rear axle ratio

Hello,

I did a search of the forums and found a few other posts but no ready answers.

I have a 1936 C-30 with a 4 speed. I do not know the rear axle ratio but it has to be numerically very high as 40mph is top end.

Doesn't look like any options for a lower (numerical) ratio to fit in the original differential.

A more modern rear end swap would be the logical choice.

Can anyone recommend a good unit as a substitute?

Something that might accommodate the original hubs/wheels since I dumped almost $1000 in new tire/tubes and flaps not so long ago.

Some later model International unit perhaps?

Maybe I should give up and go with a Ford 9" or 10.5" unit and try and sell my nearly new tires?

I am open to ideas, I want to achieve highway speeds, 60mph anyway.

Overdrive units are too expensive.

Tossing around the notion of a SBC transplant while maintaining outside originality/appearance.

Thanks!

Craig

Part of the answer will depend on how you plan to use it. Do you want to keep the full load carrying capacity that the truck has now, or will a half-ton rear axle meet your needs? What is the bolt pattern of your existing wheels?

Dean

Like Dean said, it depends on how you are going to use it normally. Your statements about the overdrive units being expensive seem to contradict your statement about going with a SBC transplant or even the rear end transplant. There are costs associated with every change. Some are up-front costs and are very obvious, but will come with a lot of hidden costs. I would think the OD would be less expensive than either a motor swap or rear end swap, given all the other costs that go along with each one.
It would seem to me that if you are prepared to transplant the rear end, that would be a first step as long as the bolt pattern can be resolved easily. Updated brakes would come along with the rear end. But with a rear end swap, there gets to be an issue whether the stock motor produces enough torque to spin that lower ratio rear end.
If all you add is the OD to a stock rear and stock motor, will the higher speeds on the road be too much for the wheel bearings at all corners and the old axle shafts/spindles. And what about the old stock brakes at higher speeds?
It really boils down to a balanced system and all the components need to work together. My point is, changing one component will affect the other elements of the system - in some cases adversely.

Points well taken, from a cost perspective my comments about an overdrive unit were predicated on my already having an SBC and a couple of choices in rear ends available to me so I would not have to spend anything further there. The labor would be all me.

The truck will not be used for any heavy hauling so it is not a great concern of mine to lose pulling power.

Updating brakes and so on as needed is doable as well.

It is not a daily driver by any means just a toy really.

Not sure what there is in terms of bolt up overdrive units available that might work. I recall searching out an overdrive unit for a '20's era auto I once had and it was cost prohibitive. Is there something from the '30's that would have been available I should be searching for?

Not trying to set speed records merely trying to avoid being a navigation hazard for others.

Craig

It used to be common to take an old 3 speed transmission, turn it around and install behind the main trans to make an overdrive. Takes some machine work, and isn't the best in terms of efficancy, but was done quite often.
Haveing any one engine lieing around is a lousy reason for an engine swap. Engine swaps should be thought out and planned so everything works together. In this case, it is pointless unless a rear ratio change is made. More money, when you add up all the small stuff (and there is a lot) to do a good swap, I doubt you could come away for much less then a gear vendors OD unit. There is an upper limit to driveshaft speed that must be considered before you start.
1st you need to know what ratio you have (there have been many threads on how to find this out), next you need to know what the rev/mile of the wheel and tires you have now, to determin how fast you need to turn the present diff to reach 60 MPH. Then you need to determine if the present brakes are good enough to stop it from that speed with the load you plan to carry if any. With this info, you can then decide if a simple OD will get you where you want to go, or if you need to completely replace the drivetrain. With a truck this old, it would be a shame to ruin the looks of the old wheels and tires, I don't think modern wheels would "look right" on a C model, just my opinion.

? What size tyres are you running are they 7.50-20's.


Trevor

Yes, 7.50 x 20 on the back.

I looked at the specs on a Model C-30. The 213 cid Engine rated at 78.5 brake HP at 3400 RPM. The options for rear axle ratio are 5.285, 6.166, and 6.60 to one. Fourth gear in the transmission is 1:1 ratio. A 7.50 X 20 Tire should measure about 37" in diameter. Given those figures your truck should run much faster than 40 mph. That is unless I am calculating wrong. Tire diameter times Pi 3.1416 divided by 12 gives circumference in feet. Circumference times max rpm times 60 minutes in an hour divided by 5,280 feet in a mile divided by gear ratio should give the top speed. So a 37" tall tire should go 56.7 mph with the lowest speed rear axle. With the 6.166 axle ratio it should run 60.7 mph. MM

I have used Gear Venders overdrive for thirty years. First I had it on my totally stock R120. A couple of things changed for the worse. First, the engine could not pull even a slight rise in the road, in OD, which is a big situation here on the West Coast where all of the land stands on end. Second, the original R120 brakes could not deal with the increased speed and after about 2.8 seconds at 60mph, the brakes all but failed because they could not get rid of the extra rapid heat buildup. I got the last set of new brake drums that DV Aldous, in Vancouver had, so I know that the brakes were as good as they could be. Third, the SD220 did not have enough power to push the R120 much past 60mph, ever. This was one of the biggest over $3500 mistakes I have ever made. The GV OD worked really well when I put a 6.9 diesel in the R, but it had enough torque. The R would easily it 85 mph, in OD, with the big Diesel pushing it. I had added Dana 44/60 axles with 3:73 ratios and four wheel disc brakes so it could stop the 5577 pounds. (with me in it) Somewhat less with me on the curb. Too much pie, I'm guessing. It is the pie that has made it next to impossible for me to get into a C series and I don't like where the tow company puts their hook when I am pulled out. I am thirty pounds short of having two postal codes for my butt.
This GV has been pressed into service in another application where the rear axle has a 4:10 ratio and the OD turns that into 3:20. This car has had four wheel disc brakes added because it is necessary in our traffic and highway speeds.

Another possibility may be to find a 5 speed with overdrive 5th.
Or a 3 spd auxillary from a. 60s vintage Loadstar dump truck like I drove at one time. That would give you under, direct and overdrive.

In my opinion the world doesn't need any more D@%n Chevy motor hot rod, street rod, rat rod things. Like looking at jugs of milk in the grocery store all basically the generic same thing.
I am not hard core against rods, one local guy has built an old Dodge with a Desoto hemi and it is pretty cool. Another has a Model A sedan with a Buick nail head and finned aluminum brake drums.

Keep the DNA all IH! Just my opinion, don't intend to hurt anyones feelings