Simple math in DB query

I am trying to query my DB and return results where the 'speed' column is 10+km/h over the 'speed_limit' column.

I am using a query builder to take variables out of the request and build them similar to this:

if($overSpeed != false){ $query = $query->where('speed', '>', 'speed_limit' + 10); }

I've tried variations of the following: $query = $query->where('speed', '>', 'speed_limit' + 10); $query = $query->where('speed', '>', '(speed_limit + 10)');

And I cannot seemt o get it right. Any help would be appreciated.