So this is my first post here puts on flame suit and also my first time working with the arduino. However I feel like this is the only thing that might work.
A couple buddies and I are going on a road trip to a few cities around the US. And I thought it would be pretty sweet if we had an LCD or screen projecting the mile count in the back window, so people could see it. I know i would need some sort of sensor by my tire to count the rotations, as I don't want to break into my dashboard. So If anyone has any suggestions that would be great.
sully
Thank you for the reply. 
It is a... 2005 chevy Trail Blazer. It has an electronic dash, not the old mechanical ones. I actually thought of the magnet approach or maybe a gps tap-in. I have seen posts of gps arduino set-ups on other pages, and I didn't know if I could make it count miles.
Lets say I either one of those approaches, I'm not asking for a walk through, however how would I start?
sully
GPS could be an option
You might consider using a GPS receiver (module) to track movement. Also depending on the car's make/model you might be able to tap into OBDII and get mileage information. (For that, you'll have to research. )
If you get really large black on grey LCD indicators, those might be readable outside of the car. Otherwise LCD probably isn't going get you anything useful.
You might be able to do something like the SparkFun clock on a smaller scale. 12ft GPS Wall Clock - SparkFun Electronics or some kind of LED Matrix Sign.
Ya I just thought of the GPS and how it would only really tell me my "as the bird flies" way. Is it possible to get vector information (direction and distance?) from the GPS? That is why I feel that the magnet would be the best rout to take. And I do like the idea of the direction to home idea. It would be a very good touch. 
yes, I think that is the path I am going to go, and I thought about that also, do you have any idea how I could store it as a non-variable?
yes, I think that is the path I am going to go, and I thought about that also, do you have any idea how I could store it as a non-variable?
http://www.arduino.cc/en/Reference/EEPROM
Everytime you have calculated that you have moved one mile, update the EEPROM. Perhaps update it with the current coordinates and distance traveled.
Every couple of days you could manually update your current mileage.
(I'm and sure one of the guys will say "but but but FLASH has limited write cycles!!!! Yeah they do. 10k-100k. Just like the program flash memory. If you only update once per mile, you'll be fine.)
thank you sir, that is some useful information. Now I am wondering a couple more things, lets say I update the lat/long every 2 minutes or maybe even every minute. How would i get that information from the gps unit. It comes in in NMEA data, in sentences.
"$GPGGA,123519,4807.038,N,01131.000,E,1,08,0.9,545.4,M,46.9,M,,*47"
the lat is 48 deg 07.038' N
and the long is 11 deg 31.000' E
now i know how to do the math to find the distance, however how do i get that tidbit of information out of that sentence for the arduino to use?
EDIT:
Oh and I was also looking around for LCD screens, I will need something somewhat large, does anyone know of something about 6by18 inches or so? maybe a little smaller. And something that the arduino can operate.
sully
sorry, i know how to calculate that, i just don't know how to get that little piece of data out of that sentence for the arduino to use.
hmm okay those look very promising. Now the only thing I'm worrying about is my coding skills... hopefully I will be able to get this done by june, when we go on the trip.
Do you think it would be useful to start coding it now before I get the supplies? And would I have to put inputs for each of the 4, 7 segment displays that I would need? possible 5 of them.
lets say after the input and the math I get that we went 1337 miles. (haha) would I have to put each number inputing for a different display or will it do that automatically?
How about this project:
Should be easy to replace the LCD with the type of display you want.
Hello
since car is new enough would you like to consider OBD interface? even this solution is little bit expensive, but with this interface you can get more information that you may use in other projects.
I moved this to the software section of the forum, because now I need to know about how to program. Thank you for all the help.