Tire Diameter Calculator

How to use this calculator

1. Enter your stock tire size. Find it on the door-jamb sticker, in the owner's manual, or stamped on the sidewall — the format is 265/70R17 for metric (passenger and light-truck) or 33x12.50R15 for flotation (lifted-truck and off-road sizes). Or pick a vehicle preset to autofill stock specs and unlock the lift estimate.
2. Enter the new tire size you're considering. Same format. Both fields recompute live — no submit button.
3. Read the result. Three hero numbers: diameter change (with %), speedometer error percent (with actual mph at indicated 60), and the change in revs per mile. Severity chip color-codes the upsize: green (≤3%), yellow (3–7%), red (>7%).
4. Check the lift estimate. If you picked a vehicle preset and the new tire is taller, you'll see roughly how much lift is needed to clear it — based on typical fender-clearance buffer for that vehicle.
5. Save it. Hit "Share" to copy a link with your sizes preserved, or "PDF" for a one-page tire spec sheet you can take to the tire shop or alignment bay.

Why this calculator is different from other tire-size tools

Most tire-size calculators online ship a single output number — height, period. The few that compare two sizes either bury speedometer error in prose, ignore lift fitment, hide behind tire-shop product funnels, or are anonymous. Here's what we did differently:

• Speedometer error and actual mph as a hero output. Most calculators discuss speedo drift in a paragraph; we put the percentage and the actual-speed-at-indicated-60 in the result panel. If your speedometer reads 7% slow, you're doing 64.2 mph at indicated 60 — that's a ticket.
• Lift estimate with vehicle presets. Pick Tacoma, Wrangler JL, F-150, Ram 1500, 4Runner, Civic, Mustang, or WRX — and the calc estimates how much lift you need to clear the upsize, based on typical fender-clearance buffers. None of the SERP-leading calculators do this.
• Side-by-side scale visualizer. The two tires render at true ratio — same scale, same baseline — so you can see at a glance whether the upsize is "looks fine" or "needs flares."
• Plus-size helper built in. Switch to the Plus-Size tab, enter your stock size, and see every alternate within ±3% diameter ranked by closest fit. The 3% threshold is the SAE-standard window where speedometer, ABS, and traction control stay calibrated.
• Three sizes parsed in one engine. Metric (265/70R17), passenger-prefixed (P215/55R17), light-truck (LT265/75R16), and flotation (33x12.50R15) all parse from the same input field. No "tire type" picker, no width/aspect/diameter tri-field UI.
• Branded PDF spec sheet. One-page download with both setups, the diameter delta, speedometer error, severity chip, and a pre-drive checklist (load index, clearance, recalibration, lug-nut re-torque). Take it to the alignment shop.
• Reviewed by an ASE Master Tech. The top SERP results are anonymous calculator-aggregators or tire-shop funnels. Mike Reeves has 15 years in the bay and signs his name to the math.
• Embeddable, free, no signup. Forum mods, Tacoma/Wrangler/F-150 build-thread pinned posts, off-road YouTubers, tire shops — copy a one-line snippet, host the same tool on your page with attribution.

How this calculator works (the math)

Tire-size math is small but easy to get backwards. Five formulas drive everything in this tool:

Section height (the sidewall)

Section height is the sidewall — the rubber from the rim edge to the tread. Aspect ratio is the percentage of the tire's width that becomes sidewall. So a 265/70R17 has a sidewall of 265 × 0.70 = 185.5 mm, or about 7.30 inches.

section_height_mm = width_mm × (aspect_ratio / 100)

Overall diameter (tire height)

Overall diameter is the full tire height from ground to top — the number tire sellers and lift-truck builders care about. It's the rim diameter plus two sidewalls (one top, one bottom):

overall_diameter_in = rim_diameter_in + (2 × section_height_mm / 25.4)

For flotation tires (33x12.50R15), the first number IS the overall diameter in inches — no math needed. The 12.50 is the tread width and the 15 is the rim, but the height is given to you directly.

Circumference and revs per mile

Circumference is just π × diameter. Revs per mile is the number of times the tire rotates to cover one mile (63,360 inches):

circumference_in = π × overall_diameter_in
revs_per_mile = 63360 / circumference_in

Bigger tires turn fewer times per mile. That's why upsizing makes the speedometer read slow.

Speedometer error

Your speedometer reads off the wheel speed sensor, which counts axle rotations. The car's computer assumes the tire is the stock diameter. If the new tire is bigger, the same rotation count covers more ground — so you're going faster than the speedometer says.

speedo_error_pct = ((new_diameter - stock_diameter) / stock_diameter) × 100
actual_speed = indicated_speed × (new_diameter / stock_diameter)

At 7% slow, indicated 60 mph is actual 64.2 mph. At 12% slow, indicated 60 is actual 67.2 mph — that's reckless-driving territory in some states. OBD2 tuners can recalibrate the speedometer once the upsize is done.

Lift estimate

For lift estimation, half the diameter increase becomes vertical clearance need (the tire grows up and down equally). Each vehicle has a typical fender-clearance buffer that the tire fits within before the lip catches:

lift_estimate_in = max(0, (diameter_change_in / 2) - fender_clearance_buffer_in)

This is an approximation — actual lift needed varies with rim offset, suspension travel, and trim level. Use it as a starting point, then verify on a flat lift before ordering parts.

Sources used

Math is cross-validated against Tire Rack's Tire Tech reference, Discount Tire's tire-size methodology, and SAE J1252 (Tire Selection for Light-Duty Vehicles). The 20 fixture cases this tool is tested against live in fixtures.json alongside the source — every release passes them.

Three real-world examples

2024 Tacoma TRD — stock 265/70R17 to 285/75R17

Popular mild-upsize for the new Tacoma. The calc shows diameter delta +2.22" (+7.0%), speedometer error 7.0% slow (indicated 60 = actual 64.2 mph), revs per mile drop 638 → 596. Severity chip is yellow — "verify clearance + recalibrate." With the TRD's typical 0.5" fender buffer, the lift estimate is around 0.6" to clear cleanly. Mike's call: install fits stock fenders with a small body-mount-chop or 1" lift, recalibrate speedo via tuner, run a wider load-range tire to handle the extra mass.

Wrangler JL Sport — stock 245/75R17 to 33x12.50R17

Common Jeep upsize. Diameter +1.53" (+4.86%), speedometer 4.86% slow (60 indicated = 62.9 mph), revs −30. Severity chip is yellow but on the lower end. Lift estimate for the JL Sport (0.75" buffer) comes out to ~0 inches — these fit stock fenders on most JL trims. Recalibration is recommended; some Jeep owners skip it on this delta because the error is moderate, but the trail-cop point on hill descents is real.

Civic Si plus-1 — 235/40R18 to 245/40R18

Width-only upsize, no aspect change. Diameter +0.32" (+1.26%), speedometer 1.26% slow (negligible), revs −10. Severity green: within the 3% safe upsize. No recalibration needed, fender-lip clearance check is the only pre-drive item. This is plus-sizing done right — the rim is the same diameter, the tire's wider but stays close to stock height. Use the Plus-Size tab on this calc to see all the safe alternates for any stock size.

What the numbers mean — plus-sizing, speedo error, RPM

What is the 3% rule?

Plus-sizing — going to a bigger rim with a lower-profile tire — keeps overall diameter within ±3% of stock. That's the SAE-standard window where speedometer, ABS, traction control, and stability control stay calibrated. Outside ±3%, the wheel speed sensors disagree with reality enough that ABS and TCS can fire incorrectly. Above 7%, you're in lift-and-recalibrate territory.

How much speedometer error is too much?

Most US states have an explicit speedo-accuracy threshold, typically ±5%. Above that, the speedometer is technically out of compliance. Practically: 1–2% slow doesn't matter, 3–5% slow is annoying on cruise control, 5–10% slow is when GPS-vs-speedo drift gets noticeable, above 10% slow you're really driving faster than you think — and getting odometer drift on top of it (your maintenance intervals come up later than they should).

What's the difference between metric and flotation tire sizes?

Metric sizes (265/70R17) give you width in millimeters, aspect ratio (sidewall as % of width), and rim diameter. Flotation sizes (33x12.50R15) give you overall diameter in inches first, then tread width, then rim. Flotation is mostly for off-road and lifted-truck applications where overall height matters more than load math. The calc accepts both formats in the same field — write what's stamped on your sidewall.

Why does my speedometer read slow with bigger tires?

Wheel-speed sensors count rotations, not actual distance. The car's computer assumes each rotation covers a fixed distance based on stock tire circumference. Bigger tire = bigger circumference = more ground covered per rotation. The speedometer reports rotations × stock-assumed-circumference, which under-reports your actual speed. OBD2 tuners can re-program the assumed circumference to fix it.

What does "revs per mile" affect?

Revs per mile changes effective gearing. Bigger tires lower revs per mile (the wheel spins fewer times to cover a mile), which raises effective gearing — same engine RPM at the same wheel speed, but you're going faster. Tow rigs and lifted trucks usually offset this with regear (e.g., from a 3.55 to a 4.10 differential ratio). On a stock-axle vehicle, large tire upsizes hurt acceleration, MPG, and towing capacity in ways re-gearing fixes.

Frequently asked questions

Where do I find my current tire size?

Three places, easiest first: the door-jamb sticker on the driver's-side B-pillar, the owner's manual (Tire Information section), or stamped on the sidewall of the tire itself. The format is something like P265/70R17 113T — the part before the load index (113) and speed rating (T) is what this calculator needs.

How much can I upsize without lifting?

Rule of thumb: about 1 inch of overall diameter on most stock vehicles, more on trucks and Jeeps with factory fender flares. A vehicle preset on this calc gives you a more specific estimate. Always test on a flat lift with full steering lock and full suspension compression before driving.

Will plus-sizing void my warranty?

Plus-sizing within ±3% diameter is generally fine — that's the SAE-standard window the manufacturer designed around. Going outside that window can affect ABS, TCS, and electronic stability calibration; some manufacturers will deny warranty claims on related systems if they trace back to a non-spec tire. Keep documentation if you go aggressive.

Do I really need a speedometer recalibration?

Within ±3% diameter, no — the error is below most state legal thresholds and below human perception. From 3–7% it's recommended; above 7%, it's necessary for accurate readings and to keep ABS/TCS happy. OBD2 tuners (handheld programmers from Bully Dog, Hypertech, Superchips) can re-flash the ECU with the new tire diameter — usually a 5-minute job on most modern vehicles.

Can I embed this calculator on my forum or build thread?

Yes — copy the embed snippet at the bottom of this page. Free, no signup, no analytics tied to the embed. The widget includes attribution to RevRated and Mike Reeves, which is part of the deal. If you're a Tacoma/Wrangler/F-150 forum mod and want a custom embed (color theme, default vehicle), email Mike via the contact page.

Related tools

• Wheel Offset Calculator + Visualizer — pair this with offset/poke math when you're swapping wheels and tires together.
• Bar to PSI Converter — convert manufacturer-spec PSI to your gauge's units.
• Browse all free tools by Mike Reeves →

Mike's recommendations for tire upgrades

Whether you're plus-sizing for handling or going taller for clearance, four product categories cover most of what you'll need around a tire swap:

• Best All-Season Tires — match your new size with a load index and speed rating that fits your vehicle. Spec mismatch is the most common reason an upsize feels wrong.
• Best Winter Tires — large upsizes often pair best with a narrower winter set on stock-diameter wheels to keep clearance through slush.
• Best Tire Pressure Gauges — bigger tires need exact PSI to get the contact patch and rolling resistance right. A digital gauge ±0.5 PSI is the floor.
• Best Portable Tire Inflators — top-off cold-morning PSI before sidewall flex steals MPG and ruins handling. Cordless is worth the price.
• Best Running Boards — big upsizes usually mean lift, and lift means higher step-in. Running boards keep the truck usable.
• Best Tonneau Covers — truck-build territory pairs naturally with bed protection. Different category, but the same buyer.

Sources & methodology

• Tire Rack — Tire Tech — canonical sidewall math + plus-sizing reference.
• Discount Tire — Tire Size Calculator Methodology — overall-diameter formula, cross-validation.
• SAE J1252 — Tire Selection for Light-Duty Vehicles — rolling-circumference and speedometer-correction baseline.
• NHTSA FMVSS 110 (Tire Selection / Rims) — fender-coverage baseline used for the lift-estimate buffer.
• Vehicle stock specs (Tacoma, Wrangler JL, F-150, Civic Si, Mustang GT, WRX, Ram 1500, 4Runner) — verified against owner-forum FAQ pins and manufacturer build sheets as of April 2026.

The 20 fixture cases this tool is tested against are in fixtures.json alongside the source. About Mike Reeves · Last reviewed April 30, 2026.

Embed this tool on your site

Free for forum mods, build-thread pinned posts, off-road YouTubers, tire shops, and personal blogs. Required attribution is included in the snippet. No fee, no signup, no analytics attached to the embed.

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  Tire height &amp; diameter calculator by
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  &middot; Reviewed by Mike Reeves, ASE Master Technician
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