Trying to understand final crawl ratio?

[organic]

I guess whats got me confused is running larger tire size. When you use the calculator to determine final crawl you have to plug in your diff ratio. What if your running a larger diameter tire with your stock ring and pinion? The bigger tire would in turn make your 4.10's more like 3.20's. So, 3.20 should be the number I plug in instead? Maybe I'm over thinking this.

 

[gribly]

Final crawl is tranny input to hubs, does not include tire size.

 

[srteach]

My understanding is:
The ratios are an input to output ratio in a gear system (trans, differential, over or under drive, transfer case). The ratio does not represent the distance traveled by one rotation of a specific size tire.

A function of ratios is power multiplication. Power (in the form of speed torque) must come in with the input shaft. It is either increased or reduced by the ratio. Theoretically, high speed energy input is "converted" to high torque output through 5.56:1 gearing, while torque is "converted" to speed through 1:5.56 gearing. Realistically, there is some friction losses.

The trick is to get the right ratio combination for the speed / power combination you need.

As gribly said: The final drive ratio includes all the gearing from the engine output to the rear drums / brakes. Each gear in the transmission will produce a different final drive ratio, because each gear has a different ratio.

Bigger tires takes more effort to turn. The torque needed to turn a tire is a function of the radius of a tire (straight line center to outer tread distance). The longer the radius, the more torque needed to turn it. You need more power input to turn bigger tires, but the final drive ratio is the same.

 

[gribly]

You're over-thinking his question. He didn't understand why there wasn't any way to put the tire size in when calculating final crawl - it's because tire size does not affect final crawl ratio. The final crawl tells you how many times the input shaft turns in relation to the hubs, the tire size will affect how many times the hubs turn to how far the truck moves.

 

[4x4junkie]

I'm guessing maybe the OP is looking for a figure something like "how many engine revolutions per foot traveled".
Unfortunately such a standard never really caught on for some reason, I'm guessing maybe due to the added calculation that would be needed for it (pretty lame reason to leave that part out IMO, if so).

My BII gets about 12.8 revs per foot (I think I have that figured out right lol)

Using the same math, with 33s on a 3.0L truck (stick) stock with 3.73s, you'd be right about 4 revs per foot.

 

[organic]

Thanks guys, I think I got it.
4x4junkie, whats the formula for revs per foot?

 

[CHKNFKR]

pi times diameter gets you the circumference of the tire, in other words feet per 1 tire revolution. invert that and you've got tire rev per foot. multiply by the crawl ratio and you get engine revs per foot.

31" tire = 8.1' per tire rev
invert and you get .12 tire revs per foot
multiply be crawl ratio (40 is a good stockish ratio) = 4.8 engine revs per foot

 

[feellnfroggy]

figure up your final crawl ratio. now write it up like this, old tire size over old speed divided by new tire size over x

old new tires
__/___ = then figure up algebraically.
old x speed

This will give you the difference between the old tire rotation and the new. From here you can do the same with your tires and crawl ratio.

Or jsut send me or one or the guys your old and new numbers and we'll figure it up for you.

 

[4x4junkie]

I got to the figure by dividing the final crawl ratio by the tire circumference in feet.

Crawl ratio = 117.37 (3.72 × 2.48 × 2.48 × 5.13)
Tire circ. = 9.13682' (34.9" dia)
Revs per foot = 12.845826

 

[organic]

I got to the figure by dividing the final crawl ratio by the tire circumference in feet.

Crawl ratio = 117.37 (3.72 × 2.48 × 2.48 × 5.13)
Tire circ. = 9.13682' (34.9" dia)
Revs per foot = 12.845826

Perfect. Thank you.