The M74 Completion scheme is on course to come in between £15 million and £20 million under budget.
Cabinet Secretary for Infrastructure and Capital Investment Alex Neil said robust management of the project by Transport Scotland on behalf of the Scottish Government had delivered significant savings for the taxpayer.
Neil also praised public and private sector partners such as contractor Interlink M74 on delivering the road substantially ahead of schedule.
He said: “The M74 Completion is the largest infrastructure project on the ground in Scotland and it’s opening eight months early and between £15m and £20m under budget.
“This will mean significant savings for Scottish taxpayers and that drivers will be able to enjoy the benefits of this crucial piece of infrastructure several months ahead of schedule.
“This is clear evidence of our robust and effective management of major projects such as this. Our record of investment in transport infrastructure over the past four years has provided a much needed boost for the construction sector and supported tens of thousands of jobs.
“We have worked closely and, effectively with Glasgow City, South Lanarkshire and Renfrewshire Councils - testament to what can be achieved when a well-planned project is executed through strong collaborative partnerships."
David Welsh Project Director for Interlink M74 JV, a joint venture between Morrison Construction, Balfour Beatty, Morgan Sindall and Sir Robert McAlpine, said: ??"It has been very satisfying working closely with both Transport Scotland and Glasgow City Council delivering a very visible and demanding project whilst retaining a strong spirit of partnership and collaboration.
"In particular we have appreciated the continuing support and engagement of businesses and communities local to the project."