There are lots of places, but from what I've heard there is not much point to getting one. What kinda car do you drive? Is it obd2 (96+)? I was going to post something about this anyway but I'll say it here.
I am going to get a boost/vacuum gauge as if I had a turbocharger on my car. For what reason? Because even without a turbo or anything my NA car can fall into vacuum, which works against the pistons and puts more resistance on the engines motion. So, when you change your speed around to make the gauge hit 0 boost and 0 vacuum, you will be getting the least resistance from vacuum pressures on the pistons. This is, I believe, the same thing as the old vacuum mpg gauge, but the vacuum mpg gauge tends to be awfully inaccurate.
They will only really work on carburated engines acurately, which don't get the best MPG to begin with. Fuel injection has a fuel return line that will make all of the numbers on these vacuum gauges not really mean much.
We're currently working on a MPG gauge for older cars that run fuel injection, which will be awesome if/when it's done.
If you are really interested in an onboard computer and you have a carburatred car, I'd try to find a Zemco computer on Ebay. You can get it for about $100, and it has MPG, trip odometers, etc. it was a product of the gas shortage in the 70s. They are a really cool machine, one which I think would be very cool to reproduce if at all possible.
That crazy zemco crap. That fuelsaver from england is back on ebay, and I really want it, I just don't have the money, but who knows if it'll ever show up again.
ARGH. You mean the one that stops your car when you idle? I want that too, but I also don't want to sit there and have my car start and stop all of the time.
I'm still concerned about the electrical load required to start the car over and over again. My understanding is that you have to drive a decent distance in order to get the charge back from the initial start.
The solution to this might be getting a small solar charger to trickle charge your battery during the day. They sell them for like $40-60 in various auto parts stores.
There was a Zemco driving computer on ebay that ended today. I bid on it but got sniped in the last two seconds (but hey, I was trying to snipe, too). It ended up going for $61 plus $8 shipping. It came with a fuel flow sensor, magnetic speed sensor, and the in car computer that displayed distance traveled, speed, instantaneous mpg and net mpg. Maybe a few more variables. I really wish I had more money to bid on this. It would be a real time saver for testing mpg, as well as increased accuracy.
<a href="http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&rd=1,1&item=8000576500&sspa gename=STRK%3AMEWA%3AIT">Zemco driving computer on eBay</a>
Fuel flow sensors are really expensive so making one myself is out of the question.
Scouring the country for an excellent condition Civic VX
What I really want you to make, though, is a zemco fuel saver. It's on ebay now too. It shuts to car off at idle and then when you whack the gas starts it back up. The VW Lupo also sports this feature.