305/45R20 Tyre Size: Dimensions, Alternatives & Speedometer Effect

Measured from the ground to the top of the tread, a new 305/45R20 tyre stands 782.5 mm tall (30.8 inches). The sidewall — the rubber between the rim edge and the tread — is 137.2 mm high, the tyre covers 2458.3 mm of road per revolution, and it completes 654.7 revolutions in a mile. Those numbers come straight from the three figures in the size code: 305 mm width, 45% aspect ratio and a 20-inch rim. This guide explains the maths behind them, lists the sizes that match this one most closely, and works out what fitting a different size would do to your speedometer reading.

This is a measurement page, not a recommendation. Whether a 305/45R20 belongs on your car depends on the placard figure, the load index and the speed rating your vehicle requires — none of which a diameter table can settle on its own. What the table can do is give you the exact size of this tyre (782.5 mm diameter, 137.2 mm sidewall, 2458.3 mm circumference) so that any comparison you make is grounded in real numbers. Work through the alternative-size table and the speedometer section before deciding anything, and treat the plus-or-minus 3% guideline as what it is: a rule of thumb, not an approval.

This is a calculation, not a safety approval. Always check your vehicle's door-jamb placard, load index and speed rating, and confirm fitment with a tyre professional before fitting a different tyre size.

What does 305/45R20 mean?

The code breaks into three parts. The first number, 305, is the section width in millimetres — the width of the tyre from sidewall to sidewall when mounted on its measuring rim. The second number, 45, is the aspect ratio: the sidewall height expressed as a percentage of the width. Multiply them (305 x 45% = 137.2 mm) and you get the sidewall height. The final number, 20, is the rim diameter in inches — 20 x 25.4 = 508.0 mm. Add the rim diameter to twice the sidewall (508.0 + 2 x 137.2 = 782.5 mm) and you have the overall diameter. That is the whole formula, and it is why every size in the chart can be checked rather than trusted blindly.

305/45R20 dimensions

Measurement Value
Overall diameter 782.5 mm (30.8 in)
Sidewall height 137.2 mm
Circumference 2458.3 mm
Revolutions per mile 654.7
Rim diameter 20 in

Start with the circumference: 2458.3 mm per revolution. That is the distance your car covers every time this tyre turns once, and it is what makes the revs-per-mile figure (654.7) what it is — a mile divided by the circumference. Fit a tyre with a larger circumference and each revolution carries you further, so at the same wheel speed the car is moving faster than the instruments report. The sidewall (137.2 mm) is the flexible part: it absorbs the first shock from bumps and potholes, and a medium-profile sidewall like this one means a balanced sidewall that splits the difference between handling response and ride comfort. These are nominal new-tyre dimensions, and real tyres vary slightly by brand, model, wear and inflation pressure.

Closest alternative sizes to 305/45R20

The three sizes below are the closest matches to 305/45R20 in the chart, drawn from the same rim or one inch either side and ranked by diameter difference. The closest, 275/50R20, differs by +0.1% (+0.5 mm). The commonly used guideline is to stay within plus or minus 3% of the original diameter, which keeps the speedometer error small and the gearing close to standard. That guideline is about the speedometer and the car's calibration, not about whether the tyre physically clears the arches — a size can be within 3% on diameter and still rub if it is much wider, so width and clearance need their own check.

Alternative size Diameter Sidewall Diameter difference Within ±3% guideline?
275/50R20 783.0 mm (30.8 in) 137.5 mm +0.1% Within the guideline
275/55R19 785.1 mm (30.9 in) 151.2 mm +0.3% Within the guideline
255/60R19 788.6 mm (31.0 in) 153.0 mm +0.8% Within the guideline

Speedometer effect of changing from 305/45R20

Take the closest alternative, 275/50R20, as a worked example. Its diameter is 783.0 mm against 782.5 mm here, a difference of +0.1%. Because the speedometer counts revolutions, the error follows the diameter ratio: actual speed equals indicated speed multiplied by 783.0 / 782.5. At an indicated 30 mph the car would really be doing 30.0 mph; at 50 mph indicated it would be 50.0 mph; at 70 mph indicated, 70.0 mph. That means the speedometer would read slightly low, so you would be travelling faster than the dial suggests. Even a small percentage matters at motorway speed, which is why the plus-or-minus 3% guideline exists.

Replacement size Diameter change Actual speed at 30 mph indicated Actual speed at 50 mph indicated Actual speed at 70 mph indicated
275/50R20 +0.1% 30.0 mph 50.0 mph 70.0 mph
275/55R19 +0.3% 30.1 mph 50.2 mph 70.2 mph
255/60R19 +0.8% 30.2 mph 50.4 mph 70.5 mph

Wider, narrower or taller than 305/45R20: what actually changes

Going wider than 305 mm while keeping the same aspect ratio also makes the tyre taller, because the sidewall is a percentage of the width — that is the single most common surprise in a size change. Going wider and dropping the aspect ratio keeps the diameter closer to standard, which is the idea behind plus-sizing. A taller tyre raises the car slightly and lengthens the gearing, so the engine turns a little slower at a given road speed; a shorter tyre does the opposite. Wider rubber generally brings more dry grip and more road noise, and it can increase fuel use slightly through higher rolling resistance. None of these effects is dramatic for a small change, but they all point the same way: change one dimension at a time and keep the diameter close. That is the point this page works through for 305/45r20 tyre size.

How to check 305/45R20 fits your vehicle

The checks are the same whatever size you are considering. First, the door-jamb placard: size, load index and speed rating. Second, the rim: a 20-inch rim of the right width for a 305 mm tyre. Third, clearance — arches, suspension components and steering lock, with the car loaded as it will actually be used. Fourth, the pressure-monitoring system, which may need a reset or new sensors after a change. Fifth, the spare: if the spare is a different size, know its speed and distance limits before you need it. None of these is a formality, and together they are what turns a calculation into a safe fitment. For the 305/45R20, the 305 mm width and 137.2 mm sidewall are part of that same check.

Mistakes to avoid with 305/45R20

The most common mistake is reading the size off the tyre already fitted and assuming it is the correct size — a previous owner may have changed it. The second is going wider without adjusting the aspect ratio, which quietly makes the tyre taller and throws the speedometer off. The third is ignoring the load index and speed rating, which matter as much as the dimensions. The fourth is fitting different sizes on the same axle, which the car was not designed for. And the fifth is forgetting that the spare, the pressure-monitoring system and the speedometer all assume the standard size — change the size and all three need a thought. For a 305/45R20, that means checking the 305 mm width, the 45% aspect ratio and the 20-inch rim against the placard before you buy.

Related 305/45R20 sizes and tools

Frequently asked questions

What is the overall diameter of a 305/45R20 tyre?

The overall diameter of a new 305/45R20 tyre is 782.5 mm, which is 30.8 inches. That is the rim diameter (508.0 mm) plus twice the sidewall height (137.2 mm). It is a nominal figure, so an individual tyre can vary slightly by brand, model, wear and inflation pressure.

How tall is the sidewall on a 305/45R20 tyre?

The sidewall is 137.2 mm tall, because the aspect ratio is 45% of the 305 mm width (305 x 45/100 = 137.2 mm). A medium-profile sidewall like this one means a balanced sidewall that splits the difference between handling response and ride comfort. The sidewall height is the main thing that decides how much cushioning the tyre gives over bumps.

How many revolutions per mile does a 305/45R20 tyre do?

A 305/45R20 tyre completes 654.7 revolutions per mile, based on its 2458.3 mm circumference. That figure matters because the speedometer and odometer count revolutions: fit a tyre with a different circumference and the readings drift by the same percentage as the diameter change.

What is the closest alternative size to 305/45R20?

The closest size in the chart is 275/50R20, which differs in diameter by +0.1% (+0.5 mm). The commonly used guideline is to stay within plus or minus 3% of the original diameter, and the alternative table on this page shows which sizes meet that. Diameter is only one check, though — load index, speed rating and physical clearance still need confirming.

Will changing from 305/45R20 affect my speedometer?

Yes. The speedometer assumes the original circumference, so a diameter change shifts every reading by the same percentage. Fitting 275/50R20 instead of 305/45R20 changes the diameter by +0.1%, so an indicated 70 mph would really be 70.0 mph. Keep the diameter within plus or minus 3% to keep that error small, and check the placard before fitting anything different.