Revving too high

[Tonymolloy]

I've just bought a 71 c10 a few weeks ago it's running a 454 aand th 400 box,12 bolt axle with 4-11 gears,at 55 mph it's revving at about 3500 rpm,what's the best solution to solve this and bring the revs down? A gear vendor overdrive? Or a different crown wheel and pinion? Cheers for any help

[68panelman]

Probably lower your rear ratio. I have 3:08's in my blazer and love-em

[n33k0]

How tall are the rear tires on your truck? I have a TH350 with 4.10 gears, so my setup shouldn't be too far off yours; with 28" tires in the rear (275/60r15) I calculate around 3000 RPM at 60mph. I don't have a tach, but that number is consistent between multiple online RPM calculators.

Either way, a lower (numerically) differential gear will get you the biggest bang for your buck. For the price of a gear vendor O/D, you could get yourself a proper 4 speed O/D transmission (200-4r, 700-R4). Taller tires will help a little, too.

[Fitz]

Originally Posted by Tonymolloy

I've just bought a 71 c10 a few weeks ago it's running a 454 aand th 400 box,12 bolt axle with 4-11 gears,at 55 mph it's revving at about 3500 rpm,what's the best solution to solve this and bring the revs down? A gear vendor overdrive? Or a different crown wheel and pinion? Cheers for any help

I used a Gear Vendors unit....lets the rpm's drop to about 2150 on the freeway at 70 AND I still have the 4.11 rear for the stoplight wars!
It's all a matter of what you want; a hot rod or a granny truck.

[LockDoc]

Originally Posted by Tonymolloy

I've just bought a 71 c10 a few weeks ago it's running a 454 aand th 400 box,12 bolt axle with 4-11 gears,at 55 mph it's revving at about 3500 rpm,what's the best solution to solve this and bring the revs down? A gear vendor overdrive? Or a different crown wheel and pinion? Cheers for any help

I swapped a 3.08 rear end in my '67 with the same engine/trans combo as you have and it worked out great. The 454 has enough low end torque to pull hard in the lower RPM range and it will lower your cruising RPM considerably.

I just replaced the whole rear axle.

LockDoc

[harrisoncole4]

Aren't gear vendor units around $3k though? seems like an expensive alternative to just re-gearing.

[Fitz]

Originally Posted by harrisoncole4

Aren't gear vendor units around $3k though? seems like an expensive alternative to just re-gearing.

$2600.00 out the door, but they DO allow you to run 4.11's on the street and, before someone starts on about 700R4's they won't take 650 ft-lbs of torque...no matter how you build them. The GV unit will handle 1000 ft-lbs! Once again, it's all about choices.

[Tonymolloy]

Cheers for all the replies,I reckon I will change the rear gears,less than half the cost,

[franken]

Why not price a 4l80?

[Tonymolloy]

Originally Posted by franken

Why not price a 4l80?

What is a 4180 ?

[leftybass209]

A 4l80 is a newer overdrive transmission. At this point, you have to weigh your cost vs. Fuel mileage vs engine longevity.

Engines used to turn at 3500 rpms regularly pre-overdrive. Some say that's how it used to be, but if that was the best way, why did things change?

Changing the rear diff ratio will net you a faster return on your investment. It would take many many tanks of gas to justify the minimal savings you would see by switching to an overdrive trans. Also, 4.11's are too low of a gear for some overdrive trans, rendering 1st gear useless for anything but heavy towing. That forces you to need to change diff gears anyways.

Switching any high dollar item (trans, engine) is most cost effective to change to something different when you have mechanical issues or failures.

[Tonymolloy]

Originally Posted by leftybass209

A 4l80 is a newer overdrive transmission. At this point, you have to weigh your cost vs. Fuel mileage vs engine longevity.

Engines used to turn at 3500 rpms regularly pre-overdrive. Some say that's how it used to be, but if that was the best way, why did things change?

Changing the rear diff ratio will net you a faster return on your investment. It would take many many tanks of gas to justify the minimal savings you would see by switching to an overdrive trans. Also, 4.11's are too low of a gear for some overdrive trans, rendering 1st gear useless for anything but heavy towing. That forces you to need to change diff gears anyways.

Switching any high dollar item (trans, engine) is most cost effective to change to something different when you have mechanical issues or failures.

Cheers for that info

[MARKDTN]

Originally Posted by leftybass209

...Engines used to turn at 3500 rpms regularly pre-overdrive. Some say that's how it used to be, but if that was the best way, why did things change?...

Not debating anything you are saying, but here is an interesting observation. If I assume a truck with 4.11 gears, direct transmission, and 265/70/16 tires (671 rev/mile) then the engine turns 2758 revolutions per mile. If I assume a 3.42 rear with 700R4 and the same tires, the engine turns 1606 revolutions per mile in overdrive. That is 58% of the direct transmission. If you rated engine life by total revolutions instead of miles, it may help explain why modern engines last about twice as long than the Gen 1 small blocks-before you take fuel injection into consideration. Also by keeping the RPMs down, you have much less inertia to stop each time the piston changes direction so will help too. Just a few random thoughts.

[leftybass209]

Originally Posted by MARKDTN

Not debating anything you are saying, but here is an interesting observation. If I assume a truck with 4.11 gears, direct transmission, and 265/70/16 tires (671 rev/mile) then the engine turns 2758 revolutions per mile. If I assume a 3.42 rear with 700R4 and the same tires, the engine turns 1606 revolutions per mile in overdrive. That is 58% of the direct transmission. If you rated engine life by total revolutions instead of miles, it may help explain why modern engines last about twice as long than the Gen 1 small blocks-before you take fuel injection into consideration. Also by keeping the RPMs down, you have much less inertia to stop each time the piston changes direction so will help too. Just a few random thoughts.

I agree. Reduction in RPM'S is a direct correlation to engine longevity.

Op, i do still think a rear gearing change is your most cost effective way to lower your operating RPMS. Worry about the trans later. There's no reason to swap a working trans for a potentially $2k+/- replacement.

[picklito]

I see you're in the UK, so I don't know how easy it is to find truck parts. But you should be able to find a used rear axle for a lot less than either a Gear Vendors or another trans. Could be an affordable way to get the RPM down and see how you like it. I've got a 3.08 in my 70 402 big block and really like it.

[Tonymolloy]

Originally Posted by picklito

I see you're in the UK, so I don't know how easy it is to find truck parts. But you should be able to find a used rear axle for a lot less than either a Gear Vendors or another trans. Could be an affordable way to get the RPM down and see how you like it. I've got a 3.08 in my 70 402 big block and really like it.

I'm going to use new parts from the states,I've got a company who is going to do the work for me,I'm going to remove axle,strip it down and get it powder coated then they will do the rest

['68OrangeSunshine]

I'd look for a 3.73:1 Posi.

[BULLzGOOD]

YOUR th400 HAS A DIRECT DRIVE HIGH GEAR. THE 454 HAS ENOUGH TORQUE TO HANDLE 3.42'S OR EVEN 3.08'S IN THE REAR. (WITH LIMITED SLIP, OF COURSE). BUT THERE'S A TRICK VERY FEW PEOPLE KNOW ABOUT. WHEN AMC STARTED USING THE th400 IN THE FULL SIZE JEEPS, THEY FOUND FIRST GEAR TOO HIGH. SO, THEY MADE THEIR OWN LOW GEARS SPECIFICALLY FOR FULL SIZE JEEPS. SO, HERE'S THE TRICK. THE GUTS OF ALL th400'S ARE INTERCHANGABLE. THE ONLY DIFFERENCE BEING THE BELL HOUSING BOLT PATTERN IN THE CASE. GET YOUR HANDS ON A th400 OUT OF AN EARLY FSJ (WAGONEER, CHEROKEE, PICK-UP) EVEN ONE THAT NEEDS REBUILDING, AS THE HARD PARTS SELDOM NEED REPLACING, AND SWAP OUT FIRST GEAR. NOW YOU'RE GOOD TO GO.

[cblock]

Bullzgood, interesting fact about the th400's that were on jeeps. But looks like tony malloys problem is with the higher speeds like road driving. And if you've a 454 on a half ton truck I dont see why you would need that low a gear on a rearend unless you were gonna go ofroading or drag racing.without an overdrive 3.08 or 3.42 are probably your least expensive and best choices.

[BULLzGOOD]

Originally Posted by cblock

bullzgood, interesting fact about the th400's that were on jeeps. But looks like tony malloys problem is with the higher speeds like road driving. And if you've a 454 on a half ton truck i dont see why you would need that low a gear on a rearend unless you were gonna go ofroading or drag racing.without an overdrive 3.08 or 3.42 are probably your least expensive and best choices.

you repeated my point, cblock, which was to use a higher gear ratio in the rear end. The fsj turbo 400 info was in case he wanted to haul a camper or some other heavy load. The lower first gear would simply ease the workload on the transmission.

[my67c20]

So here is some numbers to consider. For every inch in diameter your tire grows you gain 2 mph at a given rpm. For every inch your tire grows you reduce your rpm by 100 at a given speed. An overdrive transmission will reduce your rpm by 600. Changing rear gears to 3.42 drops rpm by 250, and 3.08 by another 250. Assuming these are available for your axle. Estimate gears to cost 1k per axle, or tires 1k per vehicle, or transmission 2k per vehicle, best bet is to go the tire route. Might be a dog off the line, but you have the granny gear, but you gain on the high side. Also with the gear ratio of the SM465, this may give you more range in 2 and 3rd. So @ 2500 rpm the max speed in 1st is 9, 2nd is 16, 3rd is 35, and 1st is 55. With 6" (34") taller tires @ 2500 rpm the max speed in 1st is 11, 2nd is 19, 3rd is 43, and 1st is 67. Just my 2 cents

[john]

Is your tach or speedometer accurate? With 4.11 gears at 3500 rpm and 28" tires you should be doing 70 mph.
http://www.angelfire.com/fl/procrastination/rear.html

[57taskforce]

Originally Posted by my67c20

So here is some numbers to consider. For every inch in diameter your tire grows you gain 2 mph at a given rpm. For every inch your tire grows you reduce your rpm by 100 at a given speed. An overdrive transmission will reduce your rpm by 600. Changing rear gears to 3.42 drops rpm by 250, and 3.08 by another 250. Assuming these are available for your axle. Estimate gears to cost 1k per axle, or tires 1k per vehicle, or transmission 2k per vehicle, best bet is to go the tire route. Might be a dog off the line, but you have the granny gear, but you gain on the high side. Also with the gear ratio of the SM465, this may give you more range in 2 and 3rd. So @ 2500 rpm the max speed in 1st is 9, 2nd is 16, 3rd is 35, and 1st is 55. With 6" (34") taller tires @ 2500 rpm the max speed in 1st is 11, 2nd is 19, 3rd is 43, and 1st is 67. Just my 2 cents

How did a 465 get drug into the conversation??? he's got a turbo 400... I'd swap the rear out to a 3:08 and be done then you can leave the tires you've got(assuming they are in good condition) and you don't have to go thru the trouble of swapping a 700r in there. This is probably the easiest and most cost effective route I can see.

[my67c20]

Sorry 57taskforce, i wrote that up for another message, and forget to edit out the stuff about a 456. Although everything else applies.