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Old 03-03-2010, 11:30 PM
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which rear end gears to run?

hey everyone, so here it is. i got a 75 chev short bed with a 454 built ( 450-500hp ) turbo 400 tranny, 275 60 15s tires in rear. my question is, from a stop, the thing is amazing, but at 50 mph or so its like at 2800 rpm. its gone through all 3 gears before i can light a smoke! this truck dosent go to any tracks or anything, and i think it has more than enough to lose some off its bottom end, for some more decent rpms on the freeway. actually, id like it if it ran like 3000 rpms or so at like 80-85....any suggestions? i did have the diff cover off, but stupidly didn't count the teeth....but at this point i think anything would be better than what i got now ( gears )

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Old 03-04-2010, 02:31 AM
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Figuring that the 275 tire is something in the 27.5-28" diameter range would give you 3.08 gear for 3000 rpm to be 80-85 mph. 26" tire will need a 2.90 gear for the same result. Calculator at www.wallaceracing.com
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Old 03-04-2010, 05:51 AM
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Sounds like a good candidate for a built 700R4 trans, best of both worlds,
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Old 03-04-2010, 07:12 AM
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my $.02:
it takes some thought and planning to solve it (or even just improve it)....

the tranny shifts based on Hg "and/or" rpms,,,which ever comes first...

your motor has so much low end rpms flywheel TQ ft/lbs of force the rpms climb extremely quick so it shifts quick due to the rpms govenor (and basically the Hg is always high calling for a shift on the vac govenor control)

yup, TQ ft/lbs force on the tires=flywheel TQ x tranny ratio x gears ratio...

but, load on the motor =TQ ft/lbs force on the tires/vehicle weight (power to weight ratio)

so,,,, here's a whole bunch of calculators for rpms/wheels/ratio's/etc to play with...
http://www.wallaceracing.com/Calculators.htm

the old 455 firebirds (less weight) had around 2.60 gears to shift right but in your heavier truck fair chance your gona be using more pedal to have slower shifting..

fairly cheap test:
go get a pair of junkyard wheels and some used "really" tall tires...
(you can plug in tires diameters on the calculators to work backward from diameters to figure what gears/with your tires= ????rpms and slower shifts)

I've got the same "to quick" shift TQ/wt problem driving normal,,, but,,,, it really doesn't hurt anything...
LOL,,,what it's telling you is due to the huge TQ power to weight ratio ya don't need a tranny!!!
I have yet to figure out a "reasonable" cost solution that won't hurt motor response or shorten tranny life such as use higher rpms shift points govenors=more load on the next gear and clutches...
(I do have manual shift option and occasional just use 2nd and 3rd,,,vac gauge on the dash= monitor carb lean/rich while accelerating motor load)

Last edited by red65mustang; 03-05-2010 at 07:16 AM.
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Old 03-04-2010, 07:59 AM
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If your truck has a 3.73 gear then you willhave to change the differential along with the gears if you go to anything from 3.42 to 2.73. The 12 bolt truck rear end uses a 4 series differential for the 3.73 gears and a 3 series for all gears from 3.42 to 2.73. An overdrive transmission would probably be a better idea.
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Old 03-04-2010, 09:14 AM
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if you are after mpg with less rpms then be aware it takes alot of miles for a rear end or OD tranny swap $$$$ cost to pay for itself...
illustration
$3gal/15mpg=$.20 per mile
$3gal/18mpg=$.16 per mile

$.04 per mile savings x 10k interstate miles per year(?) =$400 gas cost savings per year...

(intown mileage "IS" controlled by your size 12 right foot,,,not gears,,,mount a vac guage in the truck to be a "watch dog" for lean/rich gas consumption,,,keep it above 12Hg for lean while accelerating)

I would do the tall tires test to see if average mpg does improve (at all),,,the truck is a "brick wall" for air flow at hwy speed and heavy....

darn hard to judge rpms for motor wear,,,more rpms (closer to the TQ peak) is less load on the motor parts,,,less constant rpms is more load/due to less TQ but less total rotations ...

the math to do the normal driving net A/F cfm reduction per rpms is a nightmare but just to illustrate with WOT numbers:

if 60mph=2k a 460 cube motor consumes 196 total cubic feet of A/F per mile...
if 60mph=2.5k a 460 consumes 272 total cfm per mile...
looks good percentage wise until you factor in a 14/1 cruise A/F ratio,,only .7% gas "vapor" per cube
if 60mph=3k a 460 sucks in 347 cubic feet per mile!!!
it not linear for cfm versus rpms!!!

like I said in the post above it takes a whole bunch of research and planning...

Last edited by red65mustang; 03-05-2010 at 06:19 AM.
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Old 03-04-2010, 10:52 AM
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Quote:
Originally Posted by diamond mike
hey everyone, so here it is. i got a 75 chev short bed with a 454 built ( 450-500hp ) turbo 400 tranny, 275 60 15s tires in rear. my question is, from a stop, the thing is amazing, but at 50 mph or so its like at 2800 rpm. its gone through all 3 gears before i can light a smoke! this truck dosent go to any tracks or anything, and i think it has more than enough to lose some off its bottom end, for some more decent rpms on the freeway. actually, id like it if it ran like 3000 rpms or so at like 80-85....any suggestions? i did have the diff cover off, but stupidly didn't count the teeth....but at this point i think anything would be better than what i got now ( gears )
NOW:
50 MPH @2800 RPM w/28" tires = 4.67 (4.56:1, probably).

CHANGING TO:

TH700R4- w/a .7 OD the 4.56 becomes 3.2:1, or 1920 RPM (w/no converter slippage) at 50 MPH, 3082 RPM @ 80 MPH.

TH2004R- w/a .67 OD the 4.56 becomes 3.06:1, or 1836 RPM @ 50 MPH, 2938 RPM @ 80 MPH.

So either tranny swap will have you right at the 3000 RPM at 80 MPH you want.

If you want to keep the TH400, junkyard source a 3.08 rear end- complete. Bolt it in and go.

The BBC will pull a 3.08 w/NO problem. That 3.08 will also get you your desired 3000 RPM @ 80 MPH, too.

LINK to calculator.

Last edited by cobalt327; 03-04-2010 at 11:04 AM. Reason: Add link.
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Old 03-04-2010, 11:21 AM
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just a comment and food for thought:

do the flywheel TQ x each tranny gear x rear gears calculations on all possibilties because you do have 500ft/lbs of TQ!!!
EX:
put 3.08's in the rear put's alot more TQ stress on the tranny WOT so it needs to be tougher to last...
the OD tranny has a lower 1st gear ratio and can be useless with 4.56 rear gears driving normal,,,different 1st gears ratios are available but aren't inexpensive...

pretty sure there is a convertor slippage calculator on the wallace site,,,you may be slipping,,,4.56 is rare in a oem rear (3.50 is common,,,3.73 in some)

ask a cop to "shoot you" with his radar gun while you hold steady mph speed,,,at 20/40/60mph to know exact mph versus tach rpms to use on the calculators...
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Old 03-04-2010, 12:04 PM
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Quote:
pretty sure there is a convertor slippage calculator on the wallace site,,,you may be slipping,,,4.56 is rare in a oem rear (3.50 is common,,,3.73 in some)
Hey, just going by what he posted. And the 28" tire circumference is just a guess as well.

If he has a 3.50 or 3.73 and is taching 2800 RPM @ 50 mph, that's 600-700 RPM (respectively) of "slippage"! That's not slippage, that's failure. Or he has one HELL of a loose TC. If this wasn't equipped w/a GOOD cooler, it would not last.

The TH 400 will have no problem dealing w/the torque as long as it doesn't have the crummy sprag. Hopefully the tranny was built to be mated to the engine.

He says the truck isn't going for ET, just wants an easy freeway flyer.

If it were me (being budget-minded) I'd drop the old rear end in toto and install a complete 3.08 w/o a thought.

Would need a speedo correction.

Otherwise the OD will work fine, if a bit more complicated and costly. AFA 4.56 being "useless" w/the low first gear of an OD tranny, that's debatable. All the low first gear means, is the tranny upshifts quickly, under normal driving.

W/your foot in it, it will also upshift quickly, w/wheelspin. But (in my experience) it's actually easier to control wheelspin w/a low gear than a high gear. Once a high gear gets the tires broken loose, it takes a while- even when you let out of it- to get traction again. W/a low gear, the engine braking is greater and the traction can be recovered sooner.
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Old 03-04-2010, 01:38 PM
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cobalt,
to me his 50mph numbers look "fishy" and need to be checked out is all I'm saying...
(we "don't" have enough facts so I wrote "food for thought" as a preface)

my car is only 3200ft-lbs/3200lbs car weight at the tires p/w ratio in first gear and on a wet road with a 2.41 first gear yah gotta roll into the throttle to keep traction with T/A radials...
(part of why I changed it to manual shift option and start out in second,,,wet every day here and to much traffic)
on a dry road it shifts 1-2 in a heart beat (3 mph?) already driving normal....
put a OD 3.0 first gear in my car and it would shift 1-2 in a eye blink driving normal with my 3.50 gears/24" tires...

his truck weight bias is far worse than mine for traction....
if he needed to escape a accident or wants to mess around he will go nuts trying to get traction with a 3.0 first/4.56 rear combo with that much TQ...

500ft/lbs x 3.0 x 4.56 = 6840ft/lbs of force on the tires (!!!!!!) with a 60/40(?) front/rear weight bias....

that's why I wrote do all the gears combo's possibilities (easy math) and think thru all

Last edited by red65mustang; 03-04-2010 at 01:56 PM.
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Old 03-04-2010, 01:53 PM
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I just don't feel an immediate upshift is a such a bad thing, I guess. If the idea was to hold one gear for a long time, there's a couple options that come to mind- use the shifter, or use a Powerglide.

Is the OD ideal w/4.56:1 rear gears? Given his stated desire- 3000 RPM @ 80 MPH- yes, it is. But is it somehow innately unsafe? I don't believe it is- no more than many things that are done in the name of performance, IMHO. The idea that a wreck could be more easily avoided w/a higher rear gear is... debatable.

AFA pounds of torque in what gear w/what force to the tires- that is why there is a throttle connected on one end to the engine and the other end to your brain.
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Old 03-04-2010, 04:04 PM
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I guess I missed someting. Where did the 4.56 gear come from? Are you thinking that he has a 4.56? I wouldn't try to run a 4.56 with an overdrive transmission at 80 MPH anyway. The drive shaft speed is going to be crazy. A large diameter, strong drive shaft would be a MUST. A 3.73 with overdrive would be just fine. The 3.06 first gear of thee 700R4 would still be usable, but not great, and the drive shaft speed would also be reasonable. The 200 4r would also be a good choice because of the better first gear. But then I prefer 3 pedals on the floor anyway, so I wouldn't be using either one.
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Old 03-04-2010, 04:13 PM
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I was wondering that myself BGH, 3.55s = OK, 3.73s would be ideal with an O.D. trans, even a stick, maybe even 4.10s, anything beyond that would just be ridiculous,
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Old 03-04-2010, 04:59 PM
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cobalt
his truck weighs roughly 25% more than my car and you are adding more than DOUBLE the TQ force at the tires with 3.0/4.56 than my car has....

what my brain tells me is he will need a throttle pedal with about 10" of travel so "his brain" can control that TQ with 3.0/4.56 to not bust the tires loose and do loop de loops....

on a wet or dry road....

Last edited by red65mustang; 03-05-2010 at 06:24 AM.
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Old 03-04-2010, 06:29 PM
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Quote:
Where did the 4.56 gear come from? Are you thinking that he has a 4.56?
The 4.56 "came" from the OP saying he was taching 2800 R's at 50 MPH w/about 28" tires.

Quote:
I was wondering that myself BGH, 3.55s = OK, 3.73s would be ideal with an O.D. trans, even a stick, maybe even 4.10s, anything beyond that would just be ridiculous,
That's why I said at the very beginning,
Quote:
If you want to keep the TH400, junkyard source a 3.08 rear end- complete. Bolt it in and go.

The BBC will pull a 3.08 w/NO problem. That 3.08 will also get you your desired 3000 RPM @ 80 MPH, too.
and
Quote:
If it were me (being budget-minded) I'd drop the old rear end in toto and install a complete 3.08 w/o a thought.

Would need a speedo correction.
AFA an OD tranny, that was originally brought up by someone else ( cough* Rick* cough). While it wouldn't be my first choice, I did the math to compare the options to the OP.

His stated "want" was:
Quote:
actually, id like it if it ran like 3000 rpms or so at like 80-85....any suggestions?
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