# Cars - How do they measure distance travelled?

Does anyone know? Our best guess is some kind of measurement from the axle/wheel rotation. However, that would be thrown off by varying wheel sizes and so could be inaccurate.

That is correct. Changing tire size will make the odeometer reading incorrect. But it is measured at the transmission [how much the drive shaft turns]. Changing the rear axle ratio will also affect the reading. You just change a gear in the transmission to get both the speedometer and odeometer to read correctly.

Excellent, thanks. Any idea how planes do it? Triangulation from the ground?

How Odometers Work

Do planes even have odometers? I think the important measure there is engine-hours, at least for maintenance intervals and other things that you’d use mileage for in a car. If you need the distance, you should be able to get that right off of your plotted course.

But that wouldn’t work because of varying weather conditions. Strong headwinds would slow you down, etc. How do planes know how fast they are really travelling and how much distance they’ve covered? It has to be done with some sort of triangulation to stationary ground targets(ie. airports)

Hey-if your tire size is causing the odometer reading to go off, would that also affect the speedometer reading? I imagine so-and that might explain why my last car’s speedometer always read about 10% slow …

If you really need to know true ground speed in a plane, any modern Global Positioning System (GPS) will display this information. This is, in fact, done by triangulation by satelites rather than ground signals. There were a variety of older methods (some quite primitive but effective) that pilots used before GPS became widespread.

Which raises another question I have - what exactly is the difference between true ground speed and air speed?

ps. I feel damn stupid for saying ground targets and not satellites - must have been a WWII pilot in a past life.;¬)

Sure would. The computer in your car measures your speed by counting the number of revolutions the driveshaft makes in a given time, multiplying by tire circumference and then dividing by time. If you get a set of differently sized tires put on your car, you need to update the value the computer uses for tire circumference, otherwise your speedometer reading will be off (lower than actual speed for larger tires, higher than actual for smaller).

Yeah, I just traded in a 1985 Porsche 944 that was previously owned by my dad. He switched from the stock rims and the tires he bought were a different size than stock. When you do that, you’re supposed to recalibrate your speedometer, but for some reason it was never done, so it always looked like I was doing 80 when I was really doing 70.

Ground speed is the speed of the aircraft relative to the ground. It’s important if you want to know when you’ll arrive at your destination. Airspeed is the speed through the air, and is what the aircraft’s speedometer is reading[sup]1[/sup]. For example, let’s say you’re aircraft cruises at an airspeed of 150 mph. If you’re flying into a 10 mph headwind, your ground speed would be 140 mph because of the wind.
[sup]1[/sup]The airspeed indicator is actually telling you indicated airspeed, which is different than true airspeed. The instrument in the aircraft determines your airspeed from pressures it measures outside the aircraft, which can be affected by a number of factors (mainly temperature and altitude). Indicated airspeed is what the gauge is reading, and true airspeed is your actual airspeed, after correction factors have been applied. This distinction isn’t important for my simplified example, I just thought I’d note them for sake of completeness (because I’m sure someone else will come along and bring it up).

Firkin’ dingblast! That means the '95 Grand Am I just traded in only had about 170k miles on it, not 190k. And I let the dealership get away with giving me a buffalo nickel and a 1995 Mark McLemore baseball card for it! I bet I could have gotten a Rafael Palmeiro instead, if only I’d known!