Which tire is taller, 275/40 R18 or 305/30 R20?

305/30 R20 is taller by 13.8 mm (2.04%) compared with 275/40 R18.

What is the diameter difference between 275/40 R18 and 305/30 R20?

275/40 R18 measures 677 mm overall diameter, 305/30 R20 measures 691 mm — a difference of 13.8 mm (2.04%).

How much speedometer error does this change create?

Switching from 275/40 R18 to 305/30 R20 changes the speedometer reading by +2.04%. At an indicated 100 km/h, your true speed becomes about 102.0 km/h.

Is 305/30 R20 a safe replacement for 275/40 R18?

Within ±3% diameter — generally considered a safe replacement from a speedometer-error standpoint, provided wheel offset, rim width and clearance also match your vehicle.

What else should I check before changing tire size?

Beyond diameter and speedometer error, verify wheel offset (ET), rim width, hub bore, load index, brake and suspension clearance, and your vehicle's manufacturer specification. Local regulations may also limit how far you can deviate from OEM.

Fitment comparison

275/40 R18versus305/30 R20

Δ Ø +13.8 mmSpeedo +2.04%OEM-safe

305/30 R20 stands taller than 275/40 R18 — bigger rolling diameter, slightly more clearance, calmer cruise revs.

Plus-sizing from 275/40 R18 to 305/30 R20 keeps overall diameter close to factory while opening room for a larger 20-inch wheel. This alternative fitment moves rolling diameter a touch off the original spec. Expect a slight but noticeable shift in indicated speed compared to the original tires. Expect a more planted steering feel, at the cost of some of the cushioning a taller sidewall provides. Diameter change stays inside the conservative ±3% safety window — an OEM-safe fitment on most vehicles.

TakeCommon upgrade for sportier handling and a tighter wheel-gap look on the same vehicle.

Quick fitment verdict

275/40 R18 → 305/30 R20 at a glance

OEM Safe

Within ±3%

Inside factory tolerance — ABS, ESP and cruise control stay calibrated.

Fender Clearance

Likely rubs

Significantly wider/taller — rubbing risk on liners or fender lip is real.

Speedometer Impact

+2.04%

At a true 100 km/h the dash reads 102.0 km/h — negligible.

Daily Driving

Livable

Daily use is fine; expect a slightly different ride and cruise rev count.

Side-by-side telemetry

Dimensional read-out

Current

275/40 R18

Diameter: 677.2 mm
Sidewall: 110.0 mm
Wheel: 18″
Width: 275 mm

Δ delta

Ø +13.8 mm

SW -18.5 mm

RH +6.9 mm

NewNew

305/30 R20

Diameter: 691.0 mm
Sidewall: 91.5 mm
Wheel: 20″
Width: 305 mm

Real-world effects

How this swap actually feels

Steering response
90/100 · Sharper turn-in
Ride comfort
30/100 · Firmer ride
Fuel economy
30/100 · Slightly higher drag
Highway cruising
68/100 · Lower cruise revs
Pothole resistance
21/100 · Less wheel protection

Shareable card

Generate fitment card

Export a garage-grade telemetry card of this comparison — perfect for forums, Reddit and Discord.

Ride height

Lifted stance

+6.9 mm

Chassis sits higher — slightly more clearance, wheel-gap visually grows.

New tire lifts the chassis by ~6.9 mm — more clearance, slightly more wheel-gap.

Suspension travel · arch clearance

Wheel gap

Wheel gap visually increases

+6.9 mm

How the arch-to-tire gap reads from across the parking lot — the visual stance change everyone notices first.

275/40 R18

305/30 R20

Wheel-gap Δ+6.9 mm

Static · unloaded chassis

Fender relationship

Tucked · Flush · Poke

Stance language

The visual relationship between the tire's outer edge and the fender lip — the lens enthusiasts use to judge a fitment.

Tucked

Inside fender

Flush

Lip-aligned

Poke

Outside fender

Width & offset dependent

Speedometer reality

Dash reads 102.0 km/h

+2.04%

Taller rubber: at a true 100 km/h your dashboard reads optimistically high.

ABS · ESP · cruise control

Setup telemetry

How this setup changes the car

Driver-perspective read-out of the 275/40 R18 → 305/30 R20 swap — steering, comfort, stance and dash behavior in plain enthusiast language.

Steering feel

-18.5 mm sidewall

Sharper steering response

Shorter sidewall transmits inputs faster — quicker turn-in, more confident on-center feel.

Ride firmness

40% → 30%

Slightly firmer over rough pavement

Expect more chatter on broken tarmac and a sharper pothole strike — keep an eye on wheel damage risk.

Fender relationship

+30 mm width

Wheel sits closer to the fender

Wider tire pushes the contact patch outboard — flusher stance, but verify fender lip clearance at full lock.

Speedometer behavior

+2.04%

OEM-safe speedometer reading

Inside the factory ±3% tolerance — ABS, ESP and cruise control behave as designed.

Daily drivability

Ø +13.8 mm

Livable upgrade with minor trade-offs

Daily use is fine; expect a slightly different cruise rev count and a touch more road feel.

Direct answer

Is 305/30 R20 OEM-safe?

Yes. Overall diameter changes by +2.04% versus 275/40 R18. OEM-safe. Speedometer, ABS, ESP and gearing remain inside the factory tolerance.

Direct answer

Will 305/30 R20 rub?

Possibly. Width changes by +30 mm and diameter by +13.8 mm. Possible rub at full lock or full suspension compression — verify fender lip and inner strut clearance before committing.

Direct answer

Does the speedometer change?

Yes — by +2.04%. Swapping 275/40 R18 for 305/30 R20 changes overall diameter, so at an indicated 100 km/h your true speed becomes 102.0 km/h. That's within the ±3% OEM tolerance — no recalibration needed.

Direct answer

Does lower sidewall affect comfort?

Yes — firmer ride. Sidewall changes by -18.5 mm (40% → 30%). Ride becomes firmer and steering sharper, but potholes and expansion joints hit harder and wheel damage risk rises.

Current Tire

275/40R18

Width

Ratio

Rim

New Tire

305/30R20

Width

Ratio

Rim

Fitment · Scaled comparison

● Excellent fit

275/40 R18

Diameter

+13.8 mm

+2.04%

Sidewall

-18.5 mm

Speedometer

102.0 km/h

at true 100

Clearance

Excellent fit

305/30 R20

Ground line · Scaled comparison

Excellent Fit

Within ±3% — safe for daily driving

Diameter change

+13.8 mm

2.04%

Speedometer at 100

102.0 km/h

+2.04% error

Ground clearance

+6.9 mm

ride height delta

Sidewall change

-18.5 mm

revs/km: 460.7

Open visual comparisonCompare visually

Permalink for this comparison:

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Detailed comparison

Metric	275/40 R18	305/30 R20	Difference
Overall diameter	677.2 mm	691.0 mm	+13.8 mm (+2.04%)
Sidewall height	110.0 mm	91.5 mm	-18.5 mm
Circumference	2.127 m	2.171 m	+43.4 mm
Revs / km	470.0	460.7	-9.4
Ground clearance	reference	+6.9 mm	+6.9 mm
Speedometer @ 100 km/h	100.0 km/h	102.0 km/h	+2.04 km/h

Verdict: excellent

Within ±3% — speedometer, ABS and traction control should behave normally.

Dimensional comparison

Side-by-side

Scaled engineering side-profile of both tires. Width, sidewall and overall diameter are dimensioned so you can see the change at a glance — without parsing the numbers.

Current

275/40 R18

Profile: 40%
Circumference: 2.127 m

New

305/30 R20

Profile: 30%
Circumference: 2.171 m

Side-by-side fitment

Geometry

Current

275/40 R18

Section width: 275 mm
Aspect ratio: 40%
Sidewall: 110.0 mm
Wheel diameter: 18″(457 mm)
Overall diameter: 677.2 mm(26.66″)
Circumference: 2.127 m
Revs / km: 470.0

New

305/30 R20

Section width: 305 mm
Aspect ratio: 30%
Sidewall: 91.5 mm
Wheel diameter: 20″(508 mm)
Overall diameter: 691.0 mm(27.20″)
Circumference: 2.171 m
Revs / km: 460.7

Real-world consequences

Pros / cons

Wider tire (+30 mm)

Section width

More dry grip and cornering bite
Sharper steering response on initial turn-in
Bigger contact patch under braking
More road noise on coarse asphalt
Worse aquaplaning resistance in standing water
Higher rolling resistance, small MPG hit
Possible fender or strut contact at full lock

Lower profile (-10% aspect)

Sidewall

Sharper turn-in and less sidewall roll
More planted on smooth tarmac
Bigger brake / caliper visual real estate
Harsher ride over expansion joints and potholes
Higher wheel-damage risk on impacts
Less curb protection for the rim lip
More sensitive to correct tire pressure

Taller overall (+13.8 mm)

Rolling diameter

Higher ground clearance and approach angle
Longer effective gearing — calmer highway revs
Bigger contact patch lengthwise
Speedometer reads low by ~2.0%
Reduced fender, strut and bumpstop clearance
Slower 0-60, more downshifts under load

+2″ rim upsize

Wheel diameter

OEM+ look, fills the arch better
Sharper response with matching low-profile rubber
Bigger brake clearance for upgrades
Heavier wheel, more unsprung mass
Harsher ride, more wheel-damage risk
Tire and wheel cost both go up

How it changes driving feel

Seat-of-the-pants

Steering response

Sharper turn-in

Ride comfort

Harsher impacts

Road noise

Louder on coarse asphalt

Wet / aquaplaning

Reduced standing-water margin

Fuel economy

Small MPG penalty likely

Curb / pothole protection

Higher wheel-damage risk

Fitment risk check

Verify before install

Fender rubbing

Width jump >20 mm — verify fender lip and inner liner clearance at full lock.

Suspension clearance

Wider tire may contact strut or control arm on full compression.

Cluster preview

Within tolerance

Speedometer impact

At a true 100 km/h, your dashboard will read 102.0 km/h after switching to 305/30 R20 — a +2.04% offset. Use the speedometer error calculator for any indicated speed, and the speedometer error guide for the full background.

Ground clearance change

The new tire's half-diameter changes ride height by +6.9 mm. Small differences are absorbed by suspension travel, but anything beyond ±10 mm can affect headlight aim, fender clearance and bump-stop margin. See the plus-sizing guide before committing.

Back to

275/40 R18

Back to

305/30 R20