Tire Size Input
How to Use This Calculator
Choose your Audi model
Select your model from the OEM lookup tab. It lists the A3, S3, A4, S4, A4 allroad, A5, S5, A6, S6, A6 allroad, RS 6 Avant, Q3, Q5, SQ5, Q7, SQ7, Q8, SQ8, RS Q8, R8, Q4 e-tron, Q6 e-tron, SQ6 e-tron, Q8 e-tron, SQ8 e-tron and e-tron GT, using the tire sizes in Audi's USA owner's manuals. The calculator fills in your factory front tire size and its load index.
Pick your factory wheel package
Each model lists several wheel diameters and load/speed ratings. Choose the one printed on your tire sidewall and the stock size pre-populates. Staggered cars list a different rear size.
Enter the replacement or plus-size
Type the new tire size you are considering. The format is 235/55R19: width, aspect ratio, R for radial, then rim diameter. The calculator accepts slashes, dashes, and spaces.
Add load index values (especially for EV)
Enter both load indexes if you want the load check. Audi says to check the tire sidewall for the designated load rating of a specific tire, so a replacement should have at least the load index of your current tire.
Read your results
The calculator returns the diameter difference, compared with the 3% limit in Continental's guidance, speedometer error at 50 / 100 / 130 / 250 km/h, ground clearance delta, and a full spec comparison. The visual diagram shows both tires at scale.
Real-World Audi Size Change Examples
Common plus-size scenarios with pre-calculated results. Use these as a sanity check against your own calculator output.
| Model | Stock | New Size | Diam Δ | Speedo Err | AWD | Notes |
|---|---|---|---|---|---|---|
| Audi Q5 (19" to 20") | 235/55 R19 | 255/45 R20 | −0.5% | +0.5% | Under 1% different | Both sizes are in the Audi USA Q5 owner's manual (101H). Calculated difference is small. |
| Audi A4 (17" to 18") | 225/50 R17 | 245/40 R18 | −0.6% | +0.6% | Under 1% different | Both sizes are in the Audi USA A4 owner's manual. Check the load index (94 and 93/97) when choosing. |
| Audi Q3 (18" to 19") | 235/55 R18 | 255/45 R19 | −0.5% | +0.5% | Under 1% different | Both sizes are in the Audi USA Q3 owner's manual (100H). |
| Audi Q6 e-tron (20" front to 21" front) | 255/50 R20 | 265/45 R21 | +1.2% | −1.2% | Over 1% different | Illustration only: 265/45 R21 is a Q8 e-tron size, not listed for the Q6. Shows a larger difference. |
Values calculated using standard P-Metric formula. Real-world diameter may vary ±1–2mm depending on manufacturer molded dimensions.
quattro AWD Tire Diameter Tolerances Explained
Here is what Audi's owner's manuals say about tires on all-wheel-drive cars, what they do not say, and how Audi's own staggered pairs compare.
What Audi says about tires on all-wheel-drive cars
- Only use tires with the same design, size (rolling circumference) and as close to the same tread pattern as possible on all four wheels.
- On all-wheel-drive cars, all four wheels must have tires of the same brand and the same construction and tread pattern so the drive system is not damaged by different wheel speeds.
- Do not replace tires individually. At least replace both tires on the same axle at the same time.
- The ABS and ASR only function correctly when all four wheels have a similar wear condition, and different tire sizes can reduce engine power.
Source: Audi USA owner's manuals (A4 2025, Q5 2025), Wheels chapter
What Audi does not say
- The manuals we checked give no percentage tolerance for the diameter difference between axles, and do not give different tolerances for different quattro systems.
- The calculator therefore reports the percentage difference and compares it with the 3% limit in Continental's guidance. It shows any difference as a difference rather than rating it safe.
Source: Calculator method, not an Audi specification
How Audi's own staggered pairs compare
- e-tron GT 19 inch: 225/55R19 front, 275/45R19 rear, 0.0% apart (calculated).
- e-tron GT 20 inch: 245/45R20 front, 285/40R20 rear, 1.0% apart. e-tron GT 21 inch: 265/35R21 front, 305/30R21 rear, 0.3% apart.
- Q6 e-tron 19 inch: 235/60R19 front, 255/55R19 rear, 0.2% apart. Q6 e-tron 21 inch: 255/45R21 front, 285/40R21 rear, 0.2% apart. Q4 e-tron: 235/55R19 front with 255/50R19 rear (0.5%) and 235/50R20 with 255/45R20 (0.7%).
- The exception is the R8: 245/35ZR19 front with 295/35ZR19 rear is about 5.4% apart, and 245/30ZR20 front with 305/30ZR20 rear about 5.5% apart (calculated). So factory pairs do not show how much mismatch a system tolerates.
Source: Audi USA owner's manuals (e-tron GT 2024, Q6 e-tron 2027, Q4 e-tron 2024, R8 2023); differences calculated
Audi Plus Sizing Quick Reference
Plus sizing means going up one rim size while lowering the aspect ratio to keep the same overall diameter. This table uses the three Audi A4 sedan sizes listed in Audi's USA owner's manual. Diameters are calculated.
| Step | Example Size | Diameter | vs. Stock | Speedo Delta | How It Feels |
|---|---|---|---|---|---|
| +0 (Stock) | 225/50 R17 | 25.86" | None | 0% | Audi A4 17 inch size in the owner's manual |
| +1 | 245/40 R18 | 25.72" | −0.14" / −0.5% | +0.6% | Audi A4 18 inch size in the owner's manual |
| +2 | 245/35 R19 | 25.75" | −0.11" / −0.4% | +0.4% | Audi A4 19 inch size in the owner's manual |
Base: 225/50 R17 (25.86" overall diameter). Always verify your specific stock size using the OEM lookup tab above.
How the Calculator Works: The Math Behind It
Every result the calculator returns is derived from three formulas. No lookup tables, no approximations: all values are computed fresh from your input.
1. Overall Diameter (inches)
The sidewall height in mm is width × (aspect ÷ 100). There are two sidewalls, so multiply by 2. Divide by 25.4 to convert mm to inches, then add the rim diameter. Example: 235/55 R19 → (235 × 0.55 × 2 ÷ 25.4) + 19 = 10.17 + 19 = 29.17 inches.
2. Speedometer Error
Your car's wheel speed sensor fires a fixed number of pulses per revolution. With a larger tire, fewer revolutions cover the same distance: the computer under-counts and displays a speed lower than actual. Circumference = π × diameter. The ratio of the two circumferences gives the speedometer's scaling error.
3. Diameter Difference
This percentage is compared with the 3% limit in Continental's guidance. Audi publishes no percentage in the owner's manuals we checked.
Audi EV Tire Load Index Reference
Audi says to check the tire sidewall for the designated load rating of a specific tire. These are the load indexes Audi's USA owner's manuals list for the electric models, shown for the front tire. The load index to kilograms chart covers values not listed here, and the AWD tire diameter difference check and spare tire chart quoting the Audi manual are on their own pages.
| EV Model | OEM Size(s) | Min Load Idx | Per-Tire kg |
|---|---|---|---|
| Q4 e-tron (2024) | 235/55R19 F / 255/50R19 R | 105 | 925 kg |
| Q4 e-tron (2024) | 235/50R20 F / 255/45R20 R | 104 | 900 kg |
| Q6 e-tron (2027) | 235/65R18 F / 255/60R18 R | 110 | 1060 kg |
| Q6 e-tron (2027) | 255/50R20 F / 285/45R20 R | 109 | 1030 kg |
| Q8 e-tron (2024) | 255/50R20 / 265/45R21 / 265/40R22 | 106 | 950 kg |
| e-tron GT (2024) | 245/45R20 F / 285/40R20 R | 103 | 875 kg |
| e-tron GT (2024) | 265/35R21 F / 305/30R21 R | 101 | 825 kg |
Frequently Asked Questions
Factory sizes and load indexes come from Audi USA owner's manuals. Diameters, speedometer error and percentage differences are calculated. The 3% figure is Continental's guidance, not an Audi specification. Models without an owner's manual in our sources (for example the RS 4) are not listed. Verify with your tire placard and an Audi dealer. tirecalculatorhub.com is not affiliated with Audi AG.