How to Set Up a Transparent Tire Dimension Comparison Sheet

A transparent tire dimension comparison sheet is a spreadsheet where every number comes from a visible formula you can trace back to the size printed on the tire. You enter the parts of each size, such as 225/45R17, in separate cells. The sheet then calculates sidewall height, overall diameter, circumference, revolutions per mile and the percentage difference from a baseline tire. The figures it produces are nominal values, meaning they are derived from the size code. A real tire on a real rim will measure somewhat differently. The sheet shows how sizes compare on paper. It does not tell you whether a size fits or is approved for a particular vehicle.

What the Size Code Gives You

Most passenger and light truck tires sold in the US use a metric code such as P225/45R17 or LT265/70R17. Before you build any formulas, you need to know what each part of the code means.

Section width: the first number, in millimeters. It is the nominal width of the tire from sidewall to sidewall.
Aspect ratio: the number after the slash. It is the sidewall height as a percentage of the section width. In 225/45, the sidewall height is 45 percent of 225 mm.
Construction: the letter R means radial construction. It is not used in any calculation.
Rim diameter: the last number, in inches. It is the wheel diameter the tire is made for.
Prefix: P (passenger), LT (light truck), or no prefix. The prefix matters for load and inflation, but the geometric formulas treat all of these sizes the same way.

The code mixes millimeters and inches. Most calculation errors in homemade tire size converters come from this mix of units. A transparent sheet converts every value into one unit before combining them, and it labels the unit next to every row.

The Core Formulas

Every comparison in the sheet comes from a few geometric relationships. They hold for any metric size code.

Sidewall height (mm)      = section width (mm) × aspect ratio ÷ 100
Overall diameter (mm)     = rim diameter (in) × 25.4 + 2 × sidewall height (mm)
Overall diameter (in)     = overall diameter (mm) ÷ 25.4
Circumference (mm)        = π × overall diameter (mm)
Revolutions per mile      = 1,609,344 ÷ circumference (mm)
Revolutions per kilometer = 1,000,000 ÷ circumference (mm)
Diameter difference (%)   = (candidate diameter ÷ baseline diameter − 1) × 100
Actual speed              = indicated speed × candidate diameter ÷ baseline diameter

The sidewall height is doubled because the overall diameter includes one sidewall above the rim and one below it. The constant 25.4 converts inches to millimeters. The constant 1,609,344 is the number of millimeters in one mile. The speed formula assumes the speedometer was calibrated for the baseline tire. A larger tire covers more ground per revolution, so the vehicle travels faster than the speedometer shows. A smaller tire has the opposite effect.

A Spreadsheet Layout That Stays Readable

Put the labels in column A, the baseline tire in column B, and each candidate size in the next columns. Keep inputs at the top and calculations below them. With this layout, anyone checking the sheet can read each column from top to bottom and follow the logic.

Row	Column A label	Column B formula (baseline)	Notes
2	Size label	text, e.g. 225/45R17	For reference only
3	Section width (mm)	input	Typed value
4	Aspect ratio (%)	input	Typed value
5	Rim diameter (in)	input	Typed value
6	Sidewall height (mm)	=B3*B4/100	Calculated
7	Overall diameter (mm)	=B525.4+2B6	Calculated
8	Overall diameter (in)	=B7/25.4	Calculated
9	Circumference (mm)	=PI()*B7	Calculated
10	Revolutions per mile	=1609344/B9	Calculated
11	Revolutions per km	=1000000/B9	Calculated

Copy rows 6 through 11 into each candidate column. Then add a comparison block underneath. The comparison formulas use absolute references such as $B$7, so every candidate is always measured against the same baseline.

Row 13  Indicated speed (input, baseline column only)   B13: 60
Row 14  Width difference (mm)                           C14: =C3-$B$3
Row 15  Sidewall difference (mm)                        C15: =C6-$B$6
Row 16  Diameter difference (mm)                        C16: =C7-$B$7
Row 17  Diameter difference (%)                         C17: =C7/$B$7-1
Row 18  Actual speed at indicated speed                 C18: =$B$13*C7/$B$7
Row 19  Revolutions per mile difference                 C19: =C10-$B$10

Format row 17 as a percentage rather than multiplying by 100 inside the formula. This keeps the stored value as a plain ratio, so other formulas can use it directly.

Worked Example

The following is an illustrative calculation from size codes only. It is not a measurement of any specific tire. The baseline is 225/45R17 and the candidate is 235/40R18.

Value	225/45R17 (baseline)	235/40R18 (candidate)	Difference
Sidewall height	101.25 mm	94.0 mm	−7.25 mm
Overall diameter	634.3 mm (about 24.97 in)	645.2 mm (about 25.40 in)	+10.9 mm
Circumference	about 1,992.7 mm	about 2,027.0 mm	+34.3 mm
Revolutions per mile	about 807.6	about 794.0	about −13.6
Diameter difference	—	about +1.72%	—
Actual speed at 60 mph indicated	60.0 mph	about 61.0 mph	about +1.0 mph

The candidate is wider and has a shorter sidewall, but it is slightly taller overall because the rim is one inch larger. On paper, the speedometer would read a little low with the candidate size. The table shows only what the size codes imply. It says nothing about clearance, load capacity or whether the vehicle maker lists the size.

Handling Flotation and Other Non-Metric Sizes

Some off-road and light truck tires use flotation sizing, for example 33x12.50R15. In this format the first number is the nominal overall diameter in inches, the second is the nominal section width in inches, and the last is the rim diameter in inches. There is no aspect ratio to enter, so the metric formulas do not apply as written.

To compare a flotation size with metric sizes, add a separate input block for it:
1) Enter the overall diameter in inches.
2) Convert it to millimeters with =diameter*25.4.
3) If you want a sidewall figure, calculate it as (overall diameter − rim diameter) ÷ 2.
From the overall diameter row onward, the circumference, revolutions and comparison rows work the same way. Label the column clearly as flotation sizing so no one types those values into the metric inputs.

Practices That Keep the Sheet Transparent

Enter width, aspect ratio and rim diameter in separate cells. Do not parse a full size string with nested text functions. A typo in a parsing formula is harder to spot than a typo in a single number.
Keep each formula on its own visible row. Avoid long formulas that hide intermediate values such as sidewall height.
Round only through cell formatting, not inside formulas. If you round intermediate values, small errors build up in the revolutions and speed figures.
Shade input cells in one color and calculated cells in another. Protect the calculated cells so they cannot be overwritten by accident.
Add a notes column that states the assumptions: values are nominal, the speedometer is assumed to be calibrated to the baseline, and tread wear and inflation are ignored.
Record load index and speed rating as text if they matter to you. Do not treat them as dimensions or fold them into any calculation.
If you ever measure an actual mounted tire, add a separate row labeled measured. Keep it next to the nominal row rather than replacing it.

Limits of Any Dimension Comparison

Nominal dimensions are a starting point, not a specification. Two tires with the same size code from different manufacturers can differ in actual diameter and width. The real width also depends on the width of the rim the tire is mounted on. Diameter changes as tread wears, and the loaded radius under the vehicle is smaller than the unloaded diameter.

A comparison sheet also cannot capture several things that decide whether a size is suitable: load capacity, speed rating, wheel offset, suspension and body clearance, brake caliper clearance, and the effects on stability control or all-wheel-drive systems. For those questions, use the vehicle's tire placard, the owner's documentation, or a qualified tire professional. The sheet's job is narrower. It shows clearly and checkably how the size codes compare.