Severn class all-weather boats should save about 40 litres per hour
The RNLI has cut lifeboat speeds during training exercises in a bid to cut its fuel bill.
Rising costs have resulted in the charity's annual fuel costs almost doubling to an estimated £3m nationally.
Last year, South West crews rescued 1,410 people.
Crews have been asked to reduce their speed from a maximum of 24 knots to 20 knots during training and non-emergency call outs.
The fuel bill to 31 July this year was £99,800, an RNLI spokesman said.
Barry Wagstaff, the regional general manager for the South West, which includes Devon, Cornwall, Dorset and Somerset, told BBC News crews would not be restricted when responding to emergencies.
"All our crews are educated to economise on fuel and in terms of training, 20 knots is fast enough," he said.
"The Severn class all-weather boats use about 540 litres of fuel per hour at 24 knots and this should drop to less than 500 litres at 20 knots."
Mr Wagstaff said it would make a big difference to fuel costs, but the charity could not give exact figures until the end of the year.
The RNLI receives no funding from the government and is totally reliant on charitable donations and bequests.