Ferrari 308 GTB: Why Only 712 Were Fiberglass (And Why It Matters)

The Vetroresina: Ferrari's Only Fiberglass Production Car

When Ferrari launched the 308 GTB in 1975 as the replacement for the Dino 246 GT, they faced a problem: the car needed to reach market quickly, and hand-beating steel bodies in aluminum was too slow and expensive for the projected volume. The solution was vetroresina — Italian for fiberglass. Ferrari contracted Scaglietti to build the first 308 GTB bodies in fiberglass, a material Ferrari had never used for a production road car and would never use again. The vetroresina cars were built from 1975 through mid-1977, after which Ferrari switched to steel bodies. The result is one of the most distinctive and collectible Ferraris of the 1970s.

Why Vetroresina Cars Are Worth More

The vetroresina 308 GTBs command a 50-100% premium over equivalent steel-bodied cars. Three factors drive this premium: rarity (712-808 vs 2,185 steel cars), weight (fiberglass bodies are 120-150 lb lighter), and historical significance (Ferrari's only fiberglass production car). The weight difference is not trivial — it transforms the driving experience. The vetroresina cars feel more responsive, more eager to change direction, and more like the Dino 246 that preceded them. Steel-bodied cars feel heavier and more settled, which some prefer for touring but which is less engaging on a winding road.

The vetroresina cars are 120-150 lb lighter than steel cars, which translates to faster acceleration, higher top speed, and a more engaging driving experience. Values reflect a 50-100% premium for fiberglass. Data: Hagerty, 308 GTB Register.

How to Identify a Vetroresina in 10 Seconds

Telling a vetroresina 308 GTB from a steel-bodied one is straightforward if you know where to look. The differences are visible without any tools or disassembly:

1. Door jamb finish — Open the door and look at the inner jamb. Vetroresina cars have a matte, unpainted fiberglass finish (grayish-white). Steel cars have smooth body-color painted finish in the jamb.
   • This is the single fastest identification method
   • Fiberglass has a visible weave texture
   • Steel is smooth and painted
2. Wheel arch seams — Look along the inner edge of the wheel arches. Vetroresina cars have a raised fiberglass seam where the body panels were joined. Steel cars have welded seams that are ground smooth and painted.
   • The seam is visible on both front and rear arches
   • It looks like a raised ridge of fiberglass
   • Steel cars have no visible seam
3. Engine bay finish — Look inside the engine bay at the inner fender panels. Vetroresina cars show raw fiberglass with a gel coat. Steel cars show painted body-color panels.
   • Gel coat is a matte finish, not glossy
   • Fiberglass weave may be visible in some areas
4. Body number stamping — Steel-bodied 308 GTBs have a steel body number stamped on a metal tag riveted to the inner fender. Vetroresina cars do not have this tag because the body is fiberglass.
   • Look for a metal tag on the inner right fender
   • Absence of the tag indicates vetroresina
   • Some vetroresina cars have hand-written numbers in marker
5. Hood and decklid — Vetroresina hood and decklid have a slightly different sound when tapped — a duller, softer sound than steel. The fiberglass panels also have a slightly different fit at the edges.
   • Tap the hood gently — fiberglass sounds dull
   • Steel rings with a higher pitch
   • Panel gaps may be slightly wider on vetroresina

US vs European Vetroresina: Wet Sump vs Dry Sump

Not all vetroresina 308 GTBs are the same. European-market cars use dry-sump lubrication, which allows the engine to sit lower in the chassis (improving center of gravity) and produces 255 HP. US-market cars use wet-sump lubrication to meet emissions regulations, which raises the engine slightly and reduces output to 205 HP. Only approximately 100 US-spec vetroresina cars were built, making them the rarest 308 GTB variant.

The Euro dry-sump vetroresina is the most desirable 308 GTB variant. The US wet-sump vetroresina is the rarest (~100 cars) but commands a lower price than the Euro because of the reduced power output. Production figures are estimates — Ferrari has never released an official breakdown.

Buying Guide: What to Check

1. Verify vetroresina authenticity — Use the 10-second door jamb test. Also check for a Ferrari Classiche certification, which confirms the body material and originality.
   • Door jamb: matte fiberglass = vetroresina
   • Ferrari Classiche certification is the gold standard
   • Cost of certification: ~$3,000-$5,000
2. Chassis number range — Vetroresina cars have chassis numbers in the range 18677-21289. Steel cars start at approximately 20805. There is overlap in the transition period.
   • Vetroresina: s/n 18677-21289 (approximate)
   • Steel: s/n 20805-34329 (approximate)
   • Overlap period (mid-1977) requires visual verification
3. Timing belt — The 308 GTB has an interference engine. If the timing belt breaks, pistons hit valves. Belt replacement interval is 5 years or 30,000 miles. Budget $1,500-$2,500.
   • Check when the belt was last replaced
   • A car with an unknown belt history is a risk
   • Budget for immediate belt replacement if unknown
4. Fiberglass condition — Check for cracks, delamination, and previous repairs. Fiberglass cracks are repairable but should be done by a specialist. Poor repairs are visible as uneven gel coat.
   • Look for hairline cracks around stress points
   • Check for bubbles in the gel coat (delamination)
   • Previous repairs may show as color mismatch
5. Weber carburetors — The 40 DCNF carburetors need synchronization every 5,000 miles. Poor sync causes rough idle and flat spots. Budget $400-$800 for carb service.
   • Listen for uneven idle (carb sync issue)
   • Check for flat spots on acceleration
   • Weber rebuild kits: $200-$400
6. Rust — Vetroresina bodies do not rust, but the steel chassis frame does. Check the frame rails, floor pans, and suspension pickup points. Frame rust is the most serious issue.
   • Fiberglass body does not rust
   • Steel chassis frame CAN rust
   • Check frame rails with flashlight