Whoa!!!! Take a step back.....not a black and white conversation.
- Landing pages could be providing you a lot of value if you are getting conversions and converting traffic.
- If the landing pages have unique content they can add additional seo value.
- If the landing pages are all the same and you have only changed the car make....more of an issue.
- What we have done for a number of clients in the auto dealer area is created whats called long page landing pages. So what is seen on the screen win they first land is all the pertinent information for a call to action. If they decide to scroll down to the bottom they start to see much more detailed information about the cars....(less than 12% of our auto dealer landing page traffic reads down below the first screen) This would allow you to put more automobile specific content on the same page (color, model, engine horsepower, accessories, etc.) or (car reviews and other information).....this would create unique content and then actually do the opposite of what you are worried about, those 20 pages would be providing additional SEO value.
- If the landing pages are just there because someone thought they made sense and they aren't converting traffic, then it make sense to remove them.
Before you do anything why don't you do some AB or multivariate testing on the landing pages. If this is a new concept for you, use your google adwords website optimizer o create a small test and compare what the landing pages are converting at. Send some to your home page....send some to other landing pages and see what happens. This data will help you make the right choice.....
Hope this helps.