Premium gas for Smog test?

I’ve got a 1993 Range Rover that’s due for its California smog test. While the manual calls for premium fuel, I usually just run 87 octane in it for around town errands, and it seems to manage okay. For the smog test, though, should I run it on high octane gas?

Yes, and hope the engine is not too carbon-ed up running the cheap stuff to affect the emissions. Running the cheap stuff, I can guaranty the engine had to retard the timing to prevent pre-ignition, but left it with bad burn characteristics that tend to leave a carbon build-up behind. That carbon build-up will cause the bad burn characteristics to linger even after switching back to premium. And, it will show up on an emissions test. Just how bad is the question.

If it were me, I’d go with the premium along with a heavy additional dose of Techron, and take a long cruise on the freeway to get it thoroughly warmed up and cleaned out.

If the owner’s manual says it requires (vs recommends) high octane fuel, you are foolish not to follow the manufacturers instructions as using regular may damage the engine and likely will reduce the mileage so it may increase your cost of fuel as well as cost you some power as well.

Yup! Using regular-grade gas in an engine that requires premium is a good way to spend more money in the long run. You can’t afford to save this much money when you fill the tank.

So, here’s the plan: you change the air filter and the spark plugs, and change to high octane gasoline. Then, you might jus pass that ol’ 'missions test.