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Jul 21st, 2025 | | | | Joined: May 2012 Posts: 574 'Bolter | | 'Bolter Joined: May 2012 Posts: 574 | hey all, I'm looking into raising my highway speed a little without changing my driveline in my '69. what is the best gear ratio to go down the highway with the 305 v6 cruising comfortably at 70-5 mph but still have all the pulling power as before? i believe I have a 4:11 gear ratio in it now, but all I can say is it's slow. it is at about 3000 rpm at 65 mph with the wind to my back. Thanks in advance, Indy 
1949 Dodge Coronet 1955 2nd Chevy 4400 1.5 ton 1955 2nd Chevy 3100 1/2 ton 1955 2nd Chevy 3100 1/2 ton 1957 Chevy 5400 LCF 2 ton 1966 Dodge D100 Sweptline 1968 Chevy P20 stepvan 1969 GMC LWB pickup 1972 GMC Sprint 1974 CP30 shorty bus There are three things that I've learned never discuss with people: religion, politics, and the Great Pumpkin.----Linus Van Pelt Trying to understand the behavior of some people is like trying to smell the color 9  | | | | | Joined: Feb 2004 Posts: 28,674 Kettle Custodian (pot stirrer) | | Kettle Custodian (pot stirrer) Joined: Feb 2004 Posts: 28,674 | If you're referring to your V6, it's got enough torque to handle a higher gear but gas mileage will suffer. I'd suggest something in the 3.55 or 3.36 ratio range. I had a 3.73 gear in my 59 Burb, a mixmaster setup of a GMC Dana 60 housing, Ford gears, and a Dodge pinion flange. If you've got the 3/4 ton Eaton rear axle, the 4.11 will be the tallest gear that's available.
Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!
| | | | | Joined: May 2012 Posts: 574 'Bolter | | 'Bolter Joined: May 2012 Posts: 574 | Ok, just went out to see if it was a dana rear end and all I could come up with is on the back of the pumpkin (rear of truck on pass. side) a series of numbers are cast upside down. they are 12348 and directly below those are 87880. then on the other side (towards front of truck and also on pass. side) is stamped NE 3896838 it is marked GM55 and i did come up with it is a GM 12 bolt truck axle. do these numbers mean anything?
1949 Dodge Coronet 1955 2nd Chevy 4400 1.5 ton 1955 2nd Chevy 3100 1/2 ton 1955 2nd Chevy 3100 1/2 ton 1957 Chevy 5400 LCF 2 ton 1966 Dodge D100 Sweptline 1968 Chevy P20 stepvan 1969 GMC LWB pickup 1972 GMC Sprint 1974 CP30 shorty bus There are three things that I've learned never discuss with people: religion, politics, and the Great Pumpkin.----Linus Van Pelt Trying to understand the behavior of some people is like trying to smell the color 9 | | | | | Joined: May 2012 Posts: 574 'Bolter | | 'Bolter Joined: May 2012 Posts: 574 | OK, figured out that the numbers given above are on most 12 bolt rear differentials, I did find a 12 bolt axle that might swap gears, and found the correct numbers on the passenger side on the axle tube. it reads THA 32 1 and I will get the numbers off of my axle soon, but what is this ratio? I can't seem to find any info on the web of any decoding tools or charts. it is out of a 72 chevy.
1949 Dodge Coronet 1955 2nd Chevy 4400 1.5 ton 1955 2nd Chevy 3100 1/2 ton 1955 2nd Chevy 3100 1/2 ton 1957 Chevy 5400 LCF 2 ton 1966 Dodge D100 Sweptline 1968 Chevy P20 stepvan 1969 GMC LWB pickup 1972 GMC Sprint 1974 CP30 shorty bus There are three things that I've learned never discuss with people: religion, politics, and the Great Pumpkin.----Linus Van Pelt Trying to understand the behavior of some people is like trying to smell the color 9 | | | | | Joined: Jun 2011 Posts: 1,901 Shop Shark | | Shop Shark Joined: Jun 2011 Posts: 1,901 | jack it up (both rear wheels off the ground) make a mark on the driveline and have someone turn the wheel one revolution while counting the revs of the driveline. Then you'll know for sure.
Give me ambiguity
or give me something else
| | | | | Joined: May 2006 Posts: 8,351 'Bolter | | 'Bolter Joined: May 2006 Posts: 8,351 | The THA code comes up as a '71 C10, 3.07:1 ratio. I think CASO is right, try checking it counting the revolutions. Something doesn't add up if you're winding it out, especially if it does have the ratio the code says it should have.
I do have a question that might be relevant to the situation; What size tires are you running? A small diameter tire will be similar to putting a lower gear in the diff.
Bill Burmeister
| | | | | Joined: Mar 2010 Posts: 10,059 Renaissance Man | | Renaissance Man Joined: Mar 2010 Posts: 10,059 | What transmission do you have?
A 700R4 or a T-5 may solve your problem easier than swapping rear ends/gear set, and give you better gas mileage. Both have overdrive.
1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission
| | | | | Joined: Jun 2011 Posts: 5,096 Crusing in the Passing Lane | | Crusing in the Passing Lane Joined: Jun 2011 Posts: 5,096 | First thing to do is find out what ratio you really have as was pointed above. Then make an informed decision.
'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: Feb 2004 Posts: 28,674 Kettle Custodian (pot stirrer) | | Kettle Custodian (pot stirrer) Joined: Feb 2004 Posts: 28,674 | Trying to figure out the axle ratio with both wheels off the ground is a little tricky. I like to mark a tire at ground level, and roll the truck forward or back one turn of the wheel while counting driveshaft revolutions. The only surefire way is to drop the rear cover plate and count gear teeth. For instance, a 41 tooth ring gear matched with a 10 tooth pinion gives a 4.10:1 gear ratio.
Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!
| | | | | Joined: May 2012 Posts: 574 'Bolter | | 'Bolter Joined: May 2012 Posts: 574 | alright, my apologies for my delay in response, but I turned the wheel one revolution and the driveline turned 3 3/4 turns. the axle that has the 3.07 gears is not in my truck, but I can put those gears in it if they will work. my truck has the stock 2 speed turbo tranny with 16 inch tires (stock were 15). if my calculations are right my gears should be 3.73:1 right?
1949 Dodge Coronet 1955 2nd Chevy 4400 1.5 ton 1955 2nd Chevy 3100 1/2 ton 1955 2nd Chevy 3100 1/2 ton 1957 Chevy 5400 LCF 2 ton 1966 Dodge D100 Sweptline 1968 Chevy P20 stepvan 1969 GMC LWB pickup 1972 GMC Sprint 1974 CP30 shorty bus There are three things that I've learned never discuss with people: religion, politics, and the Great Pumpkin.----Linus Van Pelt Trying to understand the behavior of some people is like trying to smell the color 9 | | |
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