TireGeoCalc245/40R18 vs 255/35R19: Size and Speed Changes
Quick answer
Nominal geometry245/40R18 → 255/35R19 · original-size calibration assumed
- Diameter difference
- +0.31in
- Diameter change
- +1.21%
- Actual at indicated 60 mph
- 60.73mph
Calculated from size markings. Not measured rolling dimensions or vehicle fitment approval.
Published
This comparison is about changing the wheel as well as the tire. Moving from an eighteen inch bead seat to nineteen inches adds wheel diameter, while dropping the aspect ratio removes sidewall. The tire code has to account for both movements. Treat 245/40R18 as the original so the resulting percentage and theoretical speed have an unambiguous reference. Read the first size as the original throughout this comparison, including the speed and radius examples below.
Calculated tire dimensions and changes from the original
The first row, 245/40R18, is the original reference. All measurements are derived from the tire geometry engine. Percentages and theoretical speed values keep that same original throughout; rounding is for display only.
| Tire size | Width (mm) | Width (in) | Sidewall (mm) | Sidewall (in) | Wheel (mm) | Wheel (in) | Diameter (mm) | Diameter (in) | Circumference (mm) | Circumference (in) | Revolutions / mile | Revolutions / km | Diameter change (%) | Actual at indicated 30 mph | Actual at indicated 60 mph | Actual at indicated 75 mph | Actual minus indicated at 60 mph |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 245/40R18 | 245 | 9.65 | 98 | 3.86 | 457.2 | 18 | 653.2 | 25.72 | 2052.1 | 80.79 | 784.2 | 487.3 | 0 | 30 | 60 | 75 | 0 |
| 255/35R19 | 255 | 10.04 | 89.3 | 3.51 | 482.6 | 19 | 661.1 | 26.03 | 2076.9 | 81.77 | 774.9 | 481.5 | 1.21 | 30.4 | 60.7 | 75.9 | 0.7 |
Nominal size-code geometry. Reference: 245/40R18. Display rounding only; not fitment approval.
- Tire size
- 245/40R18
- Width (mm)
- 245
- Width (in)
- 9.65
- Sidewall (mm)
- 98
- Sidewall (in)
- 3.86
- Wheel (mm)
- 457.2
- Wheel (in)
- 18
- Diameter (mm)
- 653.2
- Diameter (in)
- 25.72
- Circumference (mm)
- 2052.1
- Circumference (in)
- 80.79
- Revolutions / mile
- 784.2
- Revolutions / km
- 487.3
- Diameter change (%)
- 0
- Actual at indicated 30 mph
- 30
- Actual at indicated 60 mph
- 60
- Actual at indicated 75 mph
- 75
- Actual minus indicated at 60 mph
- 0
- Tire size
- 255/35R19
- Width (mm)
- 255
- Width (in)
- 10.04
- Sidewall (mm)
- 89.3
- Sidewall (in)
- 3.51
- Wheel (mm)
- 482.6
- Wheel (in)
- 19
- Diameter (mm)
- 661.1
- Diameter (in)
- 26.03
- Circumference (mm)
- 2076.9
- Circumference (in)
- 81.77
- Revolutions / mile
- 774.9
- Revolutions / km
- 481.5
- Diameter change (%)
- 1.21
- Actual at indicated 30 mph
- 30.4
- Actual at indicated 60 mph
- 60.7
- Actual at indicated 75 mph
- 75.9
- Actual minus indicated at 60 mph
- 0.7
A wheel increase that does not simply add an inch of tire height
The candidate is ten millimeters wider in nominal section, but its lower ratio reduces the sidewall. The larger wheel offsets that reduction and leaves a modest net height increase. That is a different story from adding an inch to the complete tire diameter. Keep the wheel change and the outside diameter change as separate lines in your notes.
An eighteen inch tire cannot mount on the nineteen inch wheel required by the candidate. The new wheel needs the correct attachment, brake clearance, width and offset. The close overall diameter comparison does not answer any of those mounting questions. If you are comparing complete wheel packages, obtain the wheel specifications before treating the tire rows as interchangeable assemblies.
Speed, radius and wheel position in this pair
With 245/40R18 as original, 255/35R19 changes the calculated outside diameter from 653.20 to 661.10 mm. The signed difference is +7.90 mm (+0.31 inches), or +1.21%. At an indicated 60 mph, original circumference calibration would give 60.73 mph on the candidate, a +0.73 mph shift. At an indicated 30 mph the corresponding value is 30.36 mph. These numbers compare two ideal circles rather than measuring the existing instrument error.
The nineteen inch candidate replaces 17.50 mm of combined sidewall with 25.40 mm of extra wheel, leaving 7.90 mm of outside height gain. Its radius increases by 3.95 mm, so the ideal wheel center rises by much less than the inch added to the bead seat. Width also goes from 245 to 255 mm. This is a useful sport wheel comparison when appearance starts the discussion, but its two assemblies need different wheels. Check attachment, offset and brake clearance as part of that package choice; similar complete heights cannot show whether the nineteen inch wheel itself suits the car.
Put the dimensional result in context
The added width and the larger radius affect different parts of the clearance investigation. A wheel with a different offset can move the shoulders even when nominal tire width changes only slightly. Ask for the exact tire dimensions on the intended wheel and check the relevant vehicle requirements. This pair is useful precisely because it separates similar total height from a substantially different wheel and sidewall arrangement.
Frequently asked questions
When would an owner compare these complete sport wheel packages?
Someone considering a larger wheel for a sedan or coupe may compare 245/40R18 with 255/35R19 to keep outside height relatively close while changing the visible wheel proportion. The candidate still needs a nineteen inch wheel, and its lower sidewall cannot mount on the original eighteen inch bead seat. Retaining the original package may make sense when it is the documented configuration already owned; researching the candidate requires wheel and application evidence. The small height increase explains the comparison, but it does not grant interchangeability.
What nominal travel does one turn of 245/40R18 represent?
Its 653.20 mm circle gives 80.79 inches of calculated circumference. Dividing one mile by that travel gives 784.25 geometric revolutions. This refers to the ideal outside circle; a catalog rolling value under specified conditions is a different quantity and should retain its own measurement definition when you compare products.
Can the larger wheel use the original tire?
The eighteen inch original and nineteen inch candidate have different bead seats. The candidate discussion therefore concerns a complete wheel package. Obtain the wheel's attachment, offset, width and application details before purchasing rubber, even though replacing sidewall with wheel leaves the two outside heights relatively close in the nominal calculation.
Continue your comparison
These are nominal geometric calculations, not vehicle fitment approval. Check the vehicle manual and exact tire and wheel specifications, including load and speed ratings, approved rim width and clearance; consult the manual for any diameter change exceeding 3%.
Verified against manufacturer data
Published specifications for the named models, checked on the dates below. Differences mean published minus calculated, divided by calculated. These are examples, not specifications for every tire of this size. P, LT and XL designations remain in the source row; matching nominal dimensions does not make their load capacities interchangeable.
| Size / model | Calculated diameter | Published diameter | Difference | Revolutions per mile: calculated / published (difference) | Source / checked |
|---|---|---|---|---|---|
| 245/40R1897V ToyoCelsius II (243870) | 25.72 in | 25.70 in | -0.06% | 784.2 / not recorded in rev/mile | Toyo specifications |
| 245/40 R 1897V XL General TireAltiMAX RT45 (15577620000) | 25.72 in | 25.70 in | -0.06% | 784.2 / 808 (+3.03%) | General Tire specifications |
How we calculate: nominal geometry and measured rolling data can differ. This comparison is not vehicle fitment approval.
Reference tables: Load index chart and Speed rating chart.