After a snowplow passes by, the side of your car turns white. The problem isn't just that visible white mark. The real salt is stuck underneath the car where no one looks.

So every winter, I start to ponder in front of the car wash menu. Should I pay a few extra dollars for an underbody wash, or should I just cancel my monthly subscription? I did some calculations before the snow came.

I first looked at what and how much is spread on the roads in Kansas City. The city revealed last November that it has a stockpile of 36,500 tons of regular salt.

In addition, there are 11,000 tons of Ice Ban, 51,000 gallons of brine, and 26,000 gallons of calcium chloride. I noted these figures based on last year's announcement since I haven't confirmed this winter's numbers yet.

The area where this is spread exceeds 6,000 lane miles. It's not just a matter of avoiding the main roads.

During the heavy snowfall in January 2025, the city pre-treated all its jurisdictional roads with magnesium chloride before the snow fell. In contrast, the Missouri Department of Transportation only used salt and brine on the day of the storm, claiming it would wash away in the rain. Regardless of which road you take, the underside of your vehicle encounters the de-icing agents.

It's not a particularly snowy area. According to the National Weather Service, the average snowfall from October to April is 18.8 inches (based on 1981-2010 data). Still, with stockpile numbers like that, it's not a reason to feel reassured about the lack of snow.

Liquid de-icing agents are especially troublesome. According to AAA, they remain liquid at low temperatures and can penetrate into crevices. Their corrosiveness is said to be comparable to rock salt.

How much damage do they cause? AAA conducted a phone survey of 1,001 drivers in September 2016, and the data from 2017 is still the most cited. It estimates that across the U.S., the total cost over five years is $15.4 billion, exceeding $3 billion per year.

The average cost of a repair is $490. Those who have had repairs done reported an average of 1.5 repairs over five years. Since this data is from nine years ago, the current labor costs would likely be higher.

Looking at brake lines makes the impact even clearer. According to Synchrony's 2023 data, replacing four brake lines on a passenger car costs between $260 and $1,000, while for an SUV, it's between $380 and $1,260, and for a truck, it's between $428 and $1,520.

One surprising statistic stood out to me. Among drivers in snowy regions, 15% had repairs due to rust over five years.

I did some simple calculations. Multiplying 15% by 1.5 and then by $490 gives an average of $110 over five years, which breaks down to $22 per year. This is roughly the cost of one car wash per year.

What about the cost of a car wash? Honestly, I couldn't find the prices for different locations in Kansas City from public sources. Club Car Wash does list an Underbody Blast option, but you need to check the price at the location or on the app.

For reference, the Mister Car Wash in Milwaukee lists one-time washes at $10, $14, $22, and $26. Monthly subscriptions are priced at $22.99, $32.99, and $39.99. The list doesn't specify which tier includes underbody washes, so be sure to ask before paying.

Let's assume the $22 tier includes an underbody wash. From December to March, if you go twice a month, that totals eight washes at $176. The same tier subscription costs $131.96 for four months, making it cheaper.

Once you go twice a month, the subscription becomes the better deal. The issue is that whether it's $176 or $131, both amounts are significantly higher than that annual $22.

However, that doesn't mean an underbody wash is a waste of money. The $22 figure is an average that includes 85% of people who haven't had any repairs, while those who have faced repairs will see bills of $490.

That said, I couldn't find data showing how much less repairs occur with regular washes. So, be cautious of articles that claim how much you save with a subscription.

Experts seem to agree on the effectiveness. AAA recommends washing the underbody frequently, and Consumer Reports lists underbody spraying as a worthwhile option.

The frequency varies a bit. Consumer Reports suggests once a month during the snowy season and more often if your car is covered in salt. A report from WSAW in Wisconsin this January recommended washing every 1-2 weeks.

If you're looking to save money, consider self-service car washes. Consumer Reports also mentioned that if you properly spray the wheel wells and underbody, you can remove salt with a hose wash. Costs vary by guide, ranging from $3 to $10 or $5 to $15 per wash.

Choosing the right time is also beneficial. The same WSAW report advised washing when temperatures are above 40°F, or at least in the upper 30s.

Don't forget the last wash of the season. AAA recommends washing the underbody one more time in spring, as any remaining residue can continue to cause corrosion throughout the year.

There are also tasks to complete before winter. One wash, touching up any exposed metal from paint chips, and applying a layer of wax. All of these are recommended by AAA, and they specify to use car wash soap instead of dish soap.

I'll keep the safety talk brief. If your brake pedal feels soft, warning lights come on, or you smell fuel, AAA advises pulling over to a safe location and getting it towed. This isn't a matter of calculating car wash costs.

If it were me, I would do this: If I plan to keep the car for a long time, I would get an underbody wash after de-icing agents are applied on a warm day, and if I wash once a month, I would go for a single wash or do it myself.

Only those who plan to go more than twice a month should consider a subscription. If it's a leased car that I plan to return soon, I would rather spend that money on something delicious.

Getting an underbody wash is similar to cleaning the inside of a microwave. It's out of sight, so it's easy to put off, but the cost of neglecting it will eventually add up. Just remember to wash it on a warm day after salt has been applied.