Gear ratio and tire size 833 4 speed

[duster 344]

Hi everyone, yesterday in the tall gears and 4 speed trans thread, aj/forms posted some pretty interesting numbers on gears and the 833 4 speed. witch got me think about exactly what gears, and even what tires to put on my duster. So I put together this table (of sorts) to illustrate what I was really contemplating.

The top line is the standard gears in a 833, 1st through 4th. then the gears converted through the rear end gears for each set of gears. After that is a chart of engine rpm at 65 mph in fourth gear for each set of rear end gears, with different size tires in different columns.
Taking in account, when I looked up my cam, it was recommended for 3.55 gears. And best between 2500 and 5800 rpm. I decided that 3.55 gears with 28 in tires is what I would probly want to go with. Good drivability, and could probly do alright at the track on Wednesday nights. Using this table you can compute about any thing you want using fractions, for example I was wondering what it would do at 5000 rpm.


65/2769 equals x/5000

By deciding 5000 by 2769 to get 1.8
Then times 65 by 1.8 to get 117 mph at 5000 rpm’s , with a fairly stout engine you could reasonably pull 13s in a quarter I would think. you could go on to check rpm’s in different gears to give you an idea when to shift. Of course this not an exact science and every car is a little different hp, weight and what not but for a basic idea I thought it was helpful, I guess my question is. Does my logic make any sense?

 

[413]

So if you want to know what 2.76’s feel like with a 2.66 4 speed? Put 3.91 in the car and take off in second gear all day!

yes that is how silly using a 2.76 with a 4 speed is, and also illustrates why most manual trans cars had at least 3.23 gears.

here is the tag from a 273 3 speed belvy, 3.23 gears.

 

[duster 344]

I think the car I bought was originally an auto trans and got changed, the vin indicates an economy package with a 318

 

[AJ/FormS]

Manual-trans cars usually got 10% more gear to compensate for the loss of TM(Torque-Multiplication), that goes on inside the torque-convertor, mostly in first gear.
I gave you a like, lol.

Taking in account, when I looked up my cam, it was recommended for 3.55 gears. And best between 2500 and 5800 rpm

Those numbers are somewhat arbitrary. Compression ratio, cylinder pressure, and cam timing will all affect the numbers, as will the induction system, as will the engine's bore and stroke.
The cam's actual powerband will be about 1500rpm from the power peak to the torque-peak. However, some combos will carry the power at say 95% of peak for several hundreds of rpms. Similarly, a lot of cylinder pressure will extend the lower limit even more hundreds of rpms.
But, yaknow, a crappy combo can destroy your expectations pretty quickly.
What's a crappy combo?
Well that's opinion, and so;
the one that tops my list is
an 8.0Scr 318LA with a 340 cam and big ol' open-chamber big-valve heads, with log manifolds, lol.
Been there/done that/never will again.


If I may; remember this, the lack of gear only hurts at rpms where your engine is down on power. So this equates primarily to starting off from zero mph. But it repeats at regular intervals in each gear, whenever the rpm drops into the soft zone. If that happens in a particular speed zone, that you enjoy being in, that's when it really hurts.
As someone mentioned, on your chart, 3.91s and 2.76s are pretty much the same bunch of gears except offset by one gear.
If your favorite speed-window is 35 to 50 mph, and your engine makes peak torque around 4400 rpm then the Road-gear of choice might be 10.41 which gets you 4370@35, and 6240 @50mph. So then you have no choice but the 3.91s because it is the only gear that has the 10.41 ratio available.
But I gotta tell ya, if your engine lights up the tires with 3.91s, you are going nowhere fast.
So then you got a choice; to fix the traction issues, or to swap to a lessor and lessor gear until you get under the excess power.
What I mean is this, suppose you are in a shallow sweeper, in first gear, and you step on it just a lil too hard, and the tires spin. Now you are sliding and in imminent danger of wiping out. It might be better to re-gear your car into a different lower rpm, where there is less likelihood of busting the tires loose when yur not wanting them to.
Yakno, people laugh at 2.76s in a performance car, but guess what; First gear with 2.76s, will get you to almost the exact same place as Second with 3.91s will; namely 65mph at ~5800. The engine does not care which ratio goes where, it is only subject to the combination of the two, which in this case is from 7.34 to 7.50.
Obviously two gears has the potential to get there quicker than one gear, unless it spins all thru First gear, lol. But, but, how much quicker? Either gear will get to 60mph in under 6 seconds with a half-way decent engine. Maybe 5.5 seconds. Is it worth it to you to sacrifice the lower cruise rpm to use the ever so slightly quicker 3.91s? Only you can answer that.
I suppose this is why 3.23s are so popular, they are about half way in between the 2.76s and the 3.91s.

 

[duster 344]

That’s a very good take on differential gears, exactly what I was going for when I put the numbers together, only to give you an idea of what they would be, being able to see it layed out in front of you. And able to compare things like first and second gear. Changing tires before you go to the track if you want more rpm’s. Imputing your own variables to help decide what would work best for you. Not a perfect silence but a tool meybe. I’m sure the information has been on here hundeeeds of times. But I’m new, and it was fun to lay it out and look at it. hope it may be helpful to some one, and meybe spark some more conversation. It’s always good to hear logic from the guys that have been doing it awhile

 

[AJ/FormS]

The thing about tires is this;
>the tallest tire you can fit on your Duster is about 28 inches, and it sorta has to be inside the tub, cuz the outboard edge will be running pretty close to the bottom front corners of the quarters at the rockers..
> the shortest tire with decent width, is about 25.5 but that's already looking a little outta scale.
> so that gives you a range of 25.5 to 28, which is 28/25.5= about 10%, which just happens to be about the same percentage as swapping one gear size, in the range of 2.76s to 3.91s, except 3.73s which are in-betweeners. Once you get into the 4.xx series gears, the ratios spreads are closer to 5%.
> the bulk of tire sizes that you might consider for one of the large-tub performance cars like Dusters/Demons/ and Second-gen Barracudas, are gonna fall into a much narrower window. For example, and by the math;
a 325/50-15 is 28 ; these fit, but it ain't easy.
a 295/50-15 is 26.6 ; these fit right in and look right at home, but you will need to offset the springs, and get custom offset wheels, or get your rearend narrowed.
a 275/60-15 is 28; these look great too, and are adequate for straight-line work, and are a bolt-in
a 275/50-15 is 25.8; height-wise, these are already looking a lil outta scale
I see no good reason to run anything smaller with 275hp or more, nor would I ever run 14s on the back; but;
AFAIK there are no 265s in 15s
a 255/60-15 is 27; again looking good, height-wise.
So then, the window is about 27 to 28, which is 3.7%.. At 65mph, this amounts to 2.4mph, which, IMO, is not worth fiddling with; just install whatever tire that you like the look of and think will do the job, and let the cruise rpm be what it will be..

and one more thing when selecting tires; for straightline work, height is better than width. What I mean is, when faced with a contact patch limitation of say ~100 square inches; it is better to have a patch that is 8 inches wide and 12.5 inches long, than the other way around.

But if you drive like I do, slip-sliding around as many corners as I think I can get away with, a lil extra width comes in handy. When I come in too hot and I run out of steering correction, with the tires spinning, I just put a lil toe on the clutch, and when she is sliding sideways, when the patch stops spinning, it acts like a full-lock brake, and bam! she stops sliding. The only down-side is that I gotta know where my front wheels are pointing, or I might have a new problem, lol. So then, 295s are my only choice.

 

[duster 344]

So another question to ponder. Is there any benifit as far as power, to have a 3.55 gear with a 28 inch tire vs. a 3.23 gear with a 26 in tire? The rpm’s are pretty much the same. Other than meybe having a bigger foot print, is there actually a little more power to the wheel itself transferred to the road? Or does the circumference of the tire basically gear it back down again?

 

[duster 344]

Amagine one would have to go through the numbers in each gear 1 through 4 to see the real gain or loss, I would expect the over all ratio to change a bit through the spectrum

 

[duster 344]

Obviously over thinking. But an experiment that doesn’t cost the money of a lot of gears and tires