Let's Nerd Out On Some Calculations

I pulled the rev/mile spec for the tire and went from there, because it reflects the squat in the tire at max load and pressure. That should be constant with a lighter load and tire pressure. It doesn't not reflect a fully aired down tire but will be better than the circumference.

I've got the sheet for mine around somewhere. 847 rev /mile if I remember for my 7.50-16 which are roughly 32"

 

ive seen a comparison on this a deflated tire and a hard one travel the same distance per revolution if i recall correctly

 

I remembered doing a test on this subject a few years ago. I had to do some digging, but I found it.

http://www.earlycj5.com/xf_cj5/index.php?threads/143440/page-2#post-1589194


“This is the 33x12.50 tire(s) mentioned earlier.
I started with this baseline. If I backed up and it didn’t line up perfectly, I jacked it up and repositioned the tire.



Results. I did three tire rotations so I’d get definitive results. Divide the distances by three to get numbers for a single rotation.”

 

Not making any claims here, and not intending to hijack the thread, but just for fun, my Unimog's Super Crawler low gear ratio is 283:1. It's been quite a while since I did the arithmetic, but I recall that's ~80 yards an hour (0.05 mph) with the engine against the governor! At idle, forward (or rearward) motion is pretty much undiscernible.

 

Here is a calc. that determines crawl ratio when changing tire size.


Tire size change Calc.

Old tire Dia
--------------- X old axle ratio = New axle ratio
New tire Dia

31
----- X 4.88 = 4.58:1 Trans sm465 - 6.55 X 4.58 X 2.46= 73.80 with 33" tires 78.6 with 31" tires
33

Not really that noticeable. My crawl speed is 1.14 mph

 

This is cool!

 

You beat me to it!

I was going to comment that my TJ with 33s, 70:1 crawl, and a 750 rpm idle actually has a higher ground sped than my '71 with 32s, 58:1 crawl and a Buick 350 that idles at 400 rpm.

 

I stole the code for this calculator years ago because I really like the format: https://www.public.asu.edu/~grover/willys/speed.html

I added a bunch more useful gear ratios, transmissions, transfer cases, overdrives, ...etc. and have been using it for my own stuff. I've decided to post up a public copy of it.

Go here and calculate gearing and ground speed to your heart's content: https://fireball-v6.github.io/Code/

To use it, you simply need to select one checkbox for each of the following:

• Tire size
• Axle ratio
• Transfer case ratio
• Transmission ratio
• Overdrive ratio

You can adjust the engine speed and see the resulting ground speed on the right.
There is pre-populated info for most of the common conversions (and my TJ Rubicon), but you can also manually enter gear ratios and tire sizes on the right if your combination isn't present.
You can also send me a request to add a transmission or something that isn't present and I will get to it eventually.

Have fun with it! I know I have.

There is some possibility I'll need to take it down since it's someone else's code. I e-mailed the 23 year old address to see if the owner will give me permission to post it, but I'm not holding my breath that he'll respond back. My guess is nobody cares.

 

We need to put that in the tech index.

Needs some sort of pto speed output for the driveshafts and after the pto box.

 

With my aforementioned equipment and 800 rpm, I got .81 mph.

 

.7 @ 600 rpm ( about as low as it likes to go, I have lugged it to 3-400 occasionally)

 

i get a nice even 1mph

 

.64 mph at 700 rpm. That is a really cool tool. Need to get that pinned where we can find it again. Thanks for sharing, @Fireball

 

Dang, someone's got low gears.

 

Sorry man. Didn't mean to minimize good old fashioned math. In my defense, I recently did some long division on a cardboard box with a sharpie to figure lighting layout on a job. My 30-somethings apprentices thought I was nuts given I had an iphone in my pocket.

 

I deal with numbers and excel all day, I'm also human so I make expensive mistakes in the process.
I have an excel sheet at work with all the math and then some charts plotting ground speed to rpm and gears. That's how I figured up I needed to swap to 4.27 gears with the sm465 but it's limited in scope.

I can do the math easy enough, but It does not bring me joy.

 

All I can say as a former automotive engineer, is that an intuitive easy to use tool for quickly running dozens of possibilities is an intuitive easy to use tool for quickly running dozens of possibilities.

That's why I modded it for my own personal use. I've used spreadsheets and other things in the past, but this tool is just great for quickly seeing possibilities.