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Re: What is the verdict on the 3.55 conversion? [Re: tom moore] #1311174
Sun May 19 2019 06:23 PM
Sun May 19 2019 06:23 PM
S
stoveboltgator  Offline
Wrench Fetcher
Joined: Oct 2008
Posts: 44
Northern AL
Will the 3.55 differential give you a workable, usable 1st gear with the SM420 granny low? In other words, does the SM 420 become a true 4-speed for practical normal driving? It is a pretty $$$erious modification to make if it carries no real benefit.

When I finally finish my '53 truck I want to be able to drive it somewhat normally (if sparingly) and not have to be afraid of Interstates.

Last edited by stoveboltgator; Sun May 19 2019 06:23 PM.
Re: What is the verdict on the 3.55 conversion? [Re: tom moore] #1311177
Sun May 19 2019 07:01 PM
Sun May 19 2019 07:01 PM
tclederman  Offline
Boltergeist
Joined: Sep 2001
Posts: 27,925
Jarvisburg, NC
Three (+) uses/benefits of the 1st gear in an truck with an SM420:

1. Win the slowest-truck race at an old-truck show (I did).
2. Burn rubber in first gear for 20-40 feet (to display smoking rubber, and/or, tire marks - impressive, to some people).
3. Easily start from stop when carrying/towing a heavy load (towing, for example).
4. Drag a VW bug up/out of a deep ravine (I did), without burning the clutch.
5. Never worry about getting stuck in gear when downshifting (as often happens with a worn/mis-adjusted 3-speed linkage).
6. Impress your friends by driving up the side of a barn (this is just a bad joke).

It does not give you lower RPMs at high speeds, compared to standard 3-speeds both are 1:1 in high gear.


Tim
1954Advance-Design.com
1954 3106 Carryall Suburban - part of the family for 50 years
1954 3104 5-window pickup w/Hydra-Matic - part of the family for 14 years
Z-series (54/55) GMC 350 (2-ton) COE - now part of Dave's family
- If you have to stomp on your foot-pedal starter, either you, or your starter, or your engine, has a problem.
- The 216 and early 235 engine are not "splash oilers" - this is a splash oiler.
Re: What is the verdict on the 3.55 conversion? [Re: tom moore] #1311191
Sun May 19 2019 09:08 PM
Sun May 19 2019 09:08 PM
H
Hotrod Lincoln  Offline
Boltergeist
Joined: Feb 2004
Posts: 18,572
Dellrose, TN
Since 1st. gear is non-synchronized, using it as a traffic gear is pretty much a lost cause, as you have to come to a complete stop to engage it. Then a kid on a 10-speed bicycle will beat you getting away from every stoplight. 4th. gear will produce the same highway speed as the current 3-speed does, as neither transmission has an overdrive ratio. On the upside, given halfway decent preventive maintenance, it will outlast the 3 speed about 4 to 1. Chevy 3 speed transmissions could be made out of wet pasta, and it would be a noticeable improvement in strength.
Jerry


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: What is the verdict on the 3.55 conversion? [Re: tom moore] #1311217
Mon May 20 2019 12:49 AM
Mon May 20 2019 12:49 AM
5
52Carl  Offline
Master Gabster
Joined: Mar 2010
Posts: 7,969
Virginia
With 3.55, your 4 speed granny gear will still be usable for pulling stumps, and starting from a stop up an incline or out of a hole. After that, the gear ratios in 2, 3, and 4 will be very similar to 1,2, and 3 of the 3 speed transmission.
I would go with the 4 speed/3.55 rear end gears before I'd go with the 3 speed/3.55 rear end gears.
Carl

Re: What is the verdict on the 3.55 conversion? [Re: tom moore] #1311231
Mon May 20 2019 02:24 AM
Mon May 20 2019 02:24 AM
S
stoveboltgator  Offline
Wrench Fetcher
Joined: Oct 2008
Posts: 44
Northern AL
Sounds like it adds more versatility than actual usage on a day-to-day basis. I will keep this a low-priority item for now...

Re: What is the verdict on the 3.55 conversion? [Re: tom moore] #1311234
Mon May 20 2019 02:43 AM
Mon May 20 2019 02:43 AM
S
sstock  Offline
Shop Shark
Joined: Jan 2013
Posts: 1,228
Gig Harbor, WA
Mine is in now and it was the best thing that I have done to the 53. Cruising on the freeway now. The 1st gear in the sm420 is still very low, only use it at stop signs on hills.
Mine cost me only the parts, I set it up myself, I wouldn't of paid somebody to do it though.
Steve

Re: What is the verdict on the 3.55 conversion? [Re: tom moore] #1311247
Mon May 20 2019 01:00 PM
Mon May 20 2019 01:00 PM
G
Green_98  Offline
Shop Shark
Joined: Sep 2009
Posts: 160
Olive Branch (Memphis)
I can honestly say the two best upgrades done to my 1953 1/2 ton are the 261 swap for the 216, and the 4-speed swap for the 3-speed. Hands down, the floor shift wins, but the compound low 1st gear is useless for road use. I use 2-3-4 for road driving. I've run mine up to about 55, that's the fastest I want to push this driveline and don't feel the truck would be at all safe above that speed. I too have considered the 3.55 rear end conversion, but seeing that mine is not a daily driver nor am I needing to go 65 mph+ in it, so the stock 4.11's work just fine for me.


-Patrick
1953 Chevrolet 3100
261 / 4-speed / 4:11 / Commercial Red

Re: What is the verdict on the 3.55 conversion? [Re: tom moore] #1311250
Mon May 20 2019 01:23 PM
Mon May 20 2019 01:23 PM
J
Jon G  Online
Shop Shark
Joined: Mar 2014
Posts: 942
Dallas, TX
It depends entirely on what you want. For many the T5 conversion is a nice alternative but you have to give up the old torque tube and adapt a newer style rear end in there. It isn't all that difficult, actually. Just takes time and time to study which gear ratios you want to end with. If you're careful, you can have nearly the exact same ratios as the stock 3 speed in gears 1,2,3 and then 4th becomes your first overdrive with a 25% reduction in rpm and 5th becomes your second overdrive, useful on relatively flat terrain and with a roughly 14% reduction from your 4th gear. If you have a 261, the T5 should be a natural with it and gears 1-5 are synchronized. I agree going 65 or over in the AD truck isn't wise or safe...although there are some in the audience who will jump up and say "oh I drive mine at 70 or 75 all day long." The truck was not designed to drive at that speed. As Jerry once said "it handles like a hippo on a wet mud bank" or something close to that.


Jon

1952 1/2 ton with 1959 235
T5 with 3.07 rear end
Re: What is the verdict on the 3.55 conversion? [Re: tom moore] #1311251
Mon May 20 2019 01:29 PM
Mon May 20 2019 01:29 PM
tclederman  Offline
Boltergeist
Joined: Sep 2001
Posts: 27,925
Jarvisburg, NC
The 3 significant changes that I have made on my 1954 Suburban, in order of value & performance & enjoyable drive-ability are:

2000: 3.55 R&P conversion (and ALL new bearings/bushing/seals - venders sell kits - double your costs and spend the money)

1972: 1960 261 engine (cheapest available engine I could find, when my original 235 died)

1975: 4-speed SM420 rear end (when I gave up on the 3-speed jamming/locking in gear)

No regrets on doing any of them, but, I wish I knew then how to deal with 3-speed shifter-linkage parts and adjustments.


Tim
1954Advance-Design.com
1954 3106 Carryall Suburban - part of the family for 50 years
1954 3104 5-window pickup w/Hydra-Matic - part of the family for 14 years
Z-series (54/55) GMC 350 (2-ton) COE - now part of Dave's family
- If you have to stomp on your foot-pedal starter, either you, or your starter, or your engine, has a problem.
- The 216 and early 235 engine are not "splash oilers" - this is a splash oiler.
Re: What is the verdict on the 3.55 conversion? [Re: tom moore] #1311256
Mon May 20 2019 02:43 PM
Mon May 20 2019 02:43 PM
S
sstock  Offline
Shop Shark
Joined: Jan 2013
Posts: 1,228
Gig Harbor, WA
Geez Tim, you are a pioneer, doing all that back in the early 70s, back in 72, I hadn't even discovered girls yet and I'm pushing for 61 now.
Good stuff though, I'm building my 261 now, already an sm420 convert and the 3.55 I put in a few months back.

Steve

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