I had the injectors cleaned last October and had city mileage around 18.5. I changed the tires in December and the city mileage has dropped to 17.4 and I notice a lot of fine black dust at the tailpipes. Is this normal?

 

I get the carbon down the center of the exhaust tips. Mileage varies. Ethanol drops the results by as much as 3 mpg on some cars. We have no choice at this point in time. I think I get even worse mileage than you do and I have an 05 STR. A Prius it ain't.

 

I get the carbon on the pipes too. Freaks me out. I think I get around 18-19 in town 22ish hwy.

 

I'm getting around 13.8 around town and around 24 on highway trips.

 

05 STR getting 17.5 to 18.5 around town.

 

Whatever else "fat" tyres do for you, they will make the fuel consumption worse by a measurable amount. Eco they sure aren't.
Leedsman.

 

The carbon at the exhaust tips is likely due to the ECU richening the fuel ratio as a means to prevent detonation under heavy acceleration.