The proposal from Buckinghamshire councils includes a 15-mile (24km) tunnel from the M25 to Wendover.
Current government plans include some tunnels but leaves about eight miles in exposed or partially-covered cuttings.
HS2 said it had "considered" further tunnelling but stood by its plans.
Plans for the HS2 between London and Birmingham and then to Manchester and Leeds have been developed by the government under holding company HS2 Ltd.
It promises to reduce journey times between Birmingham and London from 81 to 49 minutes, and slash the trip to Manchester by an hour, to 68 minutes.
The new Chilterns Long Tunnel Route report, produced by Peter Brett Associates, was commissioned by Chiltern District Council, Aylesbury Vale District Council, Buckinghamshire County Council and the Chilterns Conservation Board to look at alternative proposals.
The group said the planned route through the Chilterns Area of Outstanding Natural Beauty (AONB) would result in an "adverse and irreversible impact" on the landscape.
HS2 Ltd's proposal includes about 13km (8 miles) in a tunnel from the M25 to Little Missenden and 12km (7.5 miles) on two viaducts, in two green tunnels and either on embankments or in cuttings.
The new plan includes a 24.2km (15 mile) tunnel from the M25 to Wendover, the first 9km (5.5 miles) of which would follow the exact alignment of the government scheme.
The report concluded a long tunnel was "technically feasible", would protect the landscape and "be a better route operationally".
It is the second time the group has proposed a tunnel scheme but it said the latest proposal would have less impact on the countryside.
The cost is estimated at Â£1.85bn, as opposed to the government's Â£1.45bn plan, but campaigners said this was "a small price to pay" to "preserve a beautiful and unique part of the countryside".
HS2 spokesman Ben Ruse said the firm was "committed to protecting the environment" and a tunnel through the entire AONB had been "explored and given extensive consideration".
He added: "However, we have clearly set out how the current proposals provide the best balance between route engineering design requirements, cost and the desire to minimise environmental impacts."

Mr Davies is widely known as Welsh Conservative leader but his authority does not extend beyond the assembly Tory group, under party rules.
Speaking on BBC One's Sunday Politics Wales, the AM admitted the Welsh party lacked clear leadership.
The Tories lost three seats in Wales at the general election, leaving them with eight.
It lost both of the seats it gained at the 2015 general election from Labour - Gower and the Vale of Clwyd - and also lost Cardiff North.
When it was put to him that there was a lack of clear leadership in the Tory campaign in Wales, Mr Davies said: "I agree with you entirely.
"We have a very strong distinct Welsh Conservative brand, but we do need to be able to make key political decisions here in Wales and have a designated leader here in Wales, similar to what Labour formalised back in March. I readily acknowledge that.
"I lead on devolution, on secretary of state aspects such as Westminster, obviously the secretary of state leads then. The chairman of the voluntary party obviously leads on the voluntary party... That cannot continue."
Mr Davies said he thought Scotland "had a very good model which has shown to be successful".
There, Ruth Davidson led a separate campaign to the Conservatives in England and Wales, largely focused and opposed to the SNP's calls for a second independence referendum.
The party gained 12 seats in Scotland, finishing with 13.
"There is a deficiency in the ability to make those key decisions from a party perspective and I do acknowledge that," said Mr Davies.
But, he added: "Let's not forget, there's a huge job of work to be done as we go forward as a country. The Conservative Party is the largest party in Westminster and its incumbent we form a government."
The Welsh Conservative campaign was marred by a row over who was meant to have represented the party in the BBC Wales Leaders' Debate.
Neither Mr Davies or Welsh Secretary Alun Cairns took part - with senior AM Darren Millar taking the podium instead.
Plans for an HS2 tunnel under the Chilterns to protect the area have been announced by campaigners opposing existing proposals for the high-speed rail link.
[NEXT_CONCEPT]
The Welsh Tories need a designated leader that can make key decisions for the party, Andrew RT Davies has said.