The month of May is officially underway, with Indianapolis Motor Speedway holding the first day of open testing for the oval versions of the new aero kit bodywork components that will have their first race outing in the 99th running of the Indianapolis 500.

This year's Opening Day at IMS saw the Penske teams predictably quick to set the pace, with Penske's Juan Pablo Montoya and Helio Castroneves topping the timesheets for Sunday's full day of testing, which saw the field run 1845 laps without incident.

CLICK: Full times from Opening Day at Indianapolis Motor Speedway

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Montoya - who won the iconic race at his first attempt in 2000 before moving to F1 and NASCAR - set the target time of 39.6874s (226.772mph) after 95 laps of the 2.5-mile speedway, just over half a tenth faster than his team mate, already significantly faster that the 223.057mph set by Will Power during last year's Opening Day, showing the impact of the new bodywork components designed to balance aerodynamic drag reduction for speed with sufficient downforce for handling.

However the teams still have some way to go over the next few weeks before they match Ed Carpenter's four-lap average speed of 231.067mph which won him pole in 2014, let alone Arie Luyendyk's all-time speed record of 237.498mph set back in 1996 with a very different car and engine specification.

"It's okay," said Montoya of his day's work. "It's is good for Verizon and for Chevy and for everybody that is paying attention, but I think the time sheet, as always, is irrelevant until you get to the race or until you get to qualifying. Everybody is posting times in the draft and it's a matter of who is doing the best in the draft."

There was encouraging news for the Honda camp has well, which has struggled to match the pace of its rivals Chevrolet in the four road and street course races so far this season which have used a different aero kit configuration. Macro Andretti proved to be third fastest in the #27 Andretti Autosport entry on Sunday, edging out Chip Ganassi Racing's Scott Dixon for third place by 0.0681s.

"The good thing about that is we're still in the hunt speed-wise," agreed Andretti, although he was quick to add that this was only the first step in a long road leading to the Indy 500 itself on May 24.

"There are definitely still some gremlins to work out," he admitted. "We have an overwhelming laundry list of changes to try. Luckily we have a lot of time to work with it, but we're going to need some time. I don't really want to go into a lot of detail, but there are a lot of things to work out still. I'm not thoroughly pleased with my car now."

"It's faster, for sure, no doubt about it. Speeds are going to be up," said Andretti's team mate and reigning Indy 500 champion Ryan Hunter-Reay. "The difficult part for teams and drivers is balancing the clouded read you get from a big tow and new tyres versus getting a read on the new car," he added after setting the ninth fastest time of the day.

The Andretti contingent expands this month to accommodate a fourth car for Justin Wilson, and a fifth for Simona de Silvestro who has already had two outings with the team so far at St Petersburg and NOLA Motorsports Park.

"The day started off really good, but as soon as it got a little hotter it got a little difficult and we started to slide around quite a bit," said Wilson. "All in all I'm quite pleased with the first day, there is still a lot to work on and lots to try and understand with the new aero kits and the new downforce levels. We're not sure what is a typical baseline for running race run trim or qualifying trim, so we're just working it all out."

"Today was a pretty good first day," de Silvestro added. "I think our speed was OK. For me, it was a really good day just being with the team and learning a little bit on the oval. I think we're all pretty happy with the test day, and for sure we have some work to do, but I think we'll be OK. I'm just happy to be back in the car here in Indy."

Also returning to the Verizon IndyCar Series for the first time this year for this month's big race are Pippa Mann (in the Dale Coyne Racing #63 entry) and Oriol Servia (in the Rahal Letterman Lanigan Racing #32.)

"It was just absolutely fantastic to finally be back in an IndyCar after almost a year sitting out on the sidelines," said Mann. "It's great to be reunited and continue the strong relationship with my team at Dale Coyne Racing.

"I'm pretty happy with our performance on day one since we were able to get over 223mph. We have some work to do next week to help polish out everything, especially the driver!" she laughed. We certainly knocked the rust off today and I am really looking forward to getting in the car on a more consistent basis come next week."

"I enjoy driving here almost every time," said Servia. "It felt really good, I felt at home immediately and it helped that I was with the team I drove with last year. Everything felt like it was only yesterday that I drove an Indy car here instead of a year ago. The team did a great, great job. It already felt better than last year.

"We have a whole week now to look at the data and basically understand the new aero package," he continued. "We have wind tunnel figures, which is good, but it's not the same as being on track so both cars tried different things to gather as much data as possible ... I'm feeling pretty good. It's quite amazing to see how different the two aero packages are and yet are still so close in speed."

Servia will be partnering RLL's full-time driver Graham Rahal, who was also sounding upbeat about his first day at work at motor sport's most famous 'office' in the world. "It was a good first day testing the aero kit," said the Rahal Letterman Lanigan driver. "We also had to mileage out an engine, which we successfully did, but we have a little work to do with the handling

"It's better than last year, but we're not where we need to be," he said, after setting the 25th fastest time of the 28 drivers at work at IMS this weekend. "It's the first on-track day of the month, which is good, so there is a lot of time ahead of us. The team is a little split up right now over the two cars, but once we get a full staff in here it will be a lot easier."

Despite the day's track activity. Opening Day and the open oval test is nonetheless something of a false start as far as the Indianapolis 500 itself is concerned. Before official practice gets underway next week there is the small matter of the Grand Prix of Indianapolis race on the 2.439-mile, 14-turn in-field road course at IMS to attend to first.

"It's going to be a little bit of work for the team this week to switch the cars over, but it is what it is, and it's the same for everybody," said Montoya of the challenge that the teams face this week.

"This is a pretty cool road course and the race last year was pretty amazing - with the long straights and the draft, it's pretty exciting and it should bring really good racing," he added.

See full times from Opening Day at Indianapolis Motor Speedway