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Jack79's avatar

What temperature do car thermometres count?

Asked by Jack79 (10994points) May 5th, 2010

I noticed yesterday that the thermometre in my car (a Ford Mondeo) showed 25.5C, whereas a thermometre in the street was showing 29C at that moment. We discussed this with my girlfriend, who reckons car thermometres show the temperature inside the car, whereas I always assumed they showed the atmospheric one.

She says that her car in the US would show 100F when she opened it in the summer, whereas the temperature outside was not that hot. And then it would drop. But I remember also the opposite: my last car (an Opel Astra) would show -1C when I was in the Czech Republic, and it was obviously not that cold inside. And the one before (a Skoda Octavia) actually had a little warning light when road temperature dropped under 4C, to tell you that the road might be frozen.

Is it possible that it depends on the brand? That different cars show different temperatures, either ground, atmospheric or inside the car?

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9 Answers

Tobotron's avatar

well on my 206 the thermometer is located on the wing mirror so I guess its depends on local effects eg parked car on a tarmac drive is of course going to give a hotter than true reading although once your driving along it should normalise.

its not an exact precision piece of equipment either…

defiantly not the inside temp unless your car has climate control then yes that would show both indoor and outdoor temps but this should be noticeable as to which one is which?!

as far as outdoor temps go though I would imagine nearly all cars have the sensor in the region of the wing mirror away from engine heat etc so they should all give a reading of whats directly outside… !?

Mat74UK's avatar

They definitely measure the outside temp. Your girlfriends could well have been sat in the direct sunlight maybe even warmed by the surface of the car and this would have decreased with the cooling effects of the breeze as she drove away.

marinelife's avatar

It depends. Some measure the temperature in the cabin and some measure the temperature outside. What does your car manual say about yours?

poofandmook's avatar

as an added comment to @Mat74UK suggesting that the thermometer was warmed by the surface of the car… my previous car was black and when it was sunny, it would always register a much higher temperature than it was. I got into the car once and it registered a full 10 degrees higher than what I had just seen looking on the internet. We all know that black absorbs heat. Maybe the color has something to do with it?

njnyjobs's avatar

Car models have different features. Most measure outsude temperature. The ones that measure cabin interior temperature are typically tied into the car’s heating/cooling system. On the Mercedes Benz, it has a thermometer for outside temp on the dash, while the interior temp is clustered around the blower controls.

robmandu's avatar

Most commercially available thermometers aren’t all that accurate. If you get three thermometers together, you’ll get three different temparatures.

Throw in all kinds of environmental factors like direct sun, shade, wind, engine temperature (radiant heat), materials involved and it would actually be mildly amazing if your car’s thermometer displayed the exact same temperature reading as the one you saw on the street.

In my Honda, the temperature in the instrument cluster, under the speedometer, shows the outside temperature. (It even says ”OUTSIDE”).

On the dashboard, in the climate control section, I have two additional temperature displays. They show the “target” temperatures that the HVAC is attempting to reach (one for driver and one for passenger).

I have no display at all that shows what the current temperature inside the vehicle is.

For example, on a hot Texas afternoon, the outside temp might be 100°F. My vehicle’s instrument display might show that as anything from 97° to 105°. Just depends.

And then on the dash, for the A/C, the last setting when I had left the car was 78°F.

Now, the interior of the vehicle when I first get in might be around 125° – 130°F… but the only way I’d know for sure would be if I brought in my own personal thermometer.

Jack79's avatar

Thank you for your answers. Yes, I also figured it would probably count the ouside temperature, I just wasn’t sure whether different models had the sensors in different places.

And obviously, they’re not that accurate, they are just meant to give you a general idea (I wasn’t surprised at the 3.5C difference at all). But I was wondering whether being on a mirror would cool it down as the car is speeding, and warm it up when it’s parked (especially in direct sunlight).

In Germany you often get electronic signs on the side of the highway telling you both the atmospheric and also the ground temperature (especially in case of snow).

Tobotron's avatar

@Jack79 well a car with a temperature system is not a mobile weather station its just a gadget really so manufacturers arn’t gonna spend time giving you all this extra information on a mobile platform?! How would a moving metal object even be able to give a ground temperature reading anyway!?

as far as being on the mirror its for practicalities as much as anything…and the probe itself isn’t directly exposed so its not going to suffer hugely from wind/chill factors, this isn’t however true of urban hot spots such as your hot driveway in the summer etc.

its only a curiosity gimmic really though, and so its in your face as to there being ice on the road, I’m sure its also being used for climate control systems to regulate the temp of the air into the car but generally I’ve not given the temperature outside much thought beyond this.

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