singular/plural

  1. The percentage of car owners was high/higher in 2002.
    1a. The percentage of car owners were high/higher in 2002.
  2. Fifty percent of the pies has disappeared.
    2a. Fifty percent of the pies have disappeared.
    Please correct the above sentences.

The correct versions are 1 and 2a.

Dozy,
These two sentences look similar structure.
1a. The percentage of car owners were high/higher in 2002.
2. Fifty percent of the pies have disappeared.
Then how do you say then #1a is wrong?
Please explain the nuance of its grammar.

They are not the same, and you have been warned by a number of people about the dangers of trying to compare structures from different contexts.

The percentage (of something) was (the verb relates to a singular percentage of something)
Fifty per cent of (the things) haveā€¦ (The verb relates to a plural amount of things)