I have a 1990 fullsized blazer. It has the TBI 5.7 and a th700r-4 transmission. It used to get around 15 to 18 mpg and now its getting between 8 to 10. It has a K&N air filter, 3 inch flowmaster exhaust and 33 inch Kuhmo Road Venture MTs. ANy Ideas on how I could gain mpgs or fix the problem thats making it only get 8 mpgs?

 

was that 15 mpg before the 33" tires?

 

Yes, that was before the 33s but it had 31s when it was getting 15 so two more inches shouldnt decrease it by half should it?

 

It might. I bumped up from the same on my fullsize 1500 and lost 3 mpg. Regearing is a helpful way to fix this. But is it worth the cost? meh if your serious with your ride and plan on keeping the 33's

 

Could the need of a new o2 sensor cause this problem?

 

bump.

 

You first have to determine what, if anything, is wrong with your truck.

How are the plugs, wires, cap, & rotor?
When was the last time a full tune up performed?
Have you hooked the O2 sensor up to an oscilliscope or digital multimeter to watch its activity?
How does the spray pattern from the injectors look?
Any codes present?
Fuel filter replaced recently?
Have you tried changing gas stations?

 

new plugs, relatively new wires cap and rotor