245/35R20 vs 255/35R20: Size and Speed Changes

Calculated tire dimensions and changes from the original

The first row, 245/35R20, is the original reference. All measurements are derived from the tire geometry engine. Percentages and theoretical speed values keep that same original throughout; rounding is for display only.

A twenty inch sedan width change with fixed wheel diameter

Because the wheel remains twenty inches, every part of the height change comes from the two sidewalls. The ratio applies to the new wider section, so the candidate is not simply the original tire stretched sideways. The table describes that proportional change while keeping width separate from diameter and ideal radius.

For a wheel package already close to the body, ask which exact tire and wheel produced the original clearance. A different product in the same code can have a different measured section or shoulder shape. The nominal change is a useful starting comparison, but it does not create a guaranteed amount of extra width on either side of the wheel.

Speed, radius and wheel position in this pair

Starting from 245/35R20, the candidate 255/35R20 has 27.03 inches of calculated height instead of 26.75. Its +7.00 mm diameter change amounts to +1.03% of the original. The speed example follows the same ratio: a 60 mph indication becomes 60.62 mph theoretically, while 75 indicated becomes 75.77 mph. Reversing the original and candidate would change the denominator, so a return to the first size requires a fresh calculation rather than merely reversing a rounded percentage sign.

The wider twenty inch sedan tire gets its entire 7.00 mm height increase from rubber: each sidewall rises from 85.75 to 89.25 mm. Ideal radius changes by only 3.50 mm, whereas the section change spans ten millimeters. This contrast explains why a small speed change may be less revealing than lateral space beside an already tight wheel package. Retaining the original preserves its reference circle; investigating the candidate needs an application reason plus approved rim data. A matching R20 code cannot establish whether the wheel width currently on the sedan is suitable for the broader product.

Put the dimensional result in context

The theoretical speed change will be small relative to many larger wheel conversions, yet it remains tied to the original calibration assumption. Do not interpret a small modeled difference as proof that the actual speedometer is accurate. Check the complete application requirements and keep any observed instrument data separate from this ideal circumference ratio.

Frequently asked questions

What is the main trade on a relatively narrow twenty inch sedan wheel?

245/35R20 to 255/35R20 adds width to a large wheel package with limited sidewall. The candidate gains a little radius as well, but the lateral space may be the more immediate question for a sedan already close to the body. An owner could investigate the candidate after identifying a documented wider wheel option. Retaining the original avoids assuming that the existing rim accepts the new section. Ask which measuring rim produced the candidate's published dimensions, because the code alone cannot position the shoulders relative to the actual wheel arch.

Is the circumference for 245/35R20 measured on a loaded car?

No loaded measurement enters this result: 2134.71 mm follows mathematically from the size code diameter. It corresponds to 753.89 ideal revolutions per mile. If your question concerns a particular tire under load, preserve the actual product and test conditions rather than treating this nominal circumference as an observed rolling distance.

What if the current twenty inch wheel already sits near the arch?

Start with the exact assembly that produced the observed clearance, including wheel width and offset. Then obtain the candidate's measured section and measuring rim information. Nominal diameter alone cannot show where a broader sedan tire will sit beside the body, even if the ideal radius increase is small.

Continue your comparison

• Calculate the change using your actual original tire
• Understand the metric to inches calculation