Click on image for the lowdown. 
====
| | | Forums66 Topics128,120 Posts1,056,325 Members48,439 | | Most Online14,466
Apr 26th, 2026 | | AD Chevy Trucks  Over 6,000 pictures Brad Allen has an awesome collection of Chevrolet factory pictures that he has set up from film strips. Check them out!
Lots of work on Brad's part ... pure enjoyment (and references) for you!
| | THANKS!
For helping keep this great
old truck web site
on the Internet for over 30 years! and not clogged down
with Ads!  Again, thanks!
| | | | Joined: Dec 2006 Posts: 1,154 Shop Shark | | Shop Shark Joined: Dec 2006 Posts: 1,154 | Putting out a few feelers for opinions: installed the SM420 from Novak it's great. Now I want to lower RPM's at 60 mph @ 2100-2400rpm ...I have 27" tires ..1957 235...3100 ..final ratio 1:1 ...1/2 ton....I don't care about gas mileage....I don't care about going 70 mph... I don't care about towing.....no heavy hauling....... I DO care about lower rpms at 55 -60 mph.......Question 3:38.1 ring and pinion a good swap for my wants and use? What can we expect with this swap???
Thanks Steve | | | | | Joined: Jun 2011 Posts: 1,901 Shop Shark | | Shop Shark Joined: Jun 2011 Posts: 1,901 |
Give me ambiguity
or give me something else
| | | | | Joined: Dec 2006 Posts: 1,154 Shop Shark | | Shop Shark Joined: Dec 2006 Posts: 1,154 | Cool.....but just would like to hear from somebody who did the actual swap. The numbers are great but too many variables. Actual use engineering you get opinions do's and dont's and the human aspect of it all.
| | | | | Joined: Sep 2001 Posts: 29,269 Bubba - Curmudgeon | | Bubba - Curmudgeon Joined: Sep 2001 Posts: 29,269 | Use calculator at the bottom of this page. http://www.csgnetwork.com/multirpmcalc.htmlOnly 4 variables. Just change one at time. 3.90 R&P ratio (your truck) 27" tires 60 mph RPM = 2912 3.38 R&P ratio 27" tires 60 mph RPM = 2554 On my 1954 1/2 ton trucks, I use 31" diameter radial truck tires on original wheels and with 3.55 R&P in original rear end. 3.55 R&P ratio 31" tires 60 mph RPM = 2309 I like it. When I have a few passengers I use 1st gear in my SM420. | | | | | Joined: Dec 2006 Posts: 1,154 Shop Shark | | Shop Shark Joined: Dec 2006 Posts: 1,154 | Hey thanks...my tires are almost new and the diameter is 27" so 60mph@ 2500 rpm seems very reasonable. Right now 2500 gets me less than 55mph. Sounds like it's worth the trouble. When these tires go I'll go bigger like you did. I guess the 3:08 r&p set would be a bit too high for everyday driving......but the numbers are nice.
Steve | | | | | Joined: Jun 2011 Posts: 5,096 Crusing in the Passing Lane | | Crusing in the Passing Lane Joined: Jun 2011 Posts: 5,096 | Do you ever plan to drive in the mountains? Might not want to go too high on gears, if so.
Ed
'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.
| | | | | Joined: Dec 2006 Posts: 1,154 Shop Shark | | Shop Shark Joined: Dec 2006 Posts: 1,154 | Great point others mentioned the same and we do have some mountains here in Northeren CA.....Tim is right on with the 3:8.1 solution others have said the same. It's not the speed so much but less strain on the 235 gives you highway speed and picking up the cat at the vets for my wife. | | | | | Joined: Jun 2006 Posts: 693 'Bolter | | 'Bolter Joined: Jun 2006 Posts: 693 | i did that swap on my 56 3100 (from 3.9 to 3.38). i love it. rpms dropped about 400 @ 60 and it goes down the road great. i don't carry a load and i don't tow so no issue for me. it launches just fine with the factory 3-spd. | | | | | Joined: Dec 2006 Posts: 1,154 Shop Shark | | Shop Shark Joined: Dec 2006 Posts: 1,154 | Hey thanks for the info. Sounds nice I'm going to have it done next week if we can get the parts quick. Cool that it's working well with a three speed I wondered about that ....nice! Yep...not looking for record breaking speed lower RPM's for the engine health and a little more road speed.
Thanks, Steve | | | | | Joined: Mar 2010 Posts: 10,281 Renaissance Man | | Renaissance Man Joined: Mar 2010 Posts: 10,281 | I can tell you from experience that a 3.38 to 1 would be better than a 3.08 to 1 with your tire height and a stock 3 speed. The 3.08 will not get you out of shallow hole with a medium sized load in the bed. You would be able to go 70 all day long with the 3.08, but there is no such thing as free lunch.
Carl
1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission
| | | | | Joined: Dec 2006 Posts: 1,154 Shop Shark | | Shop Shark Joined: Dec 2006 Posts: 1,154 | Right on Carl...The 3.38.1 is suppose to be the best gear ratio for the SM420 transmission. I understand the 3.08 is for torque tube trucks. I don't know why that is? Your right I still want to use it as a truck and just 55mph at a lower rpm. I think the power curve for a stock 235 is 2200 to 2800 could be wrong. All I know is the 3.90 is a great ratio for heavy hauling like a truck bed full of gravel but not to good to get to the gravel pit before it closes!
Thanks, Steve | | | | | Joined: May 2001 Posts: 7,438 Extreme Gabster | | Extreme Gabster Joined: May 2001 Posts: 7,438 | I've never heard of a 3.08 in any torque tube equipped truck. | | | | | Joined: Dec 2006 Posts: 1,154 Shop Shark | | Shop Shark Joined: Dec 2006 Posts: 1,154 | You maybe right I don't know never owned a torque tube truck. Patrick's has ring and pinion for cars and trucks. Take a look at their web site full of information great people! | | |
|
|
