The above question is one we get asked quite frequently and we do recognize that the terminology used in the Jaguar enthusiast community differs from what Jaguar used officially and in their advertising. We’ll try to describe the different Jaguar terms and the reasons these differences came about. (We’ll be confining our remarks to the later-model Jags that Coventry West stocks parts for, rather than cover all of the postwar Jags.)
The first Jaguar XJ6 sedan was introduced in the US as a 1968 model. About 1973 in the US, Jaguar brought out the XJ6 Series II, and by comparison the first 1968 to 1973 XJ6 became known as the Series I car. In 1979, the Series III car was introduced into the US market and was sold there until 1987. This model carried a number of obvious visual differences, including roofline, taillights, grille, door handles, etc. (It is worth noting that the Series III XJ6 body style was continued in production beyond the 1987 model year for the XJ12, until 1991. This Series III XJ12 was never officially sold in the US market, though it was sold in Canada and Europe in limited numbers. A few have made their way into the US over the years as gray market imports). For all three of these distinct models, the cars were officially XJ6s and were often referred to as simply that, without the Series designation, though some Jaguar advertising refers to Series III, etc, and downplays the XJ6 name.
In mid-1987, the “New XJ6” began to be sold in the US as a 1988 model car. This car carries both a new boxier body with rectangular taillights and a new engine/transmission combination, and is very different visually. But still, Jaguar officially called the cars XJ6s! This “new XJ6” continued through the 1994 model year. This car is sometimes called an XJ6 Series IV. The year 1995 saw another restyling of the Jaguar XJ6 sedan, again termed the “new XJ6” or the “X300” in Jaguar’s press releases and advertising. The model designation finally changed to XJ8, due to the fitting of a new Jaguar V8 engine, in 1998. What this means is that if one mentions “XJ6” without further clarification, it could refer to any Jaguar saloon (Brit-speak for sedan) between the years of 1968 and 1997! In another words, the term XJ6 is not very specific and an unofficial set of designations has evolved to make it a bit easier to communicate just what Jaguar model is being referred to.
For the first three XJ6 variants, the terms most commonly used are Series I, II, and III, so long as it is understood that it is an XJ6 being discussed, and not an E-Type. For the 1988-94 model XJ6, the term XJ40 is used. This designation arises from the fact that Jaguar’s internal program to develop the car introduced in the US as a 1988 model was labeled the XJ40 project, though this term was never used in official advertising. The XJ40 label was commonly used in internal Jaguar documentation and in the automotive press during this model’s long gestation period. The project to develop the new XJ6 introduced in the US as a 1995 model was the X300 project and likewise this designation has come into use to describe the 1995 though 1997 cars. So, if one has a 1989 XJ6, the answer to the question of whether it is an XJ6 or an XJ40 is yes! It can be referred to by either term, but calling it an XJ40 immediately communicates to a Jaguar enthusiast that what is being discussed is a car within the 1988-1994 model years.