12 Bolt 3.54/3.55

[Howitzer]

I have a 402/700R4 with a lock up converter currently turning 3.08s. It's legs are too long right now on the highway, only getting 1500 RPM at 62 mph. I'm not sure about the 3.73s and I'm worried the 3.42s will still have me below a good bottom end power band area. Does anyone know if they make a 3.54/3.55 for the GM 12-Bolt Truck rear end?

[jr1000yarder]

Originally Posted by Howitzer

I have a 402/700R4 with a lock up converter currently turning 3.08s. It's legs are too long right now on the highway, only getting 1500 RPM at 62 mph. I'm not sure about the 3.73s and I'm worried the 3.42s will still have me below a good bottom end power band area. Does anyone know if they make a 3.54/3.55 for the GM 12-Bolt Truck rear end?

my stock dana 60 has 3.54 in there so I'd say yes

[dennislbrooks]

Shows car 12 bolt and truck ---none for truck

http://www.jegs.com/p/Richmond-Gear/...50832/10002/-1


I think you would be happy with a 3.73 behind the700r4 and bbc. I run a 402 and 3.73s in my 4wd with a 350turbo until I find $1500 for a 700r4.

[RustyBucket]

Both my trucks have 3.08 open rears w/OD. Blazer's turning 2,000rpm at 80 with the TKO trans. Don't have a tach. in the GMC but it doesn't seem to be "lugging" when it shifts into OD at about 50mph. Dont have the lock-up working on the GMC. My big complaint isn't the long gear but that the non-posi makes first gear pretty usless. If you slap either one in first gear it blasts the R. rear tire and won't go. Both transmissions have pretty low first gear ratios. I'm afraid 3.73's would make the 3.27 first gear in the Blazer useless. Thinkin' Id probably go with the 3.42 set from Yukon Gear. I know I've seen 3.55's listed somewhere recently also.
Right now I don't have $2,000 to buy Eaton posi carriers & new gears for 'em so I'll cruise for awhile.

[Kennuck]

I'm running 3.73 with a 4L60E cruises nice, I wouldn't be afraid of 4:10s with the O.D. trans.

[Mike C]

The 3.73 should be fine. And they make a 3.42 for the truck 12 bolt but not a 3.5 gear.

[AnotherWs6]

I wouldnt even think about anything less than 3.73

4.11 x.7 = 2.87

I have 3.08's and no overdrive and cruise at 70 all day long. 4.11's with OD are the equivalent of running a 2.87 ratio with no OD.

People are scared of gear and shouldn't be. I have 4.88's in my Silverado with 35 inch tires. Constantly wish I had 5.12's.

Point is - do the math, dont worry about a perceived BIG ratio.

[Wide Open]

Originally Posted by Howitzer

I have a 402/700R4 with a lock up converter currently turning 3.08s. It's legs are too long right now on the highway, only getting 1500 RPM at 62 mph. I'm not sure about the 3.73s and I'm worried the 3.42s will still have me below a good bottom end power band area. Does anyone know if they make a 3.54/3.55 for the GM 12-Bolt Truck rear end?

What RPM do you want to be at (@62ish mph) and what size tire do you have?

[Howitzer]

Realistically I'll be cruising between 65 and 70 mph. My tires are 28.98 inches. I want a lower ROM for fuel consumption but don't want to be on the back side of the power curve where I'm burning more fuel to run lower RPMs as opposed to a few more revs and actually burning less.

[Wide Open]

Does it fell like it's lugging now with those gears? Are you pulling a trailer or anything?

3.42's would put you around 1880 RPM at 70 while 3.73's would put you right about 2050 RPM

While not an apples to apples comparison I've had/have NBS and NNBS trucks with 3.42, 3.73 and 4.10's and they all got about the same mileage. The Escalade is the heaviest of the bunch but also has the most powerful engine and a similar overdrive ratio as the 700 you're running. With 3.42's it gets 18 on the highway (1500 @60, 2000@80) just like my lighter trucks with more gear.

[Howitzer]

The reason I think it's low now is that my fuel mileage hasn't changed from a TH400 to the 700R4. I should have seen some improvement which makes me think that I'm below the power curve.

I'm not pulling or hauling anything. This is a SWB just meant for driving.

I'm not familiar with NBS/NNBS

[Wide Open]

Originally Posted by Howitzer

The reason I think it's low now is that my fuel mileage hasn't changed from a TH400 to the 700R4. I should have seen some improvement which makes me think that I'm below the power curve.

I'm not pulling or hauling anything. This is a SWB just meant for driving.

I'm not familiar with NBS/NNBS

That does seem weird.
1500 should be a decent mileage rpm but maybe you do need a little more gear to make up for the overdrive and lockup converter.

Sorry - New Body Style (1999-2006) and New New Body Style (2007-2014 I think). I believe they are officially named the GMT800 and GMT900 respectively.
2000 GMC Sierra extended cab shortbed with 4x4 4.10's - 18mpg
2000 Silverado regular cab short bed with 4x4 3.73's - 18mpg
2003 GMC Yukon 4x4 3.73's - 18mpg
2007 Escalade AWD 3.42's - 18mpg

I put a L92 and 6L80 in my 2000 Silverado and kept the 3.73's - yep you guessed it - 18mpg

All vehicles listed above running 31.65" tires and highway mileage listed.
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[WorkinLonghorn]

With a carb you need the revs up around 2300+. At 1500 he will be getting real crappy MPG IMO. I had a 350 that cruised at 1800 and got 10MPG. My current truck with a 350 cruises at 2400 and gets 15 MPG despite weighing 700 lbs more.

I'm no expert but I think I would go at least 3.73 -WL

[Wide Open]

Originally Posted by WorkinLonghorn

With a carb you need the revs up around 2300+. At 1500 he will be getting real crappy MPG IMO. I had a 350 that cruised at 1800 and got 10MPG. My current truck with a 350 cruises at 2400 and gets 15 MPG despite weighing 700 lbs more. -WL

Increased downdraft through the carb for better fuel atomization or just closer to the peak torque you think?
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[Howitzer]

I think the toque curves on the Big Block are relatively flat. What i need to do is find that sweet spot of RPM where i have a decent amount of torque into it where the engine is not "loaded up" but also not screaming away at 2500 RPM and turning metal just to turn metal. I was at 2500 RPM before with the TH400 in it, and the mileage wasn't that great then, nor should it be. If it was down to 2000 RPM you could calculate that you'd be revving 500 times less per minute, on say a 3 hour drive would equal 90,000 revolutions saved. I have a hard time believing that moving that amount of mass 90,000 less times wouldn't save some fuel.

I think i might start out with 3.73's and see how that goes. I'm considering machining a set of 3.55s, as the math keeps coming up on that being a nice ratio.

[Wide Open]

You can machine your own gears?? That's cool.

I found the torque curve on the GM Heritage Center site and it looks like the 400 made around 280 lb-ft at 1500rpm and around 315 lb-ft at 2000rpm so 3.55's would put you right at +35 lb-ft at your cruising speed.

https://www.gmheritagecenter.com/doc...olet-Truck.pdf

[stsalvage]

I'm still trying to get my mind around a 402 big block with a 308 gear.Unless yours was a snow gear.But if your trying to get the bang for more dollar..I'm working with a friend that has a 1971 nova with a 427 4l80E with a Gear vendor.He's looking to get his car up to 180 mph and still leave haft of these jokers with new cars that have a cut off line of 160 MPH.He's still working on the gear chart to see where his 8 speed can benefit the best.There is a company that is making a smaller version of the Allison 8 speed.But there 12000.00. cruiser speed 189 mph with still pedal to go.

The guy is right should not be afraid of gearing.Most of these guys running 900 HP unless they have gearing running the run to Vegas then cars could only wish to hold it at 110 mph before something lets go.

A Gear vendor are used in Drag car's and Sims tractor trailers Volvo used them in there station wagons for the longest time got grate gas mileage.Funny thing is if you flip it around and you have a 3.77 gear with a 700R4 it will be 5.12 in the beginning it will give you a pulling gear or a gas mileage.I love gear vendors

[WorkinLonghorn]

Originally Posted by Wide Open

Increased downdraft through the carb for better fuel atomization or just closer to the peak torque you think?
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I really think both are factors.

[AnotherWs6]

Originally Posted by stsalvage

I'm still trying to get my mind around a 402 big block with a 308 gear.Unless yours was a snow gear.But if your trying to get the bang for more dollar..I'm working with a friend that has a 1971 nova with a 427 4l80E with a Gear vendor.He's looking to get his car up to 180 mph and still leave haft of these jokers with new cars that have a cut off line of 160 MPH.He's still working on the gear chart to see where his 8 speed can benefit the best.There is a company that is making a smaller version of the Allison 8 speed.But there 12000.00. cruiser speed 189 mph with still pedal to go.

The guy is right should not be afraid of gearing.Most of these guys running 900 HP unless they have gearing running the run to Vegas then cars could only wish to hold it at 110 mph before something lets go.

A Gear vendor are used in Drag car's and Sims tractor trailers Volvo used them in there station wagons for the longest time got grate gas mileage.Funny thing is if you flip it around and you have a 3.77 gear with a 700R4 it will be 5.12 in the beginning it will give you a pulling gear or a gas mileage.I love gear vendors

I'm confused here - what is it that you're trying to wrap your mind around exactly??? And what is a snowgear???

[RustyBucket]

http://www.wallaceracing.com/Calculators.htm

Interesting selection of calculators for gear ratios and a variety of other car/truck performance stuff.

3rd gear on your 700r trans is same as your 400 was, 1-1. Looks like 2300+- at 65mph. Probably pretty close to where you'd be w/ 3.73's in OD on the 700. Cheaper to take it out of OD & run 3rd gear.
My 2wd Blazer w/mild low comp.454, peanut port head, Performer RPM, 750vac. Holley will give 15mpg if I run anywhere between 70-80mph at 1600-2000 RPM. That's w/3.08 rear, TKO500 trans. with a .67 OD ratio.
After you've got it rollin' along I don't think it takes a lot of torque to keep it going. ... even with the aerodynamics of a brick

[AnotherWs6]

He'd be less than that 65, he barely be turning 2 grand. Of course that's assuming a 28 inch tire.

Big actors here - what size tire do you have and what is the fastest speed that you cruise at?

[RustyBucket]

Originally Posted by AnotherWs6

He'd be less than that 65, he barely be turning 2 grand. Of course that's assuming a 28 inch tire.

Big actors here - what size tire do you have and what is the fastest speed that you cruise at?

Agreed on the less than 2000RPM.. said he's on 29" tire(28.98) Guess my point is the rear gear change is quite a bit of work and expense and maybe won't make a BIG difference in his mileage.... or enough to make it worthwhile. I've worked and spent money on a lot of stuff that didn't make as much of an improvement as I thought they were gonna. BBC's aren't noted for being fuel miserly.

[AnotherWs6]

IMO opinion nothing makes usable torque at 1,500. And fuel consumption automatically going down with less RPM is obviously also nonsense which the OP seems to know. I think you'd be crazy to run anything smaller than 3.73 now that I see you're running 29" tall tires.

I think 70's and 80's fuel consumption testing ruined a lot of things. It has made people think that tiny gears are good and bigger ones are bad. If you do 55mph in a straight line with zero hills and zero wind then turning 1,500 will yield great mileage.

[dennislbrooks]

My Duramax likes 1500 rpm. 8-)

[AnotherWs6]