🚗 Original Tire from Technical Specs
New Tire to Compare
📊 Technical Dimensions Comparison
How do you calculate your tire equivalency in Spain in 2026 to pass the mandatory ITV technical inspection without having to pay for expensive engineering homologation projects? According to the Vehicle Reform Manual of the Spanish Ministry of Industry, changing your car’s tire size is considered a permissible modification and does not require custom homologation, provided the new tires are equivalent to the original ones. For two tires to be declared equivalent by ITV inspectors, the total outer diameter of the replacement tire must have a difference of less than or equal to 3.00% compared to the original tire dimensions listed on the vehicle’s official technical specification card. In addition, the load index and speed rating of the replacement tires must be equal to or higher than those of the original ones. If you want to estimate your vehicle’s recurring taxes, you can calculate them using the Road Tax Calculator (IVTM) or check renovation costs with the Building Permit Cost Calculator.
🔍 Technical Rules for Tire Equivalency Under Spanish Law
For a replacement tire to pass the technical inspection without an engineering project, it must meet 4 criteria:
- Diameter Variation of ±3.00%:
- The mathematical difference between the total outer diameter of the replacement tire and the original tire on the spec sheet must not exceed 3.00%.
- Equal or Superior Load Index:
- For example, if your vehicle requires tires with a load index of 91 (615 kg per wheel), you can fit tires rated 91, 94, or higher, but never 88 (560 kg).
- Equal or Superior Speed Rating:
- If your original speed code is V (up to 240 km/h), you can fit V, W (270 km/h), or Y (300 km/h) tires, but not H (210 km/h), except in winter tires.
- Track Width & Body Clearance:
- Fitting the new tires must not alter the vehicle’s track width by more than 30 mm nor require modifications to the outer wheel arches.
📝 Worked Examples
Example 1: Changing from 16-inch to 17-inch rims
Profile: Carlos, owner of a passenger car with original 205/55R16 tires, wants to fit larger 225/45R17 wheels.
- Original size: 205/55R16 | Original diameter: (16 × 25.4) + (2 × 205 × 0.55) = 631.9 mm
- Proposed size: 225/45R17 | Proposed diameter: (17 × 25.4) + (2 × 225 × 0.45) = 634.3 mm
- Percentage difference: ((634.3 − 631.9) / 631.9) × 100 = +0.38%
- Verdict: The variation (+0.38%) is well below the ±3.00% DGT limit.
Example 2: Oversizing both rim and profile
Profile: Laura, who wants to change her car’s tire dimensions from 195/65R15 to 225/50R17 to give the vehicle a sportier look.
- Original size: 195/65R15 | Original diameter: (15 × 25.4) + (2 × 195 × 0.65) = 634.5 mm
- Proposed size: 225/50R17 | Proposed diameter: (17 × 25.4) + (2 × 225 × 0.50) = 656.8 mm
- Percentage difference: ((656.8 − 634.5) / 634.5) × 100 = +3.51%
- Verdict: The variation (+3.51%) exceeds the 3.00% maximum limit.
Example 3: Lowering profile while keeping the original rim size
Profile: Albert, who wishes to mount lower-profile tires on his original 16-inch rims (changing from 205/55R16 to 205/45R16).
- Original size: 205/55R16 | Original diameter: 631.9 mm
- Proposed size: 205/45R16 | Proposed diameter: (16 × 25.4) + (2 × 205 × 0.45) = 590.9 mm
- Percentage difference: ((590.9 − 631.9) / 631.9) × 100 = −6.49%
- Verdict: Lowering the profile reduces the diameter by 6.49% (exceeding the 3% limit).
⚠️ 4 Common Mistakes for Drivers
- Only looking at the rim size when swapping wheels: Many drivers buy larger wheels assuming that offsetting with a thinner tire guarantees compatibility. Ignoring the exact mathematical calculation of the outer diameter often leads to a difference of over 3%, forcing you to homologate the vehicle or repurchase correct sizes.
- Ignoring load and speed index ratings: Even if the outer diameter is equivalent (e.g., a variation of +0.2%), fitting tires with a lower load index (such as going from 91V to 88V) or a lower speed rating (such as W to H) constitutes a serious defect at the ITV, preventing your car from passing.
- Failing to check for physical bodywork rubbing: A tire size can be mathematically equivalent, but if it is much wider than the original, it may rub against suspension parts or wheel arches when turning the steering wheel fully. Physical rubbing or fender modification requires official homologation.
- Forgetting to replace the spare tire: If you change the diameter of your four main wheels by more than 1% but keep your original spare tire size, a puncture will force you to drive with an uneven axle. It is recommended to update the spare tire to match the new dimensions.
🏠 Special Scenarios in Spain
Exemption for Winter Tires (M+S)
Spanish vehicle regulations allow a specific speed rating exception for winter tires or mud and snow tires (M+S marking). On these tires, drivers can fit a speed rating lower than the one homologated in the vehicle’s tech sheet, down to a minimum code of Q (up to 160 km/h). If you do this, you must place a sticker in the cabin showing the speed limit.
Individual Homologation (Reforma de Importancia)
If you want to fit tires that exceed the ±3% diameter limit (common in off-road 4x4 tuning), you must undergo a formal vehicle reform. To pass the ITV with non-equivalent tires, you must submit a conformity report issued by the vehicle manufacturer or an accredited laboratory, along with a certificate from the installer workshop.
📋 What This Means for You
If you are a standard driver looking for better tire deals
When replacing tires, search for the most common equivalent sizes. Mass-market tire sizes (like 225/45R17) are often significantly cheaper to buy than smaller, less common sizes (like 205/50R17), saving you money at the workshop while passing the ITV.
If your ITV inspection is coming up soon
Ensure the sidewalls of your tires are clean so the inspectors can easily read the embossed width, profile, diameter, load index, and speed ratings. If your tires are equivalent but do not appear on your technical sheet, the inspector will note the new size on your report sheet at no extra cost.
❓ Frequently Asked Questions (FAQ)
It means that the difference between the total outer diameter of the new tire you want to install and the original tire listed on your vehicle's tech sheet must not exceed 3% (either larger or smaller).
No, you are generally forbidden from fitting tires with a lower speed code than the one homologated. The only exception is for winter tires (M+S), where you can reduce the code down to Q, provided a speed warning sticker is displayed.
No. If the replacement tires strictly meet the equivalency rules (±3% diameter, equal or higher load and speed ratings), the ITV inspectors will log the equivalent tires on your inspection sheet free of charge.
Approved sizes are listed on your vehicle's official technical inspection card (ficha técnica), under the tires section or in the approved options list at the bottom of the card.
No. You can mount equivalent tires on your original rims (e.g., changing from 205/55R16 to 215/55R16) as long as the new tire width is compatible with the physical width (rim width/junta) of your original wheel.
Even if the diameter is equivalent, tires protruding past the wheel arches are a major defect. To pass the inspection, you must install homologated fender flares that cover the tire tread.
Multiply the width (mm) by the profile (%) divided by 100, multiply the result by 2, and add the rim diameter converted to millimeters (rim size in inches multiplied by 25.4).
Driving a vehicle with unhomologated modifications (such as non-equivalent tires) is a serious infraction. It carries an administrative fine of €200 and leads to a failed ITV inspection.