Which tire is taller, 245/40 R20 or 255/35 R21?

255/35 R21 is taller by 7.9 mm (1.12%) compared with 245/40 R20.

What is the diameter difference between 245/40 R20 and 255/35 R21?

245/40 R20 measures 704 mm overall diameter, 255/35 R21 measures 712 mm — a difference of 7.9 mm (1.12%).

How much speedometer error does this change create?

Switching from 245/40 R20 to 255/35 R21 changes the speedometer reading by +1.12%. At an indicated 100 km/h, your true speed becomes about 101.1 km/h.

Is 255/35 R21 a safe replacement for 245/40 R20?

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

245/40 R20versus255/35 R21

Δ Ø +7.9 mmSpeedo +1.12%OEM-safe

255/35 R21 stands taller than 245/40 R20 — bigger rolling diameter, slightly more clearance, calmer cruise revs.

Plus-sizing from 245/40 R20 to 255/35 R21 keeps overall diameter close to factory while opening room for a larger 21-inch wheel. This setup preserves rolling diameter within a hair of the original.

Dashboard speed shifts only marginally — within the noise of normal OEM tolerance. The shorter sidewall gives the tire a firmer, more responsive feel and sharpens steering input. The wider section adds contact patch and lateral stability, while eating into fender and suspension clearance. Visually, the bigger wheel fills the arch and gives the car a more aggressive stance. 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

245/40 R20 → 255/35 R21 at a glance

OEM Safe

Within ±3%

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

Fender Clearance

Clears fender

Width and diameter stay close to stock — arch clearance unchanged.

Speedometer Impact

+1.12%

At a true 100 km/h the dash reads 101.1 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

245/40 R20

Diameter: 704.0 mm
Sidewall: 98.0 mm
Wheel: 20″
Width: 245 mm

Δ delta

Ø +7.9 mm

SW -8.8 mm

RH +3.9 mm

NewNew

255/35 R21

Diameter: 711.9 mm
Sidewall: 89.3 mm
Wheel: 21″
Width: 255 mm

Real-world effects

How this swap actually feels

Steering response
76/100 · Sharper turn-in
Ride comfort
44/100 · Firmer ride
Fuel economy
56/100 · Slightly higher drag
Highway cruising
65/100 · Lower cruise revs
Pothole resistance
37/100 · 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

+3.9 mm

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

New tire lifts the chassis by ~3.9 mm — more clearance, slightly more wheel-gap.

Suspension travel · arch clearance

Wheel gap

Wheel gap visually increases

+3.9 mm

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

245/40 R20

255/35 R21

Wheel-gap Δ+3.9 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 101.1 km/h

+1.12%

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 245/40 R20 → 255/35 R21 swap — steering, comfort, stance and dash behavior in plain enthusiast language.

Steering feel

-8.8 mm sidewall

Sharper steering response

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

Ride firmness

40% → 35%

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

+10 mm width

Fender gap reads near-identical

Width delta is too small to change stance — same visual signature as OEM.

Speedometer behavior

+1.12%

OEM-safe speedometer reading

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

Daily drivability

Ø +7.9 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 255/35 R21 OEM-safe?

Yes. Overall diameter changes by +1.12% versus 245/40 R20. OEM-safe. Speedometer, ABS, ESP and gearing remain inside the factory tolerance.

Direct answer

Will 255/35 R21 rub?

Unlikely. Width changes by +10 mm and diameter by +7.9 mm. Very unlikely to rub with OEM wheel offset.

Direct answer

Does the speedometer change?

Yes — by +1.12%. Swapping 245/40 R20 for 255/35 R21 changes overall diameter, so at an indicated 100 km/h your true speed becomes 101.1 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 -8.8 mm (40% → 35%). Ride becomes firmer and steering sharper, but potholes and expansion joints hit harder and wheel damage risk rises.

Current Tire

245/40R20

Width

Ratio

Rim

New Tire

255/35R21

Width

Ratio

Rim

Fitment · Scaled comparison

● Excellent fit

245/40 R20

Diameter

+7.9 mm

+1.12%

Sidewall

-8.8 mm

Speedometer

101.1 km/h

at true 100

Clearance

Excellent fit

255/35 R21

Ground line · Scaled comparison

Excellent Fit

Within ±3% — safe for daily driving

Diameter change

+7.9 mm

1.12%

Speedometer at 100

101.1 km/h

+1.12% error

Ground clearance

+3.9 mm

ride height delta

Sidewall change

-8.8 mm

revs/km: 447.1

Open visual comparisonCompare visually

Permalink for this comparison:

/compare/245-40-r20-vs-255-35-r21

Detailed comparison

Metric	245/40 R20	255/35 R21	Difference
Overall diameter	704.0 mm	711.9 mm	+7.9 mm (+1.12%)
Sidewall height	98.0 mm	89.3 mm	-8.8 mm
Circumference	2.212 m	2.236 m	+24.8 mm
Revs / km	452.1	447.1	-5.0
Ground clearance	reference	+3.9 mm	+3.9 mm
Speedometer @ 100 km/h	100.0 km/h	101.1 km/h	+1.12 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

245/40 R20

Profile: 40%
Circumference: 2.212 m

New

255/35 R21

Profile: 35%
Circumference: 2.236 m

Side-by-side fitment

Geometry

Current

245/40 R20

Section width: 245 mm
Aspect ratio: 40%
Sidewall: 98.0 mm
Wheel diameter: 20″(508 mm)
Overall diameter: 704.0 mm(27.72″)
Circumference: 2.212 m
Revs / km: 452.1

New

255/35 R21

Section width: 255 mm
Aspect ratio: 35%
Sidewall: 89.3 mm
Wheel diameter: 21″(533 mm)
Overall diameter: 711.9 mm(28.03″)
Circumference: 2.236 m
Revs / km: 447.1

Real-world consequences

Pros / cons

Wider tire (+10 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 (-5% 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

+1″ 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

Comparable wet behavior

Fuel economy

Small MPG penalty likely

Curb / pothole protection

Higher wheel-damage risk

Fitment risk check

Verify before install

Fender rubbing

Check fender clearance, especially with lower offset wheels.

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 101.1 km/h after switching to 255/35 R21 — a +1.12% 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 +3.9 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

245/40 R20

Back to

255/35 R21