Tire Size Calculator

Tire Size Calculator comparisons put two sidewall sizes side by side, from metric, LT or flotation codes, then show speedometer error, engine RPM change and sizes within 3 percent.

Comparing Sizes in the Tire Size Calculator

The Tire Size Calculator compares your current tire with a new size and shows how much taller and wider it is, how far off the speedometer will read, and what happens to engine speed. It also checks the load index and speed rating and lists other sizes within 3 percent of your current tire. Truck owners fitting bigger tires, drivers switching wheel sizes, and anyone replacing an odd size can use it before ordering.

Reading the Size on the Sidewall

In the Tire Size Calculator, type the size exactly as it appears on the sidewall, or tap the arrow beside each box to pick a common size. The tool reads metric sizes such as 265/70R17, light truck sizes such as LT285/75R16, and flotation sizes such as 35×12.50R17. It ignores the P, LT, or ZR letters when working out dimensions.

Discount Tire describes a flotation size as the overall diameter, then the width, then the construction letter and the wheel diameter, all in inches. A metric size gives the width in millimeters, the sidewall as a percentage of that width, and the wheel diameter in inches. Results show in inches and mph, or in millimeters and km/h with the metric setting, using 25.4 mm per inch and 1.609 km per mile.

Diameter From a Metric Size

A metric size does not state its height, so the Tire Size Calculator works it out. The sidewall is the width times the aspect ratio, and the overall diameter is the wheel plus two sidewalls, as tire sellers such as RimTyme lay out. The millimeters are then divided by 25.4 to give inches.

$$D = d_{rim} + 2 \times \frac{W \times A / 100}{25.4}$$

For 265/70R17, the sidewall is 265 × 0.70, or 185.5 mm, which is 7.30 in. Two of those plus the 17 in wheel make a 31.61 in tire. The default new size, 285/70R17, comes out at 32.71 in, which is 1.10 in taller and 3.5% larger.

Speedometer and Odometer Error

The speedometer counts wheel turns for the original size, so a taller tire covers more ground than the dash shows. Real speed is the indicated speed times the new diameter divided by the old one, the same ratio a Mopar1973Man tech article uses. At the defaults, an indicated 60 mph is really 62.1 mph, and the odometer falls about 35 miles short for every 1,000.

A Tahoe Yukon Forum member saw the same thing after a 3.6% change. The truck read about 2.5 mph low at 70 mph, which matched both an online calculator and a GPS. The Speedometer Error Table under the cards lists the real speed at common readings from 20 to 80 mph.

What Else Changes With a New Size

A new size affects more than the speedometer, so the four cards and three tables cover the other changes. The hero line shows the new diameter and the change from the current tire. Each card below it answers one of the questions drivers ask most after a size change.

Revs per Mile and Loaded Diameter

Revs per mile is the number of times the tire turns in a mile, which is 63,360 inches divided by the circumference. The default 265/70R17 turns about 638 times a mile and the 285/70R17 about 617, a drop of 22 revolutions or 3.4%. The Tire Size Calculator shows both in the third card and in the side-by-side table.

Printed sizes describe an unloaded tire, and a tire under the vehicle rolls slightly smaller. The Rolling Diameter panel lets you take up to 15% off, with a typical loaded setting of 3%, which raises the default count to about 658 revs per mile. A Rare Car Network test measured tire height on the pavement for the same reason, since the loaded height is what sets real engine speed.

Engine RPM and the Axle Ratio to Restore It

Engine speed in top gear follows from speed, axle ratio, transmission ratio, and tire diameter. The constant 336 comes from 63,360 inches in a mile divided by 60 minutes and by pi. The same formula appears in the Mopar1973Man article and on long-running Corvette and hot rod forums.

$$RPM = \frac{mph \times axle \times gear \times 336}{D}$$

With a 3.73 axle and a 0.70 overdrive at 70 mph, the default truck drops from 1,944 rpm to 1,878 rpm on the taller tires. Restoring the old engine speed would take a 3.86 axle ratio, which is the old ratio times the new diameter over the old. A Wayalife forum member running 285/70R17 tires on 3.73 gears reported the transmission downshifting on inclines until the tire size was recalibrated.

Ground Clearance, Width, and Fit

Ride height rises by half the diameter change, so the default swap adds 0.55 in of clearance under the axles. The new tire is also 0.79 in wider, and its circumference grows by 3.46 in. For the effect on crests and trail angles, the Breakover Angle Calculator takes the new tire size directly.

Discount Tire points out that a size calculator compares dimensions only and does not confirm the tire will fit a vehicle. Wider tires need room at the fender liner, the strut, and the steering stops, and wheel offset changes where that width sits. The Scrub Radius Calculator shows how a new wheel offset moves the tire relative to the steering axis.

Load Index and Speed Rating Check

The load index and speed rating follow the size on the sidewall, as in 112T. Using the standard chart published by Just Tires, a 112 index carries 2,469 lb per tire and a T rating means 118 mph. Just Tires notes that a tire with a lower load index than the original will not carry the original load.

The Tire Size Calculator compares both tires and turns the alert red if the new load index is lower than the current one. It also warns when the new speed rating is lower. The side-by-side table lists the maximum load and speed for each tire once you set them.

Sizes Within 3 Percent of Your Tire

The last table lists up to ten metric sizes within 3% of your current diameter, on the same wheel size and nearby sizes, at a similar width. Tire Rack recommends keeping overall diameter the same when changing wheel and tire sizes, and warns that large changes affect the speedometer, ABS, traction control, and stability control. The 3% limit itself is a tire retailer rule of thumb rather than a formal standard.

Tire Rack’s plus sizing guide adds about 10 mm of width and drops the sidewall by 5 to 10 percent for each inch of wheel diameter. That is why the list shows sizes such as 265/65R18 and 265/60R19 alongside the current 17 in options. Each still sits within a few tenths of an inch of the original height.

Tire Size Calculator Limits

Metric sizes must have a width of 100 to 450 mm, an aspect ratio of 20 to 95, and a wheel of 10 to 30 in. Flotation sizes must be 20 to 60 in tall and 6 to 25 in wide. The axle ratio must sit between 1 and 10, the top gear between 0.3 and 3, and the tire flex below 15%, while the load chart covers indexes 70 to 126.

Every figure comes from the nominal size, and real tires of the same size differ between brands and tread patterns. Check the maker’s published diameter and revs per mile for the exact tire you plan to buy. Fitment, rubbing, and brake clearance still need a physical check on the vehicle.

Input Mistakes That Change the Result

Light truck tires often show two load numbers, such as 121/118, for single and dual fitment. The tool’s chart covers the single-tire index, so enter the first number. Entering the dual figure understates the tire’s capacity.

Comparing tires only by wheel size hides large diameter differences, since two 17 in tires can differ by several inches in height. Relying on the dash after a big change is another trap, because the speedometer and ABS may need separate corrections. A Tahoe Yukon Forum member used one tool to correct the ABS and an aftermarket programmer for the speedometer.

Tire Size Questions From Drivers

What does 265/70R17 mean?

The tire is 265 mm wide, its sidewall is 70% of that width, the R marks radial construction, and it fits a 17 in wheel. Those numbers work out to a 7.30 in sidewall and a 31.61 in overall diameter. The Tire Size Calculator shows all of that, plus about 638 revolutions per mile.

Can I put 285/70R17 tires in place of 265/70R17?

Many truck owners run them, but the swap is 1.10 in taller and 0.79 in wider, a 3.5% diameter change. That sits between the common 3% and 5% guidelines, so check clearance at full steering lock and full compression. FJ Cruiser owners on the IH8MUD forum reported no rubbing without a lift, while noting the speedometer read low afterward.

How much do bigger tires change the speedometer?

The error in percent equals the diameter change in percent. A 3.5% taller tire makes an indicated 60 mph a real 62.1 mph and 70 mph a real 72.4 mph. The odometer under-reads by the same share, about 35 miles every 1,000.

Do bigger tires change engine RPM and shifting?

Taller tires lower engine speed at any road speed, which works like a taller axle ratio. The default swap drops engine speed at 70 mph from 1,944 rpm to 1,878 rpm. Going to 35 in tires from the same 31.61 in stock size would need about a 4.13 axle ratio to restore the old engine speed, and the nearest common set is 4.10.

Does changing tire size affect ABS and stability control?

Tire Rack warns that large diameter changes reduce the effectiveness of ABS, traction control, and stability control, because those systems rely on wheel speed. Many owners leave changes near 3% alone, as one Tahoe Yukon Forum member did at 3.6%. For bigger changes, recalibrate the tire size in the vehicle’s computer, which on some trucks is done separately from the speedometer.