Start your engines! General Motors (NYSE: GM) will report its first-quarter 2018 earnings results on Thursday, April 26. What should we expect?Analysts polled by Thomson Reuters expect GM to report first-quarter earnings of $1.28 per share, on average, down from $1.75 in the first quarter of 2017. They expect GM's revenue to come in at $34.6 billion, down from $41.2 billion a year ago.Part of what happened is that GM sold its European subsidiary, Opel AG, in a transaction that closed early in the third quarter of 2017.