# Car maintenance cost analyzer

The analyzer is available here.  The rest of this page shows illustrations of how to use it.

# Costs for a single car

We once had a nice, red Pontiac Grand Am.  It was a pleasure to drive, but a beast when it came to car repairs.  Here is a small set of data showing maintenance and repair costs as they appear when we copy them from Excel into the input field in the analyzer.

The first column shows odometer readings, the second column shows dollar amounts for maintenance (like oil changes) and repairs (like the head gasket replacement at 98,625 miles).  The total cost over 124,081 miles was \$8,011.78.  That works out to 6.46 cents per mile.  But that is not the whole story.  The first 40,000 miles were relatively cheap, while the last 40,000 miles were more expensive.  The analyzer shows how expensive the car was mile by mile, by spreading each maintenance and repair cost out over the miles before and after the actual cost was incurred.  The analyzer gives this graph:

The red line shows the average cost per mile, 6.46 cents per mile.  The blue curve shows that the cost per mile was lower over the first 40,000 miles, always below 4 cents per mile, but the last 40,000 miles were more expensive, around 10 cents per mile.  Just after 50,000 miles, which is a standard mileage for getting new tires and having standard maintenance done, the cost per mile was high for a while, but then dropped down below 4 cents per mile again.  The head gasket replacement just before 100,000 miles made a big jump in the cost per mile, but then it dropped again to below 8 cents per mile for a while.

It is common knowledge that cars get more expensive as they get older and things start to break down.  The graph shows that.  But it also shows that the cost per mile stays somewhere around 10 cents per mile, rarely going above 14 cents per mile.

# Time to buy a new car?

One great thing about buying a new car is that maintenance and repair costs tend to be low for a while, as the Grand Am example above shows.  The down side is that new car payments are high.  Imagine that you buy a new car for \$25,000 and finance it to pay it off over 5 1/2 years, which is a standard time frame.  Imagine that you drive it 12,000 miles per year.  Even if the interest rate is zero (which it certainly won't be!) you will pay \$25,000 and you will drive 5.5*12000 = 66,000 miles, which works out to 37.88 cents per mile.  That is considerably more expensive than driving a used car which costs 10 cents per mile for maintenance and repair.  Interest will increase the cost, and you will still need to pay something like 2 cents per mile for maintenance and repair anyway.

Of course there is more to it than this.  A new car will be more reliable, and that can be very important.  The resale value of a car that is 5 1/2 years old is a lot more than the resale value of a much older car.  Still, the cost per mile of driving a new car is much higher than the cost per mile of driving our Grand Am.

# Comparison of costs for two cars

After paying so much to maintain and repair the Grand Am, we bought a Honda Odyssey, hoping that these costs would go down.  Using the analyzer to make graphs of maintenance and repair costs for the two vehicles makes it easy to compare.  We set the maximum cost per mile to 18 cents on each graph, and the maximum mileage to 200,000.

With the Odyssey, the average cost per mile (red line) is lower, at 3.58 cents per mile.  It got about 75,000 miles before the cost per mile even went above 2.  The cost per mile has gone up and down quite a bit after 100,000 miles, but it is still going after 175,000 miles.  We sold the Grand Am after 126,000 miles, but it didn't last long after that.  We still have the Odyssey.

We can simplify the picture a bit by using the analyzer to make smoother graphs, using 15,000 as the smoothness parameter.

The Grand Am cost about 10 cents per mile in the last 40,000 miles, while the Odyssey has barely gotten above 6 cents per mile (at this level of smoothness) even over the first 175,000 miles.  Much better!

# Should I get the car fixed or junk it?

This is one of the hardest questions to answer when you have an old car.  Here is a way to think about it.  Suppose that today I find that the Odyssey needs a \$2000 repair job.  That will fix the problem, but it will still be an old van, and something else might go wrong with it soon.  One way to think about it is to imagine paying the \$2000 and then getting another 5,000 miles with no repairs.  Here is how we can add this scenario to the mileage and cost data we feed into the analyzer.  Today's mileage is 177,000, I pay \$2000 today, and have no further cost for the next 5,000 miles.  To make sure the analyzer understands this, I put a cost of \$0 at 182,000 miles.

Here is the graph with smoothness parameter 15,000:

This is definitely going to make a jump in the cost per mile, but still nothing like the Grand Am was.  Of course, there is no guarantee that you can get another 5,000 miles with no repair cost.  But if you make it another 10,000 miles, the graph looks like this: