The Stovebolt Site Forums The Parking Lot Big Bolts 3 speed Brownie ??

I'm still trying to figure out a solution to my low gear ratio. I was thinking about changing out the rear diff. to a two speed, until somebody mentioned a 3 speed brownie with OD. (This is for my 1966 C60). This would be a divorced tranny. installed a short distance behind the stock tranny.

Any thoughts?? Would this be a viable solution?? Anybody have any experience with this kind of set-up? Thanks...

I drove one for a lot of years in a B-73 Mack with a 262 Cummins Diesel- - - -once you get the hang of shifting it, it's hard to imagine not having one. It's particularly useful when you need a couple of intermediate speeds when pulling a heavy load up a hill, as every main box gear becomes three speeds. Once we got the truck moving, all the shifts in both boxes were done without disengaging the clutch.
Jerry

My main problem is that my truck will only drive 40 to 45 MPH due to the low gears. So would a 3 speed brownie with OD help in this situation, with the stock rear diff. and tranny? I just want to get the truck up to 55 or perhaps 60 MPH. Thanks for your help.

Most Brownie boxes have an OD ratio of around 0.8:1, or about a 20% overdrive. If you're running 45 MPH in top gear now, a 20% OD would bring the top speed up to 54 MPH or so. You might also be able to nurse a little more speed out of it by installing taller tires- - - -what size is it running now?
Jerry

Hy Iron Clad, the short answer to your last question is yes, the overdrive will give you a higher top speed, as will taller rear tires, and an overdrive main transmission, hope that helps.

Tire size on my truck is 9R-22.5. So, how does that work? I mean, with an aux. 3 speed Brownie, the gear ratio still doesn't change in the rear diff.? Sorry for the dumb questions, I'm tired and my brain is work well tonight, LOL.

First off 40-45 mph tops for a 66 model C60 sounds slow to me... do you have a two speed rear stuck in low? What is your current rear axle gear ratio?

Here's a calculator to figure speeds and RPM
http://www.onlineconversion.com/bigger_tires.htm
Your 9R22.5 tires measure about 38" tall. Plug the ratio for the auxiliary transmission in the box that says "TransCas Ratio:"

Not all auxiliary transmissions have an OD gear, some are direct and only lower ones. If it is OD there are different OD ratios available depending on the model number 0.81, 0.84, , 0.75, and 0.73 are common ones.
Here are some model numbers and ratios
http://rides.webshots.com/photo/2499436910080251109KaJEMa
http://rides.webshots.com/photo/2623210880080251109VGJOGn

An Overdrive transmission works by turning the output drive shaft faster than the input.... the output is "overdriven". So for a given engine speed you can go faster in OD than direct because the driveshaft and therefore tires turn faster.

Grigg

My Dad had a 65 C60 with a 348 4speed and a 2 speed rear end. It would run all day loaded 60. It would run up to 65 but that was pushing it.

6066gmc guy has an article on the aux with the model numbers and gear ratios. You really dont have to worry about shifting the aux. I leave mine in od when unloaded and put it in direct when loaded. I think mine is a 5531 with a .74 OD.

Hy Iron Clad, a little explanation of how the overdrive works, when in overdrive the output of the auxilliary transmission (the driveshaft to the diff.) turns faster than the input (driveshaft from the transmission) through the function of the gears in the auxilliary transmission, and hence a higher top speed at the same transmission output speed, hope that helps you understand.

Thanks folks for all of the input. Unfortunately, the 3 speed Brownie I had my eye on fell thru, it isn't heavy enough for my truck. So, this raises the question, should I countinue to search for an auxilliary transmission or do a rear diff. swap???

Grigg, I'm not sure what the gear ratio is on my truck yet but I think it's in the high sixes or low sevens. When going 45 mph, the engine RPM is very high, I'm sure well over 3,000 rpm.

Thanks 3B for the explanation, that helps.....

It really does sound like you may have a two speed and its in low gear, do you know what axle you have?

http://wellertruck.com/lit/Weller_Diff_Catalog_2011.pdf

Use this the help identify what you have, you should be able to get casting numbers off the carrier housing.

Thanks for the link Jake. Unfortunately it's a single speed axle. If you click on my 1966 Chevy C60 2-Ton link below this reply there are some pictures of the rear axle. I'm still not sure what the ratio is, however I'm thinking of lifting the rear end and marking the drive line and turn the rear tires to count revolutions, etc. to figure it out.

OOPS.....Click on the C60 link below which takes you to my Stovebolt Gallery page then click on the pictures link from there to see the rear axle.....

The rear differential swap or just the center section will be lots less work and cheaper if you can find a ratio you like. On the other hand an auxiliary can be more useful because of the extra gears... but even 0.73 OD won't do but so much to overcome exceptionally low axle gears. All depends on what you have now and what you want to do in the end or what you can live with as far as speeds and RPM.

Grigg

It's a lot simpler to just mark a tire and the driveshaft, and have someone drive the truck straight forward or back one turn of the wheel while you count the turns of the driveshaft. There's no involvement of the differential gears that way, and you'll get a much more accurate count of the pinion gear revolutions.
Jerry

Goto page 82 of that link I posted or page 92 of the PDF. Looks like you got an H150/H170 rear.

The best ratio is 5.83 and here you can find a few places that may have the ring and pinion set in stock.

http://truckpartsinventory.com/heavytruckparts/search/3869006

These are usually around $300 $450

I ran the numbers and if your tires are 36" tall and top gear is 1:1 you should run.

50mph @ 3400 RPM With 7.20 gears
50mph @ 2900 RPM With 6.17 gears
50mph @ 2750 RPM With 5.83 gears

Thanks guys, this is the kind of information I'm looking for. Another testament to how good this site is! Jake, thanks for the link, this is something to seriously think about. One way or the other, my truck will go faster, LOL. This weekend I will look for numbers on the rear axle and look into the gear ratio. This weekend I'm spending most of my time working on the truck anyway. The front bumper is coming off, (I'm building one), and I'm going to start sanding off the paint in prep. for a new paint job. I may even remove all of the bed wood. (All this after I finish stacking cord wood, winter is coming fast....)

hi guys no matter what rear - single or two speed the ratio you have is what is !! if it was me id pull the rear cover gasket and check the ratio or count teeth /usually the ring gear is marked with some kind of #s ALSO ikm sure the 90wt rear oil should be changed or checked ? drain it and see what its like !my old 66 c-60 w/ a 366 -real short dump box would run 65 mph /when i left the quarry with a load of rock i usually made all the peterbuilt,and kenworth big dump truck snobs use the slow passing lane on the right ! i would ride there bumpers till they pulled over ! had many that would later tell others i must of had a big cubic inch hot rod motor in there,never seen an old chevy pull that hard loaded on a hill ! it was a good running 366! let the group know what happens w/ your truck and ratios!

Thanks wahl4m! Cool story too. I will be working on the truck just as soon as I finish with the 6 cords of wood, LOL.....I'm close, about 1.5 cords to go and my wood shed is just about full, just in time for winter......I believe I'm going with a new ring and pinion set as Jake45 suggested. Well, and you suggested also.