Preparing a travel budget is one of the most important parts of being a happy and successful traveler and even though road trips can be less expensive than other travel options, that doesn't mean you don't have to prepare a budget. Infact there are several unique variables here that need to be taken into consideration - not the last of which is fuel price and consumption rate.
To help better predict the total cost of a road trip, I created a spreadsheet template years ago that I use for each major trip. It helps me plan out the dates of where I'll be on what dates (and then updated with various reservation information) and it also serves as a worksheet for tweaking various budget factors.
Create a Fuel Budget
Luckily, there are tools to help you find the best prices out there, but I generally find that highway-exit stations tend to all be about the same price anyhow.
Of course, gas price isn't the only factor that goes into predicting your total budget cost. Unlike buying plane tickets, your fuel cost is variable. Factors to consider include: MPG of the vehicle, Price of Gas, and Miles Driven.
Calculating Miles Driven
I generally plan out the trip using Google Maps and then take that route and add estimated additional miles. For instance, if the route takes me "To Yellowstone" I know that I'm also going to drive at least another hundred miles inside the park ... so make sure to add that to your budget too.
Calculating Fuel Economy
For a rough estimate, it is easy to use your vehicle's EPA number but in practice, factors such as additional weight of people and gear, terrain, weather, speed, and type of driving can have a significant impact on that number. For instance, while the combined highway / city number for the Ford Explorer that we drove on our epic western road trip was rated at 20 MPG, in reality, the average number was closer to 19 (despite 90% of the travel being on highways). The biggest factor for us in this situation was that the speed limits in Utah, Wyoming, Montana, and Idaho are all greater than 70 MPH and that high-speed travel tends to reduce fuel economy. Similarly, the terrain being driven was mountainous, had lots of twists and turns, and on multiple occasions, we were going off pavement too! Despite the long distances driven though, it kept us incredibly comfortable the whole time.
Determining Fuel Line Item Estimate
Once you've determined those gas-related variables, it's simply a matter of solving the equation: miles driven x miles per gallon x price per gallon.
Creating a Lodging Budget
Probably the largest line item in your budget is going to be lodging. In my travel budget worksheet, I pick a target number and then compare that against the actual numbers once accommodations are booked. There are lots of ways to save money on hotels but that's another post. However, Shell and the Fuel Rewards® program can help you here too since you can earn rewards by booking specific hotel deals.
During our trip, for instance, I tried to book with Choice Hotels properties where possible, since I could earn up to $0.20 off per gallon based on my stays with them!
Create a Food, Entertainment, and Snacks Budget
Unfortunately, these budget items related to food, entertainment, and snacks are going to be more complex since there's almost no way to predict the costs accurately. Instead, I try to look at these numbers as "what am I willing to spend" and then extrapolate that on a daily basis. For instance, I might estimate that our consumption of food and snacks is $100 per day. As the trip progresses, that means that if we want to splurge on one day then we need to make it up on the following day. The same goes for entertainment.
Verify Your Insurance Policy Benefits and Costs
You might not think of this at first, but the needs, while you are on the road, are different than when you are at home. Roadside emergencies can be expensive but often it's only a matter of a few extra dollars to ensure you're covered.
Plus, depending on how your policy is setup for normal use you might actually be in violation. For instance, you might not need roadside assistance at home because you've got friends but hundreds of miles from them, what happens if you lose oil pressure? Likewise, does your policy include extended towing and roadside assistance or maybe even a rental car when your car breaks down? If you have a cut-rate policy you might just be out of luck but if you have something like auto insurance from Allstate or another reputable company, make sure to contact them before you depart on your journey.
Monitor Your Budget as You Travel
As your trip progresses, make sure to keep an eye on your budget an update it accordingly with actual numbers vs estimates. This will help you better predict in the future - but it will also help you identify issues if you are over budget.