We've all seen the dreaded bulge at the bottom of a tire and known something was wrong. The Department of Energy estimates that consumer vehicles waste over 1 billion gallons of fuel per year due to underinflated tires. That means Americans waste between $2.8 billion and $3.4 billion a year on gas -- a lot of money wasted on a problem that is easy to see (bulge), easy to measure (gauge), and easy to fix (air).

Now what if I told you that for every dollar you lose driving to your business on an underinflated tire, the business you are driving to could be losing $140 in productivity due to a problem that's hard to see, not easy to measure, and certainly not easy to fix?

A recent Gallup report has uncovered an ongoing, hidden problem that costs businesses between $450 billion and $550 billion in lost productivity every year: disengaged employees. But let's call these employees what they really are: people who have quit without actually quitting.

When employees feel uncoached, unappreciated or unproductive, they don't always quit and move on to a new job. Instead, many disengaged employees choose to "hang in there" until the right new opportunity comes around, working hard to keep their discontent a secret. These disconnected employees quietly hurt employee morale, retention, and productivity.

To put the magnitude of this issue in perspective, the same Gallup report cited earlier estimates that a full 50 percent of all employees are "not engaged" after just six months, and an additional 20 percent of employees "actively disengaged," meaning they proactively look for ways to undermine employee morale and retention.

So when 70 percent of your employees are to some degree disengaged, your business will probably suffer from significant, preventable productivity losses that go unseen unless you take proactive steps to boost employee engagement. Here are some employee retention strategies you can implement to proactively address employee disengagement:

1. Calculate your employee Net Promoter Score (eNPS).

The Net Promoter Score is a business metric that traditionally measures the loyalty of a firm's customer relationships, but the same calculation can be used to measure employee loyalty, which is a barometer for overall morale and productivity. The idea is simple: Ask all your employees to rate "What is the likelihood that you would recommend our company to a friend or colleague?" on a scale of 0 to 10. Anyone who answers 9 or 10 is considered a "Promoter," and anyone who answers 0 to 6 is considered a "Detractor." Your score is then calculated with the following equation:

eNPS = ( Promoters / Total Respondents ) - ( Detractors / Total Respondents)

Your company's employee Net Promoter Score is an easy way to gauge overall employee satisfaction, which can easily correlate to employee productivity. You should measue it at least quarterly. And don't worry about what a "good" eNPS score is -- just work to improve yours, quarter after quarter.

2. Train managers to spot disengaged employees.

Challenge your managers to list their direct reports and identify each one as either "more" or "less" engaged. Make sure they understand there is nothing wrong with an employee being labeled "less engaged."

Once every employee has been classified, ask each manager to list three reasons why each employee fits the label of "more" or "less" engaged. This will probably be hard for them to do. Repeat this exercise monthly without looking back at previous results. The goal is not to assess individual employees each period, but to get managers thinking about and discussing employee engagement as a team, which in turn makes it a priority for the business and something managers proactively monitor in their reports.

3. Create employee challenges designed to boost productivity.

Each quarter, ask each of your employees to list what is causing them to feel frustrated or unproductive. Once a list is made, as each employee to estimate (conservatively) how much better they could perform if their issues and ideas were addressed. Whatever measurable thing an employee can articulate as an improvement to their productivity, accept it.

Next, make an agreement with the employee: If the company resolves the issues that demotivates them, how much of the performance gains they articulated are they willing to commit to during their next performance cycle as a stretch goal? It's essentially back-scratching, and everyone wins.

It's easy to fix an underinflated tire, but it's going to take a lot of work to fix a business quietly suffering from a disengaged workforce. Employee disengagement is hard to see, hard to measure, and hard to fix. It's not a tire. My experience was one of the reasons I created Goalee to address this problem. So with or without software, you should continually invest in uncovering what keeps your employees aligned, motivated and therefore performing at their best.