Tell them to do today's Saturday morning experiment demonstrating scientific method and practical math:

__HOW TO DETERMINE TIRE SIZE FOR USE IN GRIMM JEEPER ONLINE GEAR RATIO CALCULATOR__
Today, boys and girls, we are going to use Scientific Method to demonstrate something you can do in real life after the governors let us out of our houses - determine the tire diameter to input into that online gear calculator all the cool jeepers are using so you can figure out which gears to buy with the next government stimulus check.

The first rule of Scientific Method in an experiment is to

*only change one variable at a time*.

Today we are going to use that rule to our advantage by assuming the tire diameter as the unknown variable, plugging in our existing gear ratio, and using the gear calculator to tell us the diameter of our tires rather than which gear ratio to use.

Let's see what the gear calculator needs to give us useful numbers:

1. Transmission type.

2. Transfer case type.

3. Axle ratio

4. Tire diameter.

Numbers 1 and 2 are easy - just go look at your jeep or your build sheet.

Numbers 3 and 4 are where our experiment takes place. The Grimm Jeeper calculator is designed to input a specified tire diameter, standard size or revolutions/mile in order to calculate the unknown variable which for re-gearing purposes is often the rpm at a specified speed and specified gear ratio. The issue of accuracy arises because there is no way to validate published diameters or other tire attributes supplied by third parties with the potential for "garbage in, garbage out" results. However, by using the calculator "backwards," i.e. by supplying a known and verifiable gear ratio rather than a tire diameter, we can figure out the tire diameter we should be using in the calculator for the tires we currently own and use.

This experiment shows you how to use the Grimm Jeeper calculator to tell you the diameter of your tires with amazing precision. You can use that diameter in future calculations to determine the rpms at various speeds with different gear ratios, transmissions, etc.

You will need:

A. A calibrated vehicle speedometer or GPS.

B. A working tachometer

C. A straight stretch of highway where you can safely drive 65 mph or so and record the rpm's in top gear at that speed.

Tip: It is difficult if not impossible to record the exact rpm from a TJ tach unless the needle is dead on one of the marks or numbers. It works just as well to get close to the speed at which you intend to record results, say 65 mph, and then adjust your speed so that the tach needle is directly on a mark, say 2500 rpm, and record the digital mph on the GPS at 2500 rpm rather than the rpm at a predetermined speed.

In my case, after taking several 5-mile runs on a straight, relatively level lightly trafficked Interstate at my usual 26 psi, I determined that in overdrive, with Mickey Thompson Baja ATZ-P3 LT315/75R16 tires, 42RLE transmission, Rubi-Crawler underdrive, 241OR transfer case and 5.38 gears I am travelling "just under" 2500 rpm at exactly 65 mph or exactly 2500 rpm at exactly 66 mph.

I used a Garmin Drive 51 navigation unit and a Garmin inReach Explorer+ to determine speed. I took the opportunity to compare with my TJ speedo. All three matched thanks to my Speedohealer.

Now let's solve for tire diameter.

Enter all of the data required by the online calculator except tire diameter. Go to the bottom left of the calculator where it says "Road RPMs at given speed in miles per hour"and enter the speed at which you conducted the experiment, which in my case was 66 mph.

If you are good at math you can derive the formula for extrapolating tire diameter. I'm not, so I did this instead:

I plugged in 35" for tire diameter because surely Mickey Thompson wouldn't sell undersized tires. Would they?

- the result was 2352 rpm in the bottom left chart at 66 mph. Damn, they can't be 35" because at 66 mph

my rpms were 2500. They must be smaller.

Then I plugged in 34.6", the diameter published by Mickey Thompson and the industry "standard" diameter for 315/75R16 tires.

- the result was 2379 rpm at 66 mph, so they have to be even smaller in effective diameter than 34.6. WTF!

I kept going. Not 34 1/4, not 34, not even 33.5". Those rat bastards at Mickey Thompson.

__The final result__:

With my equipment as listed, the Grimm Jeeper gear ratio calculator showed that at 66 mph in overdrive at 2499 rpm I had to be travelling on tires with a diameter of . . . . drum roll please . . . . . 32.94".

Call the police!!! I've been robbed! Where's my 2.06"????

__Conclusions__:

1. The published tire diameter is a bullshit number for purposes of calculating a desired gear ratio. Don't bother to use it.

2. "True" 35" tires are like unicorns, no rational person believes they exist.

__Extra Credit__:

Just for fun, I compared the 32.94" tire diameter determined by the calculator with other methods.

Hub height aka rolling radius method at 26 psi = 16.25" x 2 = 32.50"

Level placed on top of tire, measured to ground with yardstick at 26 psi = "about" 33".

Measured horizontally at hub, 26 psi with loaded tire on ground (can't measure my spare unloaded because its a different size) = "about" 34".

Length of string around circumference of loaded tire on ground at 26 psi = 107.0" / 3.14 = 34.07"

You can draw your own conclusions from this second set of numbers.

Mine is that the 2x hub height/rolling radius method is probably close enough for rock and roll and forum debates and can point someone in the right direction at least long enough to get out on the road and actually gather data at cruising speed. The other methods not so much. I have not changed my opinion that nothing beats a test drive with the actual gear and tire combination one is considering. [Or if one is a true time management expert, simply write down the number Blaine suggests after you have answered his questions because he has done this long enough with enough different rigs and enough different tires that those numbers aren't coming out of his ass, buy those, and move on to the next conundrum of life.]

And that's the practical science experiment for today.

__Future validation of results__:

Using 32.94" as the diameter for my existing tires, and with my existing 5.38 gears, I should see the following the next time I take a long trip down I-5:

66 mph - 2499 rpm

70 mph - 2651 rpm

75 mph - 2840 rpm

79 mph - 2991 rpm (the speed over which one is likely to be ticketed on my stretch of I-5.)

We shall see.

____________________________

@mrblaine . You know its your fault that my fries are now cold.