Kriss Motors Electric Conversions

Electric versus gasoline

A comparison of the electric Porsche 356 with its gasoline-powered cousin.

Power system
TypeAC electric motorair-cooled engine
Power at peak torque (1)57 kW52 kW
Useful RPM maximum6,5006,000
Expected power train life (2)385,000 miles100,000 miles
Transaxle final gear ratio3.88 to 14.42 to 1
Energy storage
Capacity (3)23.0 kWh490.0 kWh
Weight (battery or gas tank) (4)450 lbs106 lbs
Energy density109 Wh/kg12,200 Wh/kg (5)
Storage format36 LiFePO4 cells13.8 gallon fuel tank
Top speed (6)110 mph110 mph
Useful range (7)125 miles250 miles
Efficiency (8)
Miles per gallon or equivalent (9)125 mpg20 mpg
Cost per 10,000 miles (10)$186$2,000


  1. Lithium estimate based on 4th gear at 4200 rpm; gasoline estimate based on 70 bph of the 1582cc overhead cam flat-four Porsche engine at 4500 rpm
  2. Lithium estimate based on charge cycle life calculated using 30% DOD specification for average depth of discharge; gasoline estimate based on 2,000 hour life before major overhaul at average 50 mph speed
  3. Lithium based on 3.2 nominal voltage, 200 Ah capacity, and 36 LiFePO4 cells (lithium cells lose approximately 2.7% of capacity per year, known as calendar fade); gasoline based on fuel tank capacity
  4. Lithium LiFePO4 cell weight 12.5 lbs; gasoline average weight 6.25 lbs/gallon, tank weight 20 lbs with pump and filler tube
  5. Based on average gasoline energy density
  6. Effective maximum speed based on chassis, suspension, and body air resistance
  7. Lithium estimate based on 165 Wh/mile, 10% SOC; gasoline estimate based on 20 mpg, 10% fuel residual of 1.3 gallons
  8. Assumes average U.S. residential electricity rate of $.11/kWh and $4/gal average U.S. fuel cost at 18 mpg
  9. Calculated using average $4/gal gasoline versus $0.0186/mile electric cost
  10. Based on average full charge cost of $2.70, $0.0186/mile with electricity; gasoline at $0.20/mile based on $4/gal and 20 mpg

The Cars

Porsche 356 Speedster

The iconic Porsche Speedster proved to be an excellent EV conversion candidate due to its low weight, relatively simple design, and ample front/rear compartments for mounting pristmatic lithium battery cells. Weight distribution was improved over the original rear engine configuration with better acceleration and overall performance.

Saab Sonett

The unusual fiberglass Sonett, originally powered by a Ford V4 engine, combines a low center of gravity and front wheel drive, optimal for our high performance EV conversion. Without the need to accomodate a long drive train, the space behind the two seats was used for additional batteries. The original four-speed transmission remains intact along with the braking system.