Fuel protests in the summer of 2000 caused an immediate additional drop of around 5% in the trade value of larger petrol-powered used cars, and Glass's is predicting similar falls if fresh protests commence now.
Jeff Patterson, senior car editor at Glass's, says the rise in diesel popularity will also increase as discontent about fuel prices intensifies.
"The growing popularity of new and used diesel cars will be particularly noticeable in the large saloon and SUV sectors where petrol consumption levels are at their highest," says Patterson.
Oil prices have risen by around 25% since the end of 2003, and the government has already confirmed there will be a 1.92 pence increase in fuel duty in force from the start of September.
The price of unleaded petrol now averages more than 82 pence-per-litre and could soon reach 83.5 pence-per-litre - the level that prompted hauliers and farmers to start blockading fuel depots four years ago.
One demonstration next week will see a convoy of lorries driving through Newcastle-upon-Tyne.
The Treasury said yesterday that Chancellor Gordon Brown, was not considering withholding the planned 2p-per-litre increase on petrol duty due for September.