What is the diameter difference between 255/40R19 and 275/30R21?

255/40R19 has an overall diameter of 27.03 inches (687 mm). 275/30R21 is 27.5 inches (698 mm). That's a 0.46 inch (12 mm) difference — 275/30R21 is taller.

Will 275/30R21 cause speedometer error on a car fitted with 255/40R19?

Swapping 255/40R19 for 275/30R21 produces a speedometer error of about 1.7%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 61 mph. Anything under 3% is usually acceptable without recalibration.

How many revolutions per mile does each size do?

255/40R19 turns 746 revs per mile. 275/30R21 turns 733 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.

What's the sidewall height difference?

255/40R19 has a sidewall of 102 mm (4.02 in). 275/30R21 sidewall is 83 mm (3.25 in). Lower-profile sidewalls give sharper steering response but less bump compliance.

Is 275/30R21 a safe plus-size upgrade from 255/40R19?

A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 1.7%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.

Tire size comparison

255/40R19 vs 275/30R21

Overall diameter, sidewall, revs per mile, and speedometer error — 275/30R21 is the taller tire, with a 1.7% speedo delta when swapping.

255/40R19

27.03in

687 mm overall diameter

275/30R21

27.5in

698 mm overall diameter

Diameter Δ

0.46 in

Speedo error

1.7%

255/40R19 revs/mi

746

275/30R21 revs/mi

733

Within the ±3% overall-diameter band usually considered safe for plus-sizing — speedometer and ABS calibration should stay close to factory.

Full spec

Side by side

Metric	255/40R19	275/30R21
Section width	255 mm	275 mm
Aspect ratio	40	30
Wheel diameter	19″	21″
Sidewall height	102 mm	83 mm
Overall diameter	27.03 in (687 mm)	27.5 in (698 mm)
Circumference	84.92 in	86.38 in
Revolutions / mile	746	733

Other comparisons for these sizes

245/45R18 vs 255/40R19 255/45R18 vs 255/40R19 245/40R19 vs 255/40R19 245/45R19 vs 255/40R19 255/35R19 vs 255/40R19 255/40R19 vs 265/35R20 255/40R19 vs 255/35R20 255/40R19 vs 265/35R19 265/35R20 vs 275/30R21 275/35R20 vs 275/30R21

FAQ

Frequently asked

What is the diameter difference between 255/40R19 and 275/30R21?: 255/40R19 has an overall diameter of 27.03 inches (687 mm). 275/30R21 is 27.5 inches (698 mm). That's a 0.46 inch (12 mm) difference — 275/30R21 is taller.
Will 275/30R21 cause speedometer error on a car fitted with 255/40R19?: Swapping 255/40R19 for 275/30R21 produces a speedometer error of about 1.7%. Your speedometer will read lower than your actual speed — a 60 mph indicated reading is really 61 mph. Anything under 3% is usually acceptable without recalibration.
How many revolutions per mile does each size do?: 255/40R19 turns 746 revs per mile. 275/30R21 turns 733 revs per mile. Taller tyres turn fewer times per mile, which lowers effective final-drive ratio.
What's the sidewall height difference?: 255/40R19 has a sidewall of 102 mm (4.02 in). 275/30R21 sidewall is 83 mm (3.25 in). Lower-profile sidewalls give sharper steering response but less bump compliance.
Is 275/30R21 a safe plus-size upgrade from 255/40R19?: A plus-size upgrade is considered safe when overall diameter stays within ±3% — here the difference is 1.7%. This swap is within the typical safe range; speedometer and ABS calibration should remain close to factory.