Which tire is taller, 235/55 R17 or 255/40 R18?

235/55 R17 is taller by 29.1 mm (4.22%) compared with 255/40 R18.

What is the diameter difference between 235/55 R17 and 255/40 R18?

235/55 R17 measures 690 mm overall diameter, 255/40 R18 measures 661 mm — a difference of 29.1 mm (4.22%).

How much speedometer error does this change create?

Switching from 235/55 R17 to 255/40 R18 changes the speedometer reading by -4.22%. At an indicated 100 km/h, your true speed becomes about 95.8 km/h.

Is 255/40 R18 a safe replacement for 235/55 R17?

Between 3% and 5% diameter difference — speedometer drift is noticeable. Consider recalibration and confirm clearance with your vehicle's manufacturer specification.

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

235/55 R17versus255/40 R18

Δ Ø -29.1 mmSpeedo -4.22%Borderline

255/40 R18 is shorter than 235/55 R17 — quicker gearing feel, tighter arch gap, livelier throttle response.

Switching from 235/55 R17 to 255/40 R18 is a plus-1 upgrade that wraps a shorter sidewall around a larger 18-inch wheel. This alternative fitment swings rolling diameter far enough to feel on the road.

Indicated speed will drift far enough that recalibration is worth considering. Less sidewall flex usually translates to crisper turn-in and a slightly stiffer ride over rough pavement. 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 delta falls in the cautious 3–5% range, where speedometer recalibration and a careful clearance check are worth doing.

TakeA solid pick for drivers chasing a more aggressive stance without abandoning OEM rolling diameter.

Quick fitment verdict

235/55 R17 → 255/40 R18 at a glance

OEM Safe

Borderline

Noticeable drift from OEM — drivable, but recalibration is wise.

Fender Clearance

Check at lock

Wider or taller setup — verify clearance at full steering lock and over bumps.

Speedometer Impact

-4.22%

Dash reads 95.8 km/h at a true 100 km/h — visible drift.

Daily Driving

Aggressive

Geometry deviates enough to matter — confirm clearance before daily use.

Side-by-side telemetry

Dimensional read-out

Current

235/55 R17

Diameter: 690.3 mm
Sidewall: 129.3 mm
Wheel: 17″
Width: 235 mm

Δ delta

Ø -29.1 mm

SW -27.3 mm

RH -14.5 mm

NewNew

255/40 R18

Diameter: 661.2 mm
Sidewall: 102.0 mm
Wheel: 18″
Width: 255 mm

Real-world effects

How this swap actually feels

Steering response
98/100 · Sharper turn-in
Ride comfort
22/100 · Firmer ride
Fuel economy
37/100 · Slightly higher drag
Highway cruising
43/100 · Higher cruise revs
Pothole resistance
13/100 · 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

Lower stance

-14.5 mm

Chassis drops — tighter arch gap, more aggressive stance.

New tire drops ride height by ~14.5 mm — tighter arch gap, lower stance.

Suspension travel · arch clearance

Wheel gap

Wheel sits closer to the fender

-14.5 mm

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

235/55 R17

255/40 R18

Wheel-gap Δ-14.5 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 95.8 km/h

-4.22%

Shorter rubber: dashboard reads conservatively low — you're slower than it claims.

ABS · ESP · cruise control

Setup telemetry

How this setup changes the car

Driver-perspective read-out of the 235/55 R17 → 255/40 R18 swap — steering, comfort, stance and dash behavior in plain enthusiast language.

Steering feel

-27.3 mm sidewall

Sharper steering response

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

Ride firmness

55% → 40%

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

+20 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

-4.22%

Noticeable speedo drift

Drift is visible at highway speeds; ABS still works but loses a sliver of precision.

Daily drivability

Ø -29.1 mm

Aggressive setup — verify before daily use

Geometry deviates enough to matter — check clearance, recalibrate the dash, then re-evaluate.

Direct answer

Is 255/40 R18 OEM-safe?

Borderline. Overall diameter changes by -4.22% versus 235/55 R17. Borderline. Drivable, but speedometer drift becomes noticeable and ABS calibration is affected.

Direct answer

Will 255/40 R18 rub?

Borderline. Width changes by +20 mm and diameter by -29.1 mm. Borderline — check fender lip and inner strut clearance under load.

Direct answer

Does the speedometer change?

Yes — by -4.22%. Swapping 235/55 R17 for 255/40 R18 changes overall diameter, so at an indicated 100 km/h your true speed becomes 95.8 km/h. That's noticeable drift but usually safe.

Direct answer

Does lower sidewall affect comfort?

Yes — firmer ride. Sidewall changes by -27.3 mm (55% → 40%). Ride becomes firmer and steering sharper, but potholes and expansion joints hit harder and wheel damage risk rises.

Current Tire

235/55R17

Width

Ratio

Rim

New Tire

255/40R18

Width

Ratio

Rim

Fitment · Scaled comparison

● Borderline

235/55 R17

Diameter

-29.1 mm

-4.22%

Sidewall

-27.3 mm

Speedometer

95.8 km/h

at true 100

Clearance

Borderline

255/40 R18

Ground line · Scaled comparison

Slight Difference

Within ±5% — usable, recalibration recommended

Diameter change

-29.1 mm

-4.22%

Speedometer at 100

95.8 km/h

-4.22% error

Ground clearance

-14.5 mm

ride height delta

Sidewall change

-27.3 mm

revs/km: 481.4

Open visual comparisonCompare visually

Permalink for this comparison:

/compare/235-55-r17-vs-255-40-r18

Detailed comparison

Metric	235/55 R17	255/40 R18	Difference
Overall diameter	690.3 mm	661.2 mm	-29.1 mm (-4.22%)
Sidewall height	129.3 mm	102.0 mm	-27.3 mm
Circumference	2.169 m	2.077 m	-91.4 mm
Revs / km	461.1	481.4	+20.3
Ground clearance	reference	-14.5 mm	-14.5 mm
Speedometer @ 100 km/h	100.0 km/h	95.8 km/h	-4.22 km/h

Verdict: warning

Between 3% and 5% — noticeable speedometer drift; recalibration may be advisable.

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

235/55 R17

Profile: 55%
Circumference: 2.169 m

New

255/40 R18

Profile: 40%
Circumference: 2.077 m

Side-by-side fitment

Geometry

Current

235/55 R17

Section width: 235 mm
Aspect ratio: 55%
Sidewall: 129.3 mm
Wheel diameter: 17″(432 mm)
Overall diameter: 690.3 mm(27.18″)
Circumference: 2.169 m
Revs / km: 461.1

New

255/40 R18

Section width: 255 mm
Aspect ratio: 40%
Sidewall: 102.0 mm
Wheel diameter: 18″(457 mm)
Overall diameter: 661.2 mm(26.03″)
Circumference: 2.077 m
Revs / km: 481.4

Real-world consequences

Pros / cons

Wider tire (+20 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 (-15% 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

Shorter overall (-29.1 mm)

Rolling diameter

Shorter effective gearing — perkier acceleration
Lower center of gravity, sharper transitions
More fender and arch clearance
Speedometer reads high by ~4.2%
Engine spins higher at cruise, small MPG hit
ABS / ESP recalibration may be advisable

+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

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.

Speedometer drift

~4.2% — borderline; recalibration recommended.

Reduced gearing range

Shorter rolling diameter raises cruise RPM and effective gearing.

Cluster preview

Borderline

Speedometer impact

At a true 100 km/h, your dashboard will read 95.8 km/h after switching to 255/40 R18 — a -4.22% 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 -14.5 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

235/55 R17

Back to

255/40 R18