I found an automatic that I really like....

[ClutchFork]

I wonder what formula is used to calculate rpm at a given speed?

Now ya made me have to think about what I did to get my number and so it boils down to this:

RPM = A / B where

A = 1056 * MPH * gear ratio * final drive ratio

B = 3.14 (0.00078 * Width * Profile + Wheel Diameter)

In my case: Width = 215, Profile = 75, Wheel Diameter = 14 (from tire size 215/75/14).

The 0.00078 converts the width from millimeters to inches, changes the profile to a percent, and accounts for the fact that the tire sticks out on both sides of the wheel in the total diameter.

Edit: 3.14 is accurate enough, not the 3.1416 I previously had.

[theholycow]

I wonder what formula is used to calculate rpm at a given speed?

Now ya made me have to think about what I did to get my number and so it boils down to this:

RPM = A / B where

A = 1056 * MPH * gear ratio * final drive ratio

B = 3.1416 (0.00078 * Width * Profile + Wheel Diameter)

In my case: Width = 215, Profile = 75, Wheel Diameter = 14 (from tire size 215/75/14).

The 0.00078 converts the width from millimeters to inches, changes the profile to a percent, and accounts for the fact that the tire sticks out on both sides of the wheel in the total diameter.

Here, use this to get a whole chart...
http://dl.dropboxusercontent.com/u/1956 ... /index.htm

[ClutchFork]

Here, use this to get a whole chart...
http://dl.dropboxusercontent.com/u/1956 ... /index.htm

Now ain't that handy dandy. Thanks!

[Stelcom66]

I wonder what formula is used to calculate rpm at a given speed?

Now ya made me have to think about what I did to get my number and so it boils down to this:

RPM = A / B where

A = 1056 * MPH * gear ratio * final drive ratio

B = 3.1416 (0.00078 * Width * Profile + Wheel Diameter)

In my case: Width = 215, Profile = 75, Wheel Diameter = 14 (from tire size 215/75/14).

The 0.00078 converts the width from millimeters to inches, changes the profile to a percent, and accounts for the fact that the tire sticks out on both sides of the wheel in the total diameter.

Here, use this to get a whole chart...

Thanks for both references that I'll save.
http://dl.dropboxusercontent.com/u/1956 ... /index.htm

[ClutchFork]

Regarding the formula below, I forgot to mention the factor of 1056 is the distance in inches that you will travel down the road in one minute going one MPH. This is inches because our tire diameter comes out to inches and in dividing it out to feet, the 12 flips to the numerator and then factors into the distance you travel in 1 minute at one MPH, which is actually 88 feet.

RPM = A / B where

A = 1056 * MPH * gear ratio * final drive ratio

B = 3.14 (0.00078 * Width * Profile + Wheel Diameter)

In my case: Width = 215, Profile = 75, Wheel Diameter = 14 (from tire size 215/75/14).

The 0.00078 converts the width from millimeters to inches, changes the profile to a percent, and accounts for the fact that the tire sticks out on both sides of the wheel in the total diameter.

[zenfiz6]

Regarding the formula below, I forgot to mention the factor of 1056 is the distance in inches that you will travel down the road in one minute going one MPH. This is inches because our tire diameter comes out to inches and in dividing it out to feet, the 12 flips to the numerator and then factors into the distance you travel in 1 minute at one MPH, which is actually 88 feet.

RPM = A / B where

A = 1056 * MPH * gear ratio * final drive ratio

B = 3.14 (0.00078 * Width * Profile + Wheel Diameter)

In my case: Width = 215, Profile = 75, Wheel Diameter = 14 (from tire size 215/75/14).

The 0.00078 converts the width from millimeters to inches, changes the profile to a percent, and accounts for the fact that the tire sticks out on both sides of the wheel in the total diameter.

Can we do this in furlongs and fortnights? I think that might help make the unit analysis.... better. Yah. "Better."

If only we had stayed friends with France just a *little* longer...

[Squint]

Can we do this in furlongs and fortnights? I think that might help make the unit analysis.... better. Yah. "Better."

If only we had stayed friends with France just a *little* longer...

I love the word fortnight. It's fantastic. It might give off the impression of being a little snobby... but still. Good word.

(that isn't related to your wikipedia link other than commenting on the word fortnight)

[zenfiz6]

Can we do this in furlongs and fortnights? I think that might help make the unit analysis.... better. Yah. "Better."

If only we had stayed friends with France just a *little* longer...

I love the word fortnight. It's fantastic. It might give off the impression of being a little snobby... but still. Good word.

(that isn't related to your wikipedia link other than commenting on the word fortnight)

If you ever want to impress someone, just tell them your car can do zero to 160,000 furlongs/fortnight in 5 micro-fornights.

[Squint]

If you ever want to impress someone, just tell them your car can do zero to 160,000 furlongs/fortnight in 5 micro-fornights.

Yes, I just spent 5 minutes checking your math. And my Fiesta can only do zero to 160,000 furlongs/fortnight in a little over 8 micro-fortnights.

We weren't going for the dual horse and buggy speed record or anything when we bought it.

[ClutchFork]

Regarding the formula below, I forgot to mention the factor of 1056 is the distance in inches that you will travel down the road in one minute going one MPH. This is inches because our tire diameter comes out to inches and in dividing it out to feet, the 12 flips to the numerator and then factors into the distance you travel in 1 minute at one MPH, which is actually 88 feet.

RPM = A / B where

A = 1056 * MPH * gear ratio * final drive ratio

B = 3.14 (0.00078 * Width * Profile + Wheel Diameter)

In my case: Width = 215, Profile = 75, Wheel Diameter = 14 (from tire size 215/75/14).

The 0.00078 converts the width from millimeters to inches, changes the profile to a percent, and accounts for the fact that the tire sticks out on both sides of the wheel in the total diameter.

Can we do this in furlongs and fortnights? I think that might help make the unit analysis.... better. Yah. "Better."

If only we had stayed friends with France just a *little* longer...

My brain is only comfortable with the awkward but familiar English units (which the English gave up on). What puzzles me is why tires are rated in metric for tread width and English for wheel size. In the 1970 I recall letter designations for tread width and that my car at the time took G/78/15s. There are conversion charts on the web, think G was like a 215 mm width.

[zenfiz6]

My brain is only comfortable with the awkward but familiar English units (which the English gave up on). What puzzles me is why tires are rated in metric for tread width and English for wheel size. In the 1970 I recall letter designations for tread width and that my car at the time took G/78/15s. There are conversion charts on the web, think G was like a 215 mm width.

Actually, don't the English still use miles and MPH?

The problem is that everyone tries to convert. This is like always translating English to your chosen language. Fluency comes when you stop translating and just listen to the language.

I re-set my climate controls to Celsius (nice that I can do that in the Audi and my home thermostat) and people keep asking me: "So, what's 70F in Celsius?" No. Bad question. Think like this:

0C is freezing (we all knew this, right?)
5C is kinda cold
10C is chilly
15C is cool
20C is nice (maybe a tad cool now that I'm gettin' on in years)
25C is warm
30C is hot
35C is miserable
40C is "let's sleep in the fridge tonight."

Actually, my favorite webcomic, XKCD, did this really well.

[theholycow]

^What he said.

My brain is only comfortable with the awkward but familiar English units (which the English gave up on). What puzzles me is why tires are rated in metric for tread width and English for wheel size. In the 1970 I recall letter designations for tread width and that my car at the time took G/78/15s. There are conversion charts on the web, think G was like a 215 mm width.

It was easy to change to metric for tire width. People with old tires could keep using them until they needed new tires, then they could just get a similar width. It would not be easy to change rim diameter, nobody wants to buy new wheels just so there can be a new unit used to describe diameter, and who wants to talk about 381mm wheels? 205/75-381 is a little weird.

What I don't get, and this really irritates me, is why sidewall height is a percentage of width instead of just a quantity of millimeters (or, heaven forbid, an actual diameter measurement). Who's the crackhead who had that idea? That's why we need tire size calculators and why people end up with wrong-sized tires.

[zenfiz6]

It would not be easy to change rim diameter, nobody wants to buy new wheels just so there can be a new unit used to describe diameter, and who wants to talk about 381mm wheels? 205/75-381 is a little weird.

No it wouldn't. Nobody would make a 381. It would bump to 400. Thus is the nature of the human.

What I don't get, and this really irritates me, is why sidewall height is a percentage of width instead of just a quantity of millimeters (or, heaven forbid, an actual diameter measurement). Who's the crackhead who had that idea? That's why we need tire size calculators and why people end up with wrong-sized tires.

I don't think it's bad, necessarily. But I'm not coming up with a really good reason to do aspect ratio instead of actual sidewall height. Maybe because strength depends more on aspect ratio than thickness alone... :shrug:

[Squint]

The Life Pro Tip that I discovered last year for quickly going from C to F was to multiply by 2 and then add 30. It gets close enough to the actual answer for us dumb non-Metric thinkers to get the gist of what the temperature is.

-10 C = 10 F (really 14 F)
-5 C = 20 F (really 23 F)
0 C = 30 F (really 32 F)
5 C = 40 F (really 41 F)
10 C = 50 F (really 50 F)
20 C = 70 F (really 68 F)
30 C = 90 F (really 86 F)
40 C = 110 F (really 104 F)

And there is always a relevant XKCD. Always. And it is glorious.

[tankinbeans]

Let's just switch to Kelvin.

Oh boy, it's 273! Grab your parka. (I've forgotten the exact conversion factor.