YMMV, but my experience is that dealerships do not offer you nearly as good a deal by email vs negotiating in person, getting close, then getting up ready to walk out (and often walking out, leaving your contact phone number behind). The reasons are
- that they do not see you as serious buyer until you are there and ready to sign,
- that they are very reluctant to put their best offer in writing likely to be used for leverage elsewhere,
- and that they do not have the sunk cost of the time and effort spend negotiating to pull them toward the deal.