The fourth-generation Toyota Prius is being delayed half a year from its proposed spring 2015 production launch to December 2015, say unidentified sources at Toyota.

Citing a number of vaguely expressed issues the company wants to make sure it gets right, sources told Automotive News the company is of necessity pushing things back.

Officially, Toyota said it is not commenting on future product, but the delay is otherwise confirmed for the company that last year sold 1 million hybrids and which has set a significant 10-percent-plus efficiency improvement goal for it’s updated flagship hybrid.

Issues referenced as possible reasons for the delay include more work needed in tweaking fuel economy, and possible body and chassis tweaks.

Satoshi Ogiso, managing officer for product planning would not comment on timing, but did tell Automotive News the new Prius will be a critical test of a number of new technologies including the next-generation hybrid system and Toyota’s new modular vehicle architecture.

Given that other vehicles will share components, if the company were to release cars early that later needed to be updated or recalled, it could extend to millions of vehicles, thus urgency is increased to do it right the first time.

Toyota has said its hybrid system will be lighter, smaller, and more efficient and Ogiso said the company needs to “prioritize fuel efficiency.” Thermal efficiency also from the Atkinson-cycle engine is to be raised to over 40 percent compared to today’s 38.5 percent.

When it’s overhauled, the new hybrid system will support hybridization of myriad engine and vehicle types, said Ogiso, so this is not just about the regular Prius and Camry class hybrids.

Toyota intends to build its first pre-production Prius confirmation vehicle, a car it traditionally builds around one year before full production, around November 2014.

The plug-in Prius is projected to begin production in October 2016.

Automotive News