The Tercel should have a speed sensor, as long as it's the 3E-E (fuel injected) engine. If you need help finding it, let me know, I can take some pictures of where it is.
And yes, even if you can't find it, it's still useful just to see how many gallons/hour you're using. My speed sensor connection wasn't hooked up for a few days, and you can still get a pretty good sense of how much gas you're wasting just with the injector signal hooked up.
__________________
|