need different gear ratio - The 1947 - Present Chevrolet & GMC Truck Message Board Network

johhnyboy26 · 03-10-2014, 03:23 PM

I' got a 72 chevy with 33's, 4 inch lift, and a 350. i recently put a 700R4 for better cruising rpms. But I think the gears are little to high because it keeps shifting back and forth out of overdrive. What gears should i use?

Longhorn 70 · 03-10-2014, 03:33 PM

3.73, or 4.11 are my 2 faves. I have 4.11's with 31" tires and a 700R4.

cool68 · 03-10-2014, 04:26 PM

take a look at a rpm calculator online-project33.com

Lee H · 03-10-2014, 05:12 PM

What gear ratio do you have?

hugger6933 · 03-10-2014, 06:00 PM

you may even want to consider 4:56 but with the tire height you need to run it threw the gear formula to see what the final would be in overdrive. [0.78:1]

**jocko** · 03-10-2014, 08:11 PM

Curious what you have now also. If it's hunting between 3rd and o/d, then it sounds like the combo of tire dia and o/d gearing are playing with your transmission's mind. Here's the cruising rpm for a 700R4 at 65 and 75 mph with 3 different gear ratios (all calculated with a 33" dia tire). Hope this helps you decide. Would be good to know what your current rear end is, if you know. K10 or K20? Welcome to the board! I'd say 4.10s are just about right - and 3.73 "should" be doable, but would be borderline lugging at the 50-60 mph zone.

**kwmech** · 03-11-2014, 01:12 AM

overdrive and 33s-----4.56 all the way, maybe even a 4.88 if you plan on upping the tire size at a later date. I had a 1990 suburban a couple of years back that I put 4.56s into, that also had a 700r4. 'Burb had 31 inch tall tires on it and it still wasn't enough gear when I had to tow something. With a small block and a heavy truck the average engine like to spin between 2200- and 2600 cruising down the highway

johhnyboy26 · 03-11-2014, 10:47 AM

I'm not sure on the gear ratio i bought the truck as a project and finished it but never even thought about the diffs, if it's not the diffs what could be wrong with transmission?

johhnyboy26 · 03-11-2014, 10:51 AM

Thanks for all the replies, it really helps that people with knowledge about these trucks can give you the information you need, thanks

johhnyboy26 · 03-11-2014, 10:54 AM

I'm trying to put a factory tach in and i have an old cluster with a tach in it. What do i have to do to put this in and if it doesn't work where could i get a new tach?

03-10-2014, 03:23 PM	#1
johhnyboy26 Registered User Join Date: Mar 2014 Location: Coutts AB Canada Posts: 6	need different gear ratio I' got a 72 chevy with 33's, 4 inch lift, and a 350. i recently put a 700R4 for better cruising rpms. But I think the gears are little to high because it keeps shifting back and forth out of overdrive. What gears should i use?

03-10-2014, 03:33 PM	#2
Longhorn 70 Registered User Join Date: Dec 2012 Location: Chandler AZ Posts: 1,149	Re: need different gear ratio 3.73, or 4.11 are my 2 faves. I have 4.11's with 31" tires and a 700R4. __________________ 1970 Longhorn, Front Disc, 350/4 bolt, 882 heads, HEI, Edelbrock, 700R4, HO-52/4.11. 1996 Corvette, Collector Roadster, LT4, 396, 450RWHP, 6sp, 4.11/Dana44/posi 5 point roll bar And let us not grow weary of doing good, for in due season we will reap, if we do not give up. Gal 6:9

03-10-2014, 04:26 PM	#3
cool68 Registered User Join Date: Oct 2013 Location: Stockton, CA Posts: 475	Re: need different gear ratio take a look at a rpm calculator online-project33.com

03-10-2014, 05:12 PM	#4
Lee H Registered User Join Date: Jun 2010 Location: Upland Ca Posts: 4,147	Re: need different gear ratio What gear ratio do you have? __________________ 1972 C10 SWB, Air, PS, PB, 350/350THM. Second owner. 1965 Corvette roadster, 44K miles, 327/365 SHP, 4 speed, side exhaust, knockoffs, teak, second owner (bought in 1970), Have ALL numbers matching components. My frame off restoration thread: http://67-72chevytrucks.com/vboard/s...d.php?t=556703

03-10-2014, 06:00 PM	#5
hugger6933 Registered User Join Date: Oct 2008 Location: Marianna Arkansas Posts: 7,279	Re: need different gear ratio you may even want to consider 4:56 but with the tire height you need to run it threw the gear formula to see what the final would be in overdrive. [0.78:1]

03-10-2014, 08:11 PM	#6
jocko Senior Member Join Date: Oct 2009 Location: Godley, TX Posts: 17,976	Re: need different gear ratio Curious what you have now also. If it's hunting between 3rd and o/d, then it sounds like the combo of tire dia and o/d gearing are playing with your transmission's mind. Here's the cruising rpm for a 700R4 at 65 and 75 mph with 3 different gear ratios (all calculated with a 33" dia tire). Hope this helps you decide. Would be good to know what your current rear end is, if you know. K10 or K20? Welcome to the board! I'd say 4.10s are just about right - and 3.73 "should" be doable, but would be borderline lugging at the 50-60 mph zone. Attached Images

03-11-2014, 01:12 AM	#7
kwmech Senior Member Join Date: Mar 2003 Location: Colfax-California Posts: 8,599	Re: need different gear ratio overdrive and 33s-----4.56 all the way, maybe even a 4.88 if you plan on upping the tire size at a later date. I had a 1990 suburban a couple of years back that I put 4.56s into, that also had a 700r4. 'Burb had 31 inch tall tires on it and it still wasn't enough gear when I had to tow something. With a small block and a heavy truck the average engine like to spin between 2200- and 2600 cruising down the highway

03-11-2014, 10:47 AM	#8
johhnyboy26 Registered User Join Date: Mar 2014 Location: Coutts AB Canada Posts: 6	Re: need different gear ratio I'm not sure on the gear ratio i bought the truck as a project and finished it but never even thought about the diffs, if it's not the diffs what could be wrong with transmission?

03-11-2014, 10:51 AM	#9
johhnyboy26 Registered User Join Date: Mar 2014 Location: Coutts AB Canada Posts: 6	Re: need different gear ratio Thanks for all the replies, it really helps that people with knowledge about these trucks can give you the information you need, thanks

03-11-2014, 10:54 AM	#10
johhnyboy26 Registered User Join Date: Mar 2014 Location: Coutts AB Canada Posts: 6	Re: need different gear ratio I'm trying to put a factory tach in and i have an old cluster with a tach in it. What do i have to do to put this in and if it doesn't work where could i get a new tach?



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