His car was going/doing at the speed of 30 miles an hour.

[diamondcutter]

1. His car was going/doing at the speed of 30 miles an hour.
2. His car was going/doing at 30 miles an hour.
3. His car was going/doing at 30 miles.
4. His car was going/doing at 30.
5. His car was going/doing 30.
(By me.)

Are all these sentences correct?

 

[tedmc]

I'd write:
1. His car was going at 30 mph. (It is 48 km/h in most countries except the US/Canada and the UK.)
2 His car was doing 30 mph.

 

[Skrej]

#1 only works with 'going'
#2 only works with 'going' (and even then I find it unnatural)
#3 doesn't work either way
#4 doesn't work either way (the problem is 'at' in both cases)
#5 works with either verb, and is the most natural

You can't use 'at' with 'doing' in this context.