Many car companies believe

[Bassim]

I am wondering if my sentence is grammatically correct.

Many car companies believe that in about 20 years they would be able to completely phase out petrol cars and replace them with electrical.

 

[emsr2d2]

I'd use "will", not "would". They're "electric" cars, not "electrical". You need to end with either "electric cars" or "electric models".

 

[SoothingDave]

It's not just petrol (gasoline), it's diesel cars as well. So, saying "internal combustion" or "fossil fuel" cars is probably better.

 

[Tdol]

And you could use vehicles to avoid repeating cars.

 

[Matthew Wai]

I think 'electric ones' would suggest 'electric cars'.