It's not. Approximately 15% of the lifecycle energy cost goes into production and disposal assuming a 95% metals recycling rate and a 50% plastic recycling rate. You can read the details here.

Take a look at table 5.4 on page 112 of the pdf. Do yourself a favor and read the entire document. It's a little dated but a very good primer.

The numbers would say that if you can reduce the carbon impact of your operating cycle by greater than 15% (or approximately thereabouts) you should upgrade your car.