The Stovebolt Site Forums The Shop Area Driveline Least effort to go 65 mph?

I have a 3600 with a 3 speed column shift.
Would changing the differential gears be enough to achieve 65 mph?

Thanks everyone!

An overdrive transmission might get you there.
This is a good calculator for making those comparisons
http://www.onlineconversion.com/bigger_tires.htm

For driving comfortably at 65 and not just occasionally getting it up to 65 you may also need a rear axle gear change.

If you're then regularly driving that fast also consider how good and reliable your brakes are or are not.

To be safe might take more than just the "least effort"

Other than finding the extremely scarce 4.10 ratio HO-72 rear end which swaps into the 3/4 ton axle housing, you don't have many other options. Be aware the complete center section must be changed- - - -HO-52 and HO-72 gears do not swap. Another possibility would be to install a Dana type 60 3/4 ton rear axle, which is available under Dodge, Ford, International, and GMC trucks. That one has several ratios of replacement gears available.

Be sure your brakes and steering are capable of handling and stopping at the higher speed.
Jerry

If you get as far as an entire rear axle swap...
The GM 14 bolt rear axle in my opinion is a better swap for the AD 3/4 and 1 tons than a Dana axle.
One common configuration of the 14 bolt is exact same width as the original axle.

Much to read about this conversion in the 1 ton section.

Okay thanks for the tips folks. It sounds like it's a pretty big job no matter which option I go with.

If/when you find 4.10 gears for the existing axle that is an easy weekend project to swap with good noticeable results. Better, but not likely to achieve a 65mph all day cruising speed.

You could also consider a 3 speed OD trans (especially a Saginaw with first gear synchro). Worked great on my 3600, cruised all day at 65 70 mph. A very easy swap.
don

Change your speedo gears to make it fast - lie to yourself. It's the cheapest option.

I switched to the 4.10 rear end and installed factory 19.5" rims with 8R19.5 radials on my '57 3600 with a 235 about three years ago. Can easily run 65 on the highway, but I don't do it much. It has more to give at 65, but I have never taken it over that.

Matt

Better late than never.. I ended up using a shop called Hudlow Axle in Rossville, GA to swap my original axle for a 1977 F-250 (Dana 60 with the ratio changed to 3.55).
My original 8 lug rims fit just fine and still the original 3 speed transmission. Only the e-brake was a little challenging to configure.

I'm very please with the results - the truck easily cruises at 60 mph with 2800 rpms.

Thanks all for your great advice!

Dave

If you could give some details on how you met the challenges of the Ebrake, some of us would really benefit from that knowledge.

I swapped out the entire rear end in my 49' 3600 with a dana 60 from a 69' F250. It's the almost the exact same width from backing plate to backing plate and the 8 lug rims bolt right up. i had to have the driveshaft shortened and the u joint yoke changed out to mate up to the dana 60. It was a pretty simple project really and now I have a a rear end and brakes that I can readily get parts for and gear ratios if I decided to ever change it. I'm at 3.73 gearing, 16" rims with the biggest tires I could put on it and still look appealing. Tire diameter is almost 30" I can cruise 55 at 2300 RPM. I'm still running the stock suspension, all drum brakes, and stock steering. So I don't want to go any faster. I adapted the dana 60 e brake cables since they are held to the frame rails by a clamp it wasn't hard to do.

BZT - do you still have the SM420? Did you have to do any mod's to use the stock AD suspension - springs and shocks? Are drums and shoes readily available for the '69?

It has the original 4 speed tranny. I had to weld spring perches at a different angle on top of the existing ones to get the correct angle for the dana 60 to sit at. The old lever action shocks went out the window and I'm using standard shock absorbers. In fact I put on the same shocks the 69' F250 would use. And yes new parts are readily available over the counter for Dana 60's. I think I took pics along the way. I will see if I can dig them up. I marked the drive shaft for length and took it to a shop. The guy was great. I told him I was mating up to a dana 60 and he knew exactly what u joint yoke I needed. He cut it down and welded on the new yoke, new u joints and balanced it for a $100 bucks.

Thanks. RockAuto has them (rear drums) for $40/each in that 8 lug 6 1/2 pattern. Appreciate if you can dig out the pics of your fab work.

I have a line on a rear-end out of a '71 1-Ton. What are the possibilities that it would bolt into my 3/4-Ton axle?

Wheel mounting surface to wheel mounting surface, review bztguy's last thread.

Ed

Sorry, I should have clarified, I'm looking to swap the gears out of it, not the whole axle. The guy's not sure what it has though, any info I can provide him to help determine it?

"The guy's not sure what it has though, any info I can provide him to help determine it?"

Is the confusion on which rearend or the gear ratio? I'm assuming the 71 is a GM product. If so it Should be an Eaton HO52 or HO72, in which case the differential is an easy bolt in deal on your truck. Lots of photos online to help identify.

If the question concerns gear ratio, then: If lucky you May find a tag on a cover bolt, or you May find a stamped set of numbers on the top of the axle housing just to the right of the differential. Since they're hand stamped they will likely be shallow, dim and covered with rust and or grease. Short of finding a tag the easy way if the rearend is still in the truck is raise the truck so both rear wheels turn. With the tranny in neutral turn one wheel, if the other turns in the SAME direction, it has a locker or limited slip rear. In that case mark the drive shaft (ds) and tire, turn a wheel one complete revolution while counting the ds rotations. A very little past four ds rotations is a 4.10 gear, a little past 4 and a half turns is a 4.56 or 4.57.

When one wheel is turned and the other goes in the opposite direction, then it's an open or non limited slip gear set. In that case you need to turn the marked wheel 2 complete turns and count ds rotations. Or, turn one time and multiply the ds rotations by two in order to determine final drive ratio.

You can always pull the differential cover then count and divide the ring gear tooth count by the number of pinion teeth.

Thanks Moparguy, that is a wealth of info. Hopefully I can walk this guy through it to determine if it's a match for mine.

He sent some pics. I'm going to say this looks suspiciously like an Eaton.

https://goo.gl/photos/BhdDPxdgJXqWyrcm8
https://goo.gl/photos/YNY8MLSMP1kusSM67

If I can get him to confirm the gears at 4.10, what would a list of all the parts needed from the donor be? This will likely be stripped by him and shipped, so I just want to be sure it's everything.

thanks!

Yes on Eaton, looks to be an HO72, image of the front of the differential is not the best. Really, all you need is the differential. Now, if you plan to go to a different transmission and a one piece driveshaft, then it'd be nice to have his drive shaft. However, not sure but it may well be a two piece.....but if not you might be able to have it modded to work in your truck.

If I've overlooked anything with my advise I'm betting someone will chime in. This is a pretty easy/common upgrade.

Here is my report on changing to 4.10s in my 3600 earlier this year.

Added in edit; just a thought but you might ask the seller to include a few of each (or all) the fasteners removed while pulling the differential. It wouldn't be unusual for some of your 48's to need replacing due to rounded shoulders, buggered threads or such.

Good Luck, RonR

Thanks RonR,

I will report back on how this progresses. In the meantime, does anyone have links to a detailed gear swap? I'd like to read-up on the process as much as possible.

I can give the cliff notes version, no torque values or such.

First order of business is to inspect the replacement differential to make sure it's good. If in doubt seek help in making the determination. Look for obvious chipped teeth, rotate the pinion, is may be a bit stiff but should be smooth. Rock it back and forth checking for freeplay. Just look things over for anything that looks hokey.

If the new unit passes inspection;

Lift the rear of the truck, clearing both tires and lower to jack stands.

I pulled the wheels.

Disconnect and get the driveshaft out of the way. Don't forget to wrap tape around the u-joint caps.

Spend a few minutes with a screw driver and scraper of choice removing decades of grime and gunk from the circumference of the differential and the bolts you'll be removing.

Place a drain pan under the differential and remove a couple of the bottom bolts on the front side.

Remove the 8 axle bolts on each side, be prepared for a small amount of lub to run out.

You can now slide the axles out 10" or so.

When the lubricant has stopped draining, remove all but one of the bolts from the front of the differential.

Now place a floor jack or better a transmission jack under the diff. A helper would be nice here, but not necessary if you're careful.

Remove the last bolt, lower the diff and roll it out from under the truck.

I don't think I pulled the rear cover since I'd had it off recently and cleaned the housing while changing the rearend lub. With the diff out of the way you can see what kinda crud is laying in the bottom and after a bit of cleaning tell if the rear cover needs to come off for a thorough wipeout/cleanout.

If you are very careful and lucky and didn't damage the differential gasket, you might be able to reuse it. I did. I also reused the metal axle gaskets, taking care to put them back in exactly the same orientation they were originally in.

Best plan is to have new gaskets and lub on hand as you reverse the process and put things back together.

I highly recommend 6 pt sockets since the fasteners will likely be quite tight and crud welded in place. The axle bolts have shallow heads and you don't want to round em off.

A Google search will pull up lots of information to fill in the blanks in my quicky how-to.

Good Luck

RonR

That is awesome and a different procedure than I was expecting. Thanks RonR, I will be printing that off when the time comes. I won't be able to inspect the gears myself when they're in the donor, but hopefully I will get some good pics from the seller to post here for opinions.

Got the guy to open up the rear, he says it's actually a 5.14 gear. Is this out of the question for my stock 216?

https://photos.app.goo.gl/X5bcG289PDlFHLTo2

Are those 15" or 17" wheels you have? If they are 17", 4.56 gears are about all you can pull with a 216. And it will go 65. The old engines were built for higher rpm than modern engines. Have you checked to see what ratio you have now?

Goach, too bad this wasn't a separate topic....that said your truck "Should" have 4.56 gears now. A 5.14 gear will slow you down. Now if you were pulling a 50's style loaded horse trailer in the Rocky's then 5.14s would be a nice improvement.....more towing power.....less top end on the flats.

Thanks RonR, I thought this ratio might be going in the wrong direction. Too bad, I will keep looking.