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Old 05-02-2006, 09:08 AM   #1
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What If I Replace The Transmission?

We just returned from a 2000 plus mile trip in Chummy. We travelled through the Southern Georgia mountains on several back roads and not many interstates with the exception of getting there and back. Chummy handled great. the engine is more powerful than I would ever expect but he goes only 55 and 60 is tops. I can climb the steepest mountain without loosing speed but not go any faster than 55.
My question is what would be the advantage of replacing my old three speed transmission with a new five speed? I have this new powerful advanced engine mated to the old transmission. Would I be able to travel at a higher speed on the interstates? Would the fuel economy increase? We averaged 15 mpg on the trip. Is it even possible to do do the swap?
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Old 05-02-2006, 09:10 AM   #2
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Probably wiser and cheaper to change the gear ratio in the rear end. Should be a Chevy p-30 rear end and should be lots of parts available.
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Old 05-02-2006, 09:51 AM   #3
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...or a Gear Vendors splitter with OD
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Old 05-02-2006, 10:06 AM   #4
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CK, this is going to sound odd coming from a shadetree mechanic, but here it goes.....

If the MPG isn't too bad, you're not towing anything, not going faster than 55-60 and it's working, don't mess with it. The cost just isn't worth it.

Now if your diesel is not getting good MPG, you plan on towing a big thing behind it or you plan to hit 75mph regularly, climb mountains, then by all means, look at an OD unit firs, then rear gears, then at the trans, but only do it if there is a good reason to crack it open and spend the $$$. I went through a rear gear change and there are just so many things that can go wrong. GM takes the cream of the crop and I've heard many folks who bought great OEM gears complain of sounds from the rear end, etc.

On my Impala SS, in retrospect I don't think I'd have swapped the gears if I had to do it all over again. Of course hind sight is always 20/20.

I guess what I'm tryin' to say is that make sure you're thinkin' of modding for the right reasons. Old trans mated to a new engine isn't a bad thing if the trans is solid and working well.
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Old 05-02-2006, 10:19 AM   #5
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Hi Chaplain Kent--Just a couple of thoughts on speed of a 24-year old motor home chassis. Most of your 2000 mile trip on non interstate roads was probably 55mph max, with plenty of power on the grades. Do you really want to be rolling along the interstate at 65 or 70mph? Do you really want to spend the money for a transmission up grade? If it were me I'd be saving my money for tires, brakes, starter, alternator, and steering component repair. Just a thought.--Frank S
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Old 05-02-2006, 11:21 AM   #6
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Quote:
Originally Posted by Frank S
Hi Chaplain Kent- Do you really want to be rolling along the interstate at 65 or 70mph? Do you really want to spend the money for a transmission up grade? Frank S
There are times when rolling along the interstate ate 55 top speed and at an entrance ramp a yo yo can not decide whether to speed to merge and ends up trying to share my lane or you meet a fellow grandfather doing 50 and you wish to pass him. I am doing the max speed and can only slow down for safety and sometimes the yo yo who can not decide to merge does not understand how long it takes to stop 15, 000 pounds. The ability to speed past the 55 limit would be nice and I think a safety factor in light of the high speeds people drive today.
What is involved in changing the gear box?
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Old 05-02-2006, 11:45 AM   #7
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Chaplain Kent,
Changing the gear ratio in the rear end usually means replacing the ring and pinion gears, although sometimes it is just as well to replace the entire differential. If it's a P-30 differential, there should be lots available.

But an overdrive unit (like Gear Vendors) will give you extra gears and (maybe) better fuel economy.

I would go for the overdrive if costs were similar.
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Old 05-02-2006, 12:03 PM   #8
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hi chaplain kent

so you are enjoying that diesel power it seems.....
that is a great mod for the moho.....
but expensive for most who don't have some help with the costs...
as you did

i've been down the moho mod road.....
improving handling and power is fun just like cars and trucks....

as you are learning most diesels aren't high rpm'ers.....
and that is limiting your top end....

you make no mention of rpms at 55
or shift points you currently have....
these issues matter alot
and help determine trans mod issues...

and while driving 55-60 is noble....
these air moho's cruise soooooo smoothly
that at 55 one is tempted to set the controls
and go make coffee....
or use the head....
so 55 gets 'slow' real fast
and you really would blend better,
moving with traffic at 65-70

the overdrive is your most direct solution....
like the gear vendors set up...which is proven and reliable
this would give you better top end cruise/passing speed
and keep the engine in its sweet spot

this mod could bring you 50-100k of trouble free miles
and extend the life of your current tranny
which has how many miles?
the major reasons/advantage for selecting a new tranny are
breakdowns, a warranty and extending moho useful life
but only if the current one shows signs of failing...
p30 trans are plentiful as rebuilts.....
and the gear vendors unit can still be used....

no your mpgs will not go up.....
in reality mpgs willl go down.....
without getting into the math
most fuel usage is related to cd/frontal area and pushing air...
except for start/stopping and hill climbs....
and the "55" was selected back when based on the sweet spot for economy
sure there was politics.....but mostly it's about wind resistence as speed increases..
issues related to pushing air double (approx) between 55 and 75mph....
so expect your mph to decline to approx 12-13 mpg while driving at 75...
just a guess

so adding an overdrive isn't 'economical' but sure would be nice for longer trips.....

and you will be less tempted to get up from the pilot's chair...
while driving


cheers
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Old 05-02-2006, 12:54 PM   #9
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Ratios

Chaplin - I think 2air is right, we need a baseline. What are your current rear end gears, and speed vs. revs. I think with your torque, it would be easy to change your rear end for a lower numerical gear ratio. It should pull the lower (1st and 2nd) fine. That said, you would get your same acceleration, AND a better top speed with either a four speed or Gear Vendors OD. Personally, 55 would kill me fast.

I had my rear end changed (now THAT sounds funny!) at Randy's Ring and Pinion. I also agree with Silvertwinkie - it's not a shadetree thing. Getting the shims and angles just right in the pumpkin is key to making this work well. That said, Randy's is known for their excellent service - they were also cheaper that good ol Mr. Goodwrench (by about $500!!!). I doubt the dealer would have done such a good job.

Visit www.randysringandpinion.com They have a gear calculator to figure out what your revs would be with various tire size, gears, and and rpms vs. speed.

That tool should help.
Marc
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Old 05-02-2006, 01:01 PM   #10
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2air,
Are you saying that mileage will be worse at a given speed (say 55) with an overdrive than without?

Seems a bit counter intuitive. Without OD the engine turns X RPM at 55 MPH, with the overdrive it will turn, say, .85X RPM at 55 MPH. I don't see why you expect greater fuel consumption at .85X RPM vs X RPM when doing the same amount of work.

Of course if you're doing X RPM with OD (hence going faster and doing more work) you are likely to use more fuel than doing X RPM at 55 MPH.
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Old 05-02-2006, 02:05 PM   #11
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I read 2Airs post as swapping gears not getting any boost in MPG. I think overdrive could better the econ of the moho. I know for me personally when I towed our Airstreams with my car, I programmed the TC to lock sooner than later so towing in OD in my case added about 1-2 mpg compared to towing in 3rd. Before I did that mod, I swapped my 2.93s for 3.73s and lost about 1.3mpg city, though clearly the trans was a happier beast as the gears did the torque multiplcation so the engine and trans didn't have to work as hard, though RPMs increased in my case somewhere around 700 rpms at 55-60. For me it was a close to a wash, lost MPG by the gears, but locking the TC and towing in OD gained the mpg back compared to towing in 3rd...trans temps consistantly stayed about 175 degrees with a few small, short spikes in temp on moutainous towing.

I also think CK's moho was diesel already out of the box. He had a new engine installed after an oil change was not done correctly and destroyed the diesel engine that was in there to start with.
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Old 05-02-2006, 04:51 PM   #12
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Hi Chaplain Kent. I agree with your move. 55mph tops can be a drag but also, I think having a few more mphs available is much safer. Good luck on your decision. joe
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Old 05-02-2006, 05:10 PM   #13
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Joe,
Good Point. There are times when the safest thing to do is SPEED UP. If you can't, it could be bad.

Kent,
What RPM are you at when you top out? Could it be that your new engine has a governor that could be adjusted? I have a cousin who is a truck driver and all of their trucks have top speed governors.
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Old 05-02-2006, 05:52 PM   #14
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speed vs fuel economy

hi guy 99 and others

my notation related to going faster.....
at 75 fuel economy will go down even with an overdrive.
increasing speed from 55 to 75 means pushing lots more air with the airstream.
so if he goes with gearing that will allow for greater speed,
greater speed will reduce mpg....

we cannot predict what new gearing will do to the mpg,
at the same 55-60mph without more info
yes fuel economy has the potential to go up slightly,
but need to know current gearing, tire size and power band for the new engine...


as i recall best mpg is at or near peak torque.....all other things being equal.



cheers
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