For cars first registered before 1998 (whether in the UK or overseas) the car benefit charge is calculated on the basis of engine size and not CO2 emissions. CO2 emissions are not taken into account for any cars first registered before 1998

The guidance on this page explains which cars will have an approved CO2 emissions figure.

Only cars first registered on or after 1 January 1998 can have an approved CO2 emissions figure for the purposes of the car benefits legislation. All cars first registered before 1 January 1998 are dealt with separately, even if they are one of the few with a CO2 emissions figure, see EIM23415.

Cars sold in the United Kingdom and in other European Union (EU) countries are required to meet safety and environmental standards under European legislation. Each car must conform to a type (of which a sample has been submitted by the manufacturer or importer) that has been approved by a national certification agency. The level of CO2 emitted by the car on a prescribed test cycle is one of the factors recorded during the type approval testing procedure. Very small manufacturers (for example, TVR at the time of writing, February 2003) are excluded.

read more here...

http://www.hmrc.gov.uk/manuals/eimanual/eim23360.htm

EIM23415 - Car benefit: car without approved CO2 emissions figure but first registered on 1 January 1998 or later
Section 140 ITEPA 2003

This page relates to step 5 of the method statement in Section 121(1) ITEPA 2003, see EIM23101.

It should be unusual for a car first registered on 1 January 1998 or later not to have an approved CO2 emissions figure. This will probably only be the case for rare and one-off models of car (see EIM23385) and for some cars that have been individually imported from outside the European Union (see EIM23380).
Cars powered by petrol

For cars first registered on 1 January 1998 or later that do not have an approved CO2 emissions figure the appropriate percentage used to calculate the car benefit charge is based on engine size.
Engine Size (cc)

Appropriate percentage

0 to 1,400


15%

1,401 to 2,000


25%

2,001 and more


35%

If a car has no cylinder capacity (i.e. it does not have an engine size measured in cc) the appropriate percentage will be:

* if the car runs solely on electricity (see EIM23435 for definition of electric car)

o 15% until 2010/11
o 9% thereafter (primary legislation changed to retain the same net percentage despite the abolition of all reductions)

* 35% otherwise, for example a rotary engine.

Cars powered by diesel or alternative fuels

The adjustments to the appropriate percentage detailed below apply, except for any part of the reduction that depends on the CO2 emissions figure. The appropriate percentage from the table above is used unless the car falls within one of the categories for which adjustments are required. For details of adjustments required, see

* EIM23420 for years from 2002/03 to 2005/06,
* EIM23421 for years from 2006/07 to 2010/11, and
* EIM23422 for years from 2011/12.

http://www.hmrc.gov.uk/manuals/eimanual/EIM23415.htm