The Stovebolt Site Forums The Shop Area General Truck Talk how to tell what gear ratio?

hello again everyone

I hope all our florida 'bolters are keeping out of the way of the hurricane. Crossing my fingers for you.

I was going through my boxes of old removed parts I got from the PO of my 53 1/2 ton. I saw in one box two pieces that look suspiciously like the ring/pinion pieces that several online retailers sell to give the old trucks better speed.

When I was driving it back from purchasing it, I noticed it seemed to be really good at 50 ish miles an hour, no struggling or high reving and the tach stayed usually at or below 2000 rpms. I kept thinking "this old girl wants to go faster". But of course I didn't give in to the impulse But I am thinking that the gearing might have been replaced.

I was wondering if there is an easy way to tell what gear ratio is on the truck? I am btw completely ignorant, but working on becoming only slightly so.

Also, some of these old parts look like they're in good shape---I have no idea what most of em are, but is there an advantage to keeping it all? Or should I try and start giving them away? like an big old heater thingie (PO put in vintage heat/air) and what looks like a couple of fuel pumps, buncha brackets etc.

I can call the PO if I can dig up the old number, but thought it would be interesting to learn, also.

I don't know how you check the ratio on a closed driveline but I amd sure someone here does. I guess that it would involve turning the engine over by hand with one rear wheel off the ground.

The heater is another issue. If it is a round heater with no external intake, it is a recirculating heater. If it is the type that bolts up to the passenger cowl and takes air from the outside, it is a fresh air heater. Both units do fairly well on Ebay but the fresh air unit is more valuable.

If you're rpm is 2000 @ 50 mph, someone has probably replaced the stock 4.11 gear set with a 3.55 ratio.
You can play around with an rpm calculator and see what you've got.

Vehicle speed comes up 50.3 mph if you plug in
2000 rpm, 1:1 transmission ratio, 3.55 gear ratio and 15" tire radius(not sure exactly what yours would be).

How sure are you of your speedometer?
I gotta tell ya, 2000 @ 50 sounds absolutely perfect. That gives you an all-day fast cruise of 60 - 65 @ 2600 which, to my way of thinking, is just about ideal in every way. I don't know the numbers for your engine, but mine develops peak torque at 1500 rpm. Just above peak torque is the most ideal engine speed to sustain at a light throttle setting for best fuel economy.
This is to say that everything in your truck is completely relaxed and working at peak efficiency at 50 - 55mph. What could be better? If you got to a hill you needed to pull hard on, you'd downshift at 50 mph and come into 2nd at around 3100 which, again, is perfect for a high-demand engine speed, comfortably below the redline, but well into its power band.

I dunno. Any of you OF's think I'm wrong on this?
If it's working right, there's nothing to fix.
As to your spare parts, hang on to 'em all until you know what you have, and if you'll be needing them.

Just pull the rear cover off and count the teeth on the gears.

Hey Hotshoe! Thanks for the input...as far as the speedometer, not sure at all since it doesn't work I'm basing my guesses about RPM on the drive home when I 1st purchased...a friend drove in front of me so I wouldn't get above 50. I'd check in at stops about how fast we were going at our fastet. It just seemed like it wasn't struggling at all or revving high. So...in your opinion what is a good 'shifting strategy'? I know its nerdy but I am not sure about the difference on shifting these old trucks compared to modern manual shifters. 1st gear I shift out of really fast, which 2nd gear seems to want, but I haven't quite figured out 3rd...so you'd downshift to 2nd on a steep grade @ 50 or so?

4 on the floor - The rear cover...um. You mean to the differential? I guess calling and asking the manufacturer about the teeth is a good idea. This sounds dumb but you can just take that cover off w/out problems? And I'll check the shop manual...but I think they are skipping over lots of stuff that they assume any idiot knows. They never met me, clearly.

Ghostrider -- thanks for the link, I'll play w/ that right away.

Jack the rear of the truck up. Put it in neutral. Mark the driveshaft and mark a tire with chalk. Rotate the tire 20 times. Count how many times the driveshaft turns. If the driveshaft turns 41 times you've got 4.11's. (whatever the result is of rotations of the driveshaft, divide by 10).

Edit. If you've got a Positraction, simply rotate 10 times instead of 20 and divide by 10.

You would need to drain the rear axle oil before, or as you removed the cover. I don't think I would bother just yet, as there is an easier way.

First count the teeth on the extra ring and pinion you have, then divide and let us know what ratio that is. Do they look used? if they look used, and turn out to be the ratio you should have had stock, then it's a safe guess that the faster 3.55 gears are installed already.

To find the ratio, without removing the cover, I would jack up one rear wheel, put the transmission in high, (1:1), pull the spark plugs so the engine turns easier.
Then have someone turn the engine over by hand as they count revolutions of the crankshaft. At the same time you count the revolutions of the rear wheel, and when it has turned 20 times stop, and ask the guy turning the engine how many turns that was.
now for figuring purposes lets say he turned it 35.5 times, divide that by 10 and you get the ratio of 3.55.

Now don't get confused by the 20 turns of the wheel and then only dividing by 10. The reason is the other wheel was still, and the differential action makes the one wheel that does turn turn twice as fast as it should. It would be less confusing mathematically to jack up both wheels and let them turn together, but practically it's hard to keep them turning together, another person would be helpful if you did.

You need to turn the engine with the transmission in high because you have an enclosed driveshaft and can't see it turn.
For those of you that have an open driveshaft just leave the transmission in neutral and turn the drive shaft instead of the engine.

Confusing enough?

Grigg

Taking the differential cover off is easy but you'll want to drain/siphon the gear oil out into a container first or it will end up all over the ground. You might want to have a new gasket and fresh gear oil ready for reassembly before you break into it.

Okay. Speedometer doesn't work, tachometer exists and does work, properly. Right? All that crawling around under the truck sounds like dirty and unnecessary work unless you have good reason to believe that your ratio is inadequate, and you want to know what you have, so you can speculate on what you need.

Let's say, just for the sake of argument, that your engine is a '55 235 thriftmaster. Maximum torque is 200 pounds-feet at 2,000 rpm. Maximum horsepower is 112 at 3700 rpm. Therefore, your sustained-full-load operating range (outside of which you will probably hurt your engine, if you are in the habit of driving the bag off your truck) is 2,000 - 3,700 rpm. Above that range, your valves will begin to float (not fully close) and your engine will begin to hammer itself apart. Below that range, upper-cylinder temperatures will quickly rise and your engine will begin to burn inside.

These numbers are not set in stone, even for the '55 235. I'm just quoting them as guidelines, so as to make a point.
Let's say you're working your truck for a brick plant, doing deliveries. These trucks are so pretty, sometimes at this late date it is difficult to remember that they were designed to work like oxen. So, okay. You work your truck hard. You need to keep up with traffic as much as you can while you're loaded, but you also want it not to wear out for a long time. What you do, then, while you're running with your foot on the floor all the time is avoid overspeeding your engine by picking a higher gear, and avoid lugging (underspeeding) your engine by downshifting. Now, a lug condition exists when you are running with an open throttle, close to the bottom of your powerband, and the engine is not picking up speed.
If you are in the midrange of your power band, say, from 2400 rpm to 3300 rpm, you are okay.
If you are not at full throttle, you are okay.
To make your engine go further between overhauls, avoid the top end of your powerband except when you absolutely need to, as when lifting a full load of bricks from a traffic light up the steepest grade you can find around where I live, which is plenty steeper than anything around D/FW, unless you have acquired some of these;

[img]http://thumb11.webshots.net/t/55/455/1/17/97/2005117970096215731NlbdoZ_th.jpg[/img]

of which I didn't see any the last time I was down there.

So to answer your question - at long last - for the kind of driving you'll most likely be doing - empty, on flat ground - since you'll rarely be using full throttle, drive for economy: Use the least throttle and the least engine speed to get you up to your cruising speed, then maintain a steady speed. From the traffic light up to 50, you can probably shift at 10 and 35 mph. Or, since your tach is working, shift up at 2500, and down at 1500. Use a light throttle, just enough to pick up speed. If you need more power, use more throttle and more engine speed. You won't hurt it: Mine likes being run hard when it is loaded.

The best fuel mileage and longest life that can be achieved by any engine is to be found near its peak torque speed, with a steady, light throttle.

The above quote is from another thread regarding re installation of a missing parking brake. I think this is why GirlDriver is asking about gear ratio.
Parking Brake Thread

wow, a lot to think about. Thanks so much.

I can't wait to get this brake issue handled, then move on to the fun stuff.