TJ Tire Size

Where did you come up with 1270?

The correct way to express the formula is ((((A* B)/100) * 2) / 25.4 + C), where A = Size, B = Ratio, and C = Rim, and A, B, and C all have the same row number, A1, B1, C1, or A2, B2, C2, etc.

The beauty of Excel is that it starts on the left of any expression, and applies the functions it encounters as it moves to the right, this might not be correct from a mathematics point of view, but it's predictable and if you expect it to behave that way, then you can write your formula to fit. I don't know how it handles rocket science, but my guess is that you can't get to the moon with a spread sheet, so who cares?

An alternate way to express the tire size calculator is A * (B / 100) ...

In this case, it would make the Aspect Ratio into a percentage before it multiplied by the size.

=(((B4*(C4/100))/25.4)*2+D4)

Open a new spreadsheet. Label B3, Width, C3, Ratio, and D3 Rim, then in B4, enter 225, in C4, 75, and in D4, 15, then in E4, insert the formula above, and label E3 Overall Diameter, if you want to. The data shown in E4 will be the result of the formula having been applied to the other cells.

Now, select cell E4 and drag the bottom right corner straight down. The formula will automatically change B4 to B5, then B6, etc., as is appropriate and you can enter lots of sizes to see what happens as you play with the numbers. The calculator is especially handy with changing from a 15" rim to a 17" or 18" rim and you want to find the new aspect ratio that will leave you with the same size tire so that you don't have to mess with the speedometer or the gearing.

Our formulas end up doing the same thing. I just eliminated a few steps. Your formula divides by 100 to convert the ratio to a whole number, multiplies by 2 for the sidewalls, and divides by 25.4 to convert mm to inches. This is the same as dividing by 1270. ;-)

Chris

Got it. I was wondering where 1270 came from. If we multiply the the two divisions to cancel them out, we get 2540, then divide that by 2 and we get