Romain Grosjean topped the timesheets on the final day of testing in Barcelona, but Lotus were more impressed by the E23's reliability.
With Pastor Maldonado fastest on two of the first three days, Grosjean put the icing on the cake on Sunday as he took P1 ahead of Mercedes' Nico Rosberg with a time of 1:24.067 on the medium tyres.
The Frenchman also completed 111 laps, which is a far cry from their struggles of 2014 when they could barely manage a session without breaking down.
"It was good to have Romain in the car so we could add his feedback to that we’ve already received here from Pastor and Jolyon," Technical Director Nick Chester said. "Today was particularly windy, making it harder to work on car balance assessments, but we conducted a productive race simulation in the afternoon as well as other performance evaluations."
He added: "We’ve enjoyed a solid first test in Barcelona with good reliability and encouraging performance. The biggest positive we have seen so far is the reliability of the E23 which is enabling us to push along with our assessments and evaluations over the course of 361 laps or 1680km."
Although pleased to see his name at the top, Grosjean knows you can't read too much into it.
"It was great to get behind the wheel of the E23 once more," he said. "We made some good improvements with the balance and set-up over the course of the day and it felt a lot better after we made changes for the afternoon. We completed a very promising mileage and it’s always good to see your name at the top of the timing monitors, even if it is far too early to know what everyone else is doing."