Roadshow

How to decode your car's dashboard

After speed and fuel, your comprehension may run out of gas.

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As part of my series on stuff that many drivers feel is too stupid to ask, let's do a quick tour of your car's instrument panel.

  • Speedometer. This is the easy one. It tells you how fast you are going. It can read up to some really silly speeds even if they are beyond you or your car's ability. Back in 1979 it was U.S. law for a minute that car speedos could read no higher than 85 MPH in an effort to get us to drive slower and conserve gas. It didn't work, but carmakers kept installing 85 MPH speedos well into 90's. By the way, your car's speedometer only need be accurate within +/- 5 MPH at 50 MPH in the U.S. European regulations have a similar slop factor. Try using a GPS speed app on your phone and compare the reading to your car's dashboard.
  • Tach. If the speedometer is the most used gauge on the dash, the tachometer is the least useful. It shows how fast your engine is rotating. Right about here a lot of you are thinking "so what" and for good reason: A modern car with an automatic transmission needs zero tending of its RPMs. But we love tachs because they are great theater, swing up and down in athletic concert with the engine sounds. If you do have a manual transmission, you might use the tach to keep the engine between lugging (under 1,000 RPM) and over-revving (at or near the indicated "redline" on the gauge). Or you can do what most seasoned drivers do: Just listen.
  • Turbo/Boost. Like the tach, this is just instrument theater for most drivers. If you have a turbocharged car this gauge may be present, showing how hard your turbo is working to ram more fuel and air into your engine's intake system. #whocares.
  • Fuel. Needs no explanation (I hope) but there is some nuance around the concept of "reserve". Fuel gauges tend to be very conservative so you aren't likely to run out of gas and find yourself on the side of the road, hating your car. Read your owners manual to divine out how much fuel likely remains when the "low fuel" light comes on, and then when the needle actually is at the bottom line. Also, notice how much fuel your car takes when you fill it back up from that bottom position and compare that to the actual fuel capacity of the car, also noted in the owner's manual. The difference between those two numbers is a shadow reserve that the gauge pretends isn't even there. 
  • Temperature. Poor old temp gauge. Back in the day, you used to always keep an eye on this one. Then, sometime in the 90s, cars just stopped overheating. Now the temp gauge has often been replaced by an idiot light that tells you the engine is cold so you don't hammer it until the oil has warmed up. And that's it.
  • Ammeter/Voltmeter. These are pretty archaic today. If you have one of these you probably drive a truck, a high-performance car, or a pretty old car. They ammeter is the more useful of the two, telling you if your battery is getting charged or not. But problems here are about as likely as temperature issues discussed above.
  • Engine/Check Engine. These lights differ by about $10,000. "Check Engine" usually means an emissions equipment problem and often just a non-sealing gas cap at that. "Engine" often means low or no oil pressure or a similar major issue that could soon cost you an engine. Read your car's manual to know how they divvy up faults between these two cryptic lights.

If you just love automotive gauges (like I do) consider buying a Bluetooth OBD-II data dongle that fits under the dash of your 1996 or later car. It can send live data from your car to a dashboard app on your phone. You'll be amazed how many new virtual instruments your car can feed this way.