Most people have heard the old adage that failing to plan is planning to fail. Stepping onto a car lot having done no market research and with no plan to secure a good deal is tantamount to wearing a shirt with "Fleece Me" emblazoned across the front. Smart car buyers conduct extensive research before they set foot on a car lot. They compare makes and models, read consumer reports and ascertain average sale prices for specific vehicles. They also decide what percentage of their monthly income they can spend on a car, and over how many years they wish to finance the payment. Based off this information, a buyer can select the right car for his needs and know exactly how much money he is willing to pay for it.

Several tools help car buyers through the research process, and arguably none in more important than an auto loan calculator. This tool uses sales price, interest rate and loan term to instantly calculate a buyer's monthly car payment. The best auto loan calculators also factor in things such as sales tax and trade-in value. The following are the best online auto loan calculators that include tax.

Cars.com features an auto loan calculator loaded with features to help car buyers get the best deal. The main tab has a standard loan calculator that computes monthly payment based on vehicle price, down payment, trade-in value, sales tax, interest rate and loan term.

The next tab allows buyers to work backward from a monthly payment to arrive at a vehicle price. The inputs are the same, except instead of entering the vehicle price, the buyer enters his desired monthly payment. This is a highly useful feature for car buyers who know how much they can afford per month, and roughly what interest rate they qualify for based on credit, and want to know exactly how much car they can afford.

Other features include a side-by-side comparison to determine whether a buyer should accept a cash-back offer or reduced-rate financing, and a loan versus lease comparison tool. Buyers who familiarize themselves with the auto loan calculator on Cars.com and use it extensively before working a car deal are almost guaranteed to avoid getting into loans that are wrong for them.

Autotrader.com offers an auto loan calculator similar to the one found at Cars.com. The main tab asks for the same inputs. One small difference is for the monthly term; Autotrader.com only allows buyers to select a term length in one-year increments, starting with two years and up to six years. On Cars.com, buyers can input any number of months they like. Considering many finance companies offer terms of seven years, or 84 months, or longer, not to mention terms in half-year increments, some buyers might find the calculator at Autotrader.com limiting in this regard.

Like Cars.com, the Autotrader.com calculator also features an additional tab for calculating a purchase price from a desired monthly payment. However, it does not feature some of the additional tools found at Cars.com, such as the loan versus lease comparison.

The auto loan calculator at Bankrate.com is unique because it provides bar graphs for comparing a low-rate financing offer alongside a manufacturer's rebate. Like Cars.com, this calculator allows buyers to enter any term length they like. The only thing missing is the feature that enables buyers to start with a monthly payment amount and arrive at a car price. To work backward like this, which is highly beneficial in many car-buying scenarios, a buyer should use the calculator at Cars.com or Autotrader.com.