4.56's!!!

[shelby987]

Did the raise and rotate trick to figure out the gearing in the 3/4t I just bought......well, the gears are 4.56's....I thought possiblye 3.73 or 4.10....nope 4.56's.....but I am making 13-14mpg on the highway.......how is this possible? (250/3spd)
I have no tach so I have no idea what the engine is spinning at when i am doing 70mph....nor do i care to find out at this time....I was thinking about putting an overdrive in there.....but now I am thinking when I put my built 250 in there....maybe I will just drop in 3.73's.....as this will give me almost a 20% reduction right there.....and the increased power and torque of the engine should allow the 250 to work fine with the new gears.

any thoughts?

[stueber68]

change the gears and drop in a 700r4. The 700r4 has the lowest 4th gear of any automatic aside from the allison. You will have all of the torque that you need and have the mileage. Now if you do go overdrive make sure you have the lockout torque converter set up right, you don't want it to engage to early on a 250.

[shelby987]

Appreciate the input, but I specifically hunted down a truck with a manual trans.....I would rather have a manual with no overdrive than an automatic with overdrive.....I was thinking about using an older watson under/direct/overdrive unit so I could keep my current transmission and running gear, but I haven't been able to find one.

[FormerMember]

I am not sure I would mess with it, if you're getting that fuel economy. Unless the motor's tourqe curve meets the gear ratio?

Dropping RPM doesn't always mean better fuel economny.

[Yukon Jack]

70 mph with 4.56 gears usually takes some pretty high rpms. If you have say 29 inch tall tires, you wold be pushing about 3700 rpms at 70 mph.

How did you figure out your gearing when you put the rear end in the air? Do you have an open differential? I think I recall that an open differential requires you to rotate the tires two full revolutions to figure out the gearing, where as a locked rearend is much simplier, you just rotate once and count the driveshaft rotations and that is all there is to it.

Either way, that is some great gas mileage especially if you can get that mileage going 70 mph with 4.56 gears.

[shelby987]

I just jacked up one wheel, and counted the revolutions of the the driveshaft yoke for one revolution of the tire.....could I be wrong? (option sheet doesn't list a posi rear....so its likely an open diff (very original farm truck), so its doubtful any changes or upgrades have been made.

The gearing change will be required when I put in my new engine.....it will definately suck down the gas....right now its a 4bbl 250, high compression, roller rockers, forged bottom end, and true dual headers, but it will be brought down to a 2bbl before the install is done.....either way, at 3700 rpm, its likely to be a little more thirsty!

[FormerMember]

Sounds like you may want to check again.

Does the diff have a tag or stamp on it?

I'd match the cam to the gears. You may find that it's good around 3.54 to stay in the torque curve of the motor. Lug a motor down and it's not healthy or economical.

[wolfthing2000]

Does it have the Eaton rear. If so, my bets are that it is 4.56.

[70rs/ss]

Originally Posted by Yukon Jack

70 mph with 4.56 gears usually takes some pretty high rpms. If you have say 29 inch tall tires, you wold be pushing about 3700 rpms at 70 mph.

How did you figure out your gearing when you put the rear end in the air? Do you have an open differential? I think I recall that an open differential requires you to rotate the tires two full revolutions to figure out the gearing, where as a locked rearend is much simplier, you just rotate once and count the driveshaft rotations and that is all there is to it.

Either way, that is some great gas mileage especially if you can get that mileage going 70 mph with 4.56 gears.

Pull the diff cover and look at the numbers on the ring gear. It will either have the ratio stamped (3:73) or the mathematical version (14:63 for example as I cannot remember the exact numbers) in this case you devide the bigger number by the smaller number and it ends up with 3.732 or 3:73. Spinning the tires and counting revolutions is not as acurate, and heck you can change your diff lube while your at it!

[Coleman]

I've got a 68 K20 with the old round (large 10 bolt) rear end. It has a 4.56 rear end and with this type of differential I've been told you can only get a 4.11 grear. I did see that someone was making a 3.73 gear for this rear end but it's not readily available. I've got 30" tires and I can barely get up to 65mph with my 4 speed manual (it has the granny gear). At 70mph it would be screaming and probably blow up if I ran it there for more than a minute or so. Plenty of peddle left but the rpm's are way, way up there. I'm now looking for a used 14 bolt corporate rear end so I can move to the 3.73 grear and get better highway speeds.

[Yukon Jack]

Welcome Coleman. Sounds like you have the Eaton rear axle.

Shelby987 - if you don't pull the cover, you can always put a mark on your driveshaft and have someone watch it and push the truck one full tire rotation, this will give you the ratio accurately.

My 69 has 4.56 gears - luckily I have large tires so it helps bring the rpms down a bit. I don't normally go more than 55-60 in it - 55 is right at 2500 rpm for me.

[Longhorn Man]

4.56 is stock on an inline with a manual...odds are that's what he has.
I have a 4.10 center section I have been trying to sell... 100 bucks. figure shipping from 43228 (columbus ohio) @ 150 pounds.
Coleman... you say you can hardly get to 65? Whats wrong as to keep you from that? Even with those kinds of gears, you should be able to get clost to 100 MPH.

[70rs/ss]

Coleman... you say you can hardly get to 65? Whats wrong as to keep you from that? Even with those kinds of gears, you should be able to get clost to 100 MPH.



I think he means not without it sounding like its gonna blow! 100mph with 4:56's will wind it up pretty tight, 65mph is more reasonable and within reasonable RPM's. Hell drag racers hit 200-300MPH with 4:56's through 5:?? and 6:?? gears, but for only 10 seconds or less!! So it is more about the motor being at a happy RPM than ability, IMO.

[shelby987]

with a 70 I should be looking at a dana 60 rear and a dana 44hd front right? How much am I looking at to change the gears to 3.73's (can I go lower than that??).

The decision of which gears to buy will likely be based off of how the new engine feels/possible dyno run.. for those concerned about me just throwing in gears for the sake of getting away from the 4.56's

[Coleman]

I probably need to start a new thread so I don't distract the shelby987 topic, my mph per gear are:
1st (granny) ~0-5mph, usually start off in 2nd
2nd I can run up to 20mph.
3rd I can run up to 45 and shift into 4th.
4th gear, my tranny is screaming at 55/60mph, I might have some other issues going on here. Like I had mentioned, there is still plenty of peddle left, I can run the truck up to 75mph but wouldn't want to keep it there long. So loud you can't carry on a conversation. Kind of stuck between trying to find an overdrive unit or start replacing the differentials, rear and front. Costly either way.

Specifics:
68 K20
rear: HO72(?)
Engine: 327, needs some help, a little weak
Trans: sm465
Transfercase: Rockwell T221

[Longhorn Man]

OK... my fault. Disregard my comment.
Also, the gears I have, i failed to notice you have a 4X4.

[stllookn]

"Even with those kinds of gears, you should be able to get clost to 100 MPH." - LonghornMan

ROFLMAO...Ask him how he knows!!!!!!! I love those speedo shots!!!!!!!

[roger0080]

Check this out:

http://www.angelfire.com/fl/procrastination/rear.html

4.56 @ 3000RPM with 29" tires is approximately 57MPH

That is pretty much what my Factory tach shows for with 4.10's but I think my tach is off a little I've always suspected.

That calculator link says you are doing about 3800RPM at 70MPH! That ol 6 banger must be shak'n and ready to blow apart at that speed! Heh heh heh.

[Robert1970C20rstbukt]

Randy's Ring and Pinion had been making 3.90 gearsets for the HO52/72 rears but I don't think they have them anymore. The best you can do is find a set of 4.10's or get bigger tires.

[tommy graham]

You may already know this formula. Hope it helps.
Gear ratio x MPH x 336 divided by tire diameter = RPM

As Roger0080 stated:
(4.56 gears x 57mph x 336 = 87333.12) divided by 29" = 3011 rpm
(3011 x .7 overdrive ratio = 2108 rpm @ 57 mph

tg