wideband o2 sensor and other questions

hey all, just wondering about my o2 sensor. I know our cars run on the rich side, but my gas mileage is pretty shitty even off boost. I'm thinking i need to de-carb the engine too, but would an o2 sensor be faulty and not trip the engine light and not throw a code? If so, i'll probably replace the o2 sensor. I just started looking but online they are not the cheapest of course. approx $140cdn. there are universals out there for less than $100 but i don't know much about o2 sensors. If i'm gonna pay almost $200 for a new sensor, i was thinking i may as well get a wideband kit (innovate unit) they are a little more expensive but i rather upgrade for the benefits of a wideband.

Here's my question. Does one of these wideband units take the place of the original o2 sensor on the dp on our cars? If so, i take it these kits have the wire leads required to hook up to the stock system for the ecu.

any info would be great.

off topic, i'm still trying to figure out what i need to convert my jdm 180km speedo with a US/Cdn DM 260km speedo. Other than the gauge face, i assume i need to replace whatever the speedo hardware is behind it since its calibrated for a 180km face. Do you know what this speedo part is called and where to get one?

You will have to change the whole cluster, or the actual speedo gauge. The wideband o2 does not replace the stock sensor, although i think there are some that have a narrowband output to run on the stock computer. Im sure you can get one from someone off the boards in good condition.