Tell me more!! Do you have one installed or know of anyone that has one on their Clipper? Let me know.

I appreciate the advice from you guys.

[uncleleroy]

Well, I've driven the Clipper with the 3.54 gears for about 300 miles now.  The good news is that it is more comfortable and quieter to drive, and if anything it performs better than with the 4.10 gears.  My information indicates that the later 440's produced max torque at around 2400, and that's squarely where I'm running at 60 mph.  It's true that the tranny uses second gear a little longer when starting from zero, but at no time have I had any indication of an over-stressed engine.
I don't have an answer on the fuel economy thing yet.  My driving has not been typical, in that there have been no stretches of steady driving for more than about 20 miles, and lots of cold starts.  I haven't got my speedometer/odometer calibrated to the new gears yet, either.  So far, I've spent $525.00 on this "experiment".  Granted, that's quite a lot of gas.   uncleleroy

[uncleleroy]

One more update on the gear ratio change.  I drove from Astoria to Portland and back yesterday.  Tried to keep as near straight - up 60 mph as possible.  Used 22.2 gallons of fuel in 206 miles.  That's about a 12% increase over what I was getting with the 4.10 rear end.  If that holds up it will take lots of miles to recover costs.  I climbed the hills at 55 mph without shifting down or using full throttle, so still have plenty of power.  Would I do it again knowing what I know now??  Guess I'd like some more time on that one.  uncleleroy

[DCI]

http://www.gearvendors.com/

Here is another option that would give you the best of both worlds. It is kind of pricey but if you plan on keeping the motorhome for a long time it would probably work out to be $$ in you pocket.

Tom

[rjm]

how about an allison 6 speed and 3.07's 

rjm