My 98 v 70 is getting around 20 mpg which is 6-10 mpg less thatn it has been getting. It is a nonturbo with about 85k miles. I am in northern IN and it has been cold so that might attribute to some but I wouldn't think that much. I can still here the air pump run on ocasion. Would th O2 sensor be a potential culprit?

pump up the tires if you can handle the hard ride.for every 10F drop in temps you automatically lose 1psi due to expansion/contraction formula not air loss.
use 5w30 motor oil which can give you 1-3mpg.sometimes mid-grade or premium gas will give you a few more mpg if you dont have a lead foot.low temp do influence mpg becauce gas doesnt vaporize as well as in warm moist air.dont panic.it will warm up and your mpg will slowly improve.

Hi I live in Canada (quebec more precisely) during summer time at 100km/h i consume about 7 to 8 liters (per 100km) of gas on the highway but now in the winter at -20 celsius it went up to 8.5 to 9.5 liters per 100km.My pressure tire is fine and the car is in perfect condition. I have a v70 t5 1998....Hope this helps. Dan.

It could be the cold. Check your tire pressure (that affects it about 2-3 mpg if it's low). Have you had the plugs, wires, dist. cap and rotor changed? That will help. If it's the O2 sensor, the check engine light likely would come on.

Check engine light was on I had it reset because I had changed a thermostat and didn't know if that might have tripped the light. I had put on at least 400mi since and it hasn't come back on.

If ist been as cold there as here in New england (0 at night, teens during the day)it WILL affect mileage but yours seems extreme!