Mid-grade gas. OK for long distance cruising?

[Barry M Wheeler #2189]

In getting ready for San Marcos, with the increased cost of premium gas these days, I was wondering whether I could get away with using mid-grade gas on the trip at higher, steady speeds? I'll be driving the 4.9 in the '91 Seville.

I'm not so parsimonious that a few cents per gallon will matter that much against not feeding the car what it works best with. (And it's been twenty some years since we drove the '95 SDV that I'd leased while working in the dealership and have forgotten what I used on the road then.) So I will welcome advice in this vein.

[Carl Fielding]

Use the highest octane available, unless you will be driving at elevations of Denver or higher for prolonged periods. Particularly in hot environments, with a high sun angle. The roads get very hot in that situation, and re-radiate up into your engine. Factor in the better mileage your car will get on the proper gasoline, and the potential damage to the 4.9, and you may find it a false economy to save those nickels and save those dimes at the pump.

Also : WHAT TIRES ARE YOU RUNNING, AND WHAT IS THE MAX LOAD PRESSURE SPECIFIED ON THE SIDEWALL ?

I sure wish I could be there with you all. Drive safely, and NEVER drive if you are drowsy !!!!      -  Carl

[Barry M Wheeler #2189]

I am running stock size tires. (205/70R) Coopers. I didn't stoop down to find the air pressure rating. Thanks for the answer. It was one of those things, "just wondering."

And thanks for the reminder about being tired. I have no problem pulling off the super-slab and finding a quiet side road or parking lot to take a nap. (Doors locked of course.) I'm deciding about whether to take two or three days. I'm starting to lean towards three as we want to get there in time for Gay to attend the Cadillac Queens affair Wed. afternoon.

[Caddyholic]

What does the book call for?

[gkhashem]

Premium unleaded in a 4.9L Cadillac , my mileage in my 1991 Eldorado seems to be low 20s at best on the highway. Driving around town 18-19.

Not bad, but not great.

[Carl Fielding]

My experience with the (slightly more advanced ?), '93 4.9s is higher 20s mpg at steady legal freeway speeds. In the wide open range country out here in the West, I slow way Way down at night. I try to hold 40 - to less than 45 mph. Ever hit livestock.?  I get WELL UP into the higher 30s of mpg. If you have more time than money, such patience might be considered.  Always slow down at night.   -  CC

[jaxops]

My wife drive my 1990 Grand Marquis across country and had to use regular gas in it with its 302 V8 in the middle of nowhere midwest.  It sputtered and jerked until they found a station with premium.  Then it ran fine. 
I wouldn't recommend it. Save money at the pump and possibly have to pay for a tow later.

[cadillacmike68]

If the Owners Manual says Premium fuel recommended or required, i would only use premium from a toptier gas place. your engine and fuel system will thank you.

Many more retailers are now toptier vendors. a few years ago it was mainly Shell, Sunoco, Exxon-Mobile & Chevron, but the list has grown.

Toptier gas companies promise in writing to adhere to a higher additive level across All grades of gasoline.

https://www.toptiergas.com/