If you pay for a year's worth of auto insurance at once, you will almost always receive some form of a discount. This premium markdown is often available for a few reasons. Initially, most car insurance companies demanded the full amount upfront, however, many people were unable to meet this requirement so monthly payments became an option. While this change was ultimately made to benefit the customers, it still costs insurers both money and time in order to process payments and update accounts on a monthly basis versus just once a year. To compensate for this convenience, there are usually processing fees and others of the sort added to monthly installments. Depending on the company, these fees can affect your payment substantially, especially over a year's time. In other cases, you may be looking at a few dollars or so added to your monthly bill.
Not only would you opt out of paying a monthly processing or service fee if you were to pay up for the year, but many insurance companies will also offer additional savings for getting your payment out of the way ahead of time. Insurers grant these cost reductions because receiving these payment upfront reduces the risk of late payments, canceled policies, and extra correspondence throughout the year to make sure that payments are consistently made on time.
Whilst paying in advance and avoiding fractional payments will usually save you some cash, the amount ultimately varies from company to company. To get a more on-target calculation, contact your insurance company directly.