They should absolutely be combined into one - there is NO reason for there to be a separate "Jetta TDI" category. If all you want is to see the Jetta diesels, then YOU CAN STILL DO THAT WITH THE FILTER FUNCTION. Ta da:
Providing two options is giving the exact opposite of the intended result. Instead of having all Jetta TDI's together and getting an accurate representation of just one model, they are split across two groups (currently 38 diesels
under the "Jetta
" category and currently 10 gassers
under the "Jetta TDI
" category), and so the data is being contaminated.
Further, it seems that the EPA data is only associated with the "Jetta" option, and not the custom "Jetta TDI" option, so users opting for the latter don't get that feature.
Creating the Jetta TDI category was a mistake - especially w/o removing the Diesel engine option from the principle "Jetta" category, or the Gas option from the "Jetta TDI" category.
I see absolutely no good reason to keep them separate - move all "Jetta TDI" diesels to the Diesel L4 option under "Jetta" and Keep It Simple.