Research conducted by the Lloydspharmacy Online Doctor found that nine out of ten men said they could find the car's oil dipstick but only half were able to name and identify parts of their anatomy.
Dr Tom Brett, sexual health expert at Lloydspharmacy Online Doctor, said: "We all know that two of the most important things for many men are their sex life and their car.
"If the car's not performing properly, the British male is usually pretty confident he can get under the bonnet and solve the problem.
"But when it comes to the mechanics of their sex organs, men are clearly less likely to spot a potential problem."
Only 6% of participants could cite the common causes of erectile dysfunction. A third falsely believed it was caused by wearing tight jeans.
The survey of 1,500 men revealed that when asked to find key sex organs only 52% were correct. Bizarrely, one in 12 thought they were in the ear.