oldfrat

I used search here, also Google, but didn't get exactly what I was after.

What octane do you recommend for a 1981? Owner's manual says 87 is minimum, but what is optimum? I'd like to use optimum in this car, and minimum in my daily driver.

What kind of MPG and range can I count on for interstate driving? I'm thinking about a TX to CA drive, going by way of Phoenix instead of Flagstaff to avoid snow, There's lots of empty space between gas stations in some places. I have a 1981 stock automatic. I've seen highway MPG numbers between 17 and 22 which, for a 23 gallon tank, give me a range of 391 to 506 miles. But I'm not sure if even the 17 MPG is trustworthy.

Thanks!

Brad

The Forgotten On

I have an 81 US and I get about 20 mpg with the 3 speed auto. 87 is great, I never had any knocks and because of the low compression it can run on anything. \

If you advance the timing, use 91 to prevent knocking.

yaskota

I run premium (91) in my 84, no issues, but some seriously over due brake work is keeping my MPG miserably low- 14 MPG at most recent fill-up...

I can say that when the fuel gauge reserve light comes on, it sure can go for quite a while. I'm Actually tempted to carry an extra gallon or two in a fuel can next time my light is on just out of curiosity!

Other than that, life on an island makes me jealous of your 928 worthy trip plans! Enjoy!

Five-8

I have only done two lengthy highway runs since i have owned the car, i roughly calculated my fuel mileage to be 24mpg on both occasions, running between 70 mph and 80 mph.
I have always put 94 octane in it.

Cheers,

LT Texan

Before I swapped in Euro S stuff, I always used regular and got 20mpg or so on the highway if cruising was kept in the 70mph range.

With the higher compression, I now use premium and get north of 20 mpg at that speed.

And I'd take the flagstaff route. What a great drive you will have!

Wisconsin Joe

The lower compression allows use of 87. Anything higher is just a waste of money. It won't make the car run any better, or generate more power or anything like that.
Engines with higher compression need the higher octane rating to prevent detonation (or having the knock sensors retard timing to prevent detonation, reducing power).

I'm paranoid about running out of gas, and I don't trust any fuel gauge, so I only run a bit below 3/4 of a tank. that usually gives me around 300 miles +/-.
I've been tempted to take a gas can along and find out exactly how much it holds, like Yaskota mentioned, but I haven't yet. I haven't even found out where the light comes on.
Overall, I get 17-19 mpg in normal everyday city/highway mix. I've done a few good roadtrips, and extended highway cruising (~80mph) gets around 21 (a little less with headlights up due to drag).

Keep in mind that condition of the car, weather, terrain, driving style & speed, and a whole lot of other factors can affect mileage and range.
I'd strongly suggest doing a couple shorter trips to get a good idea of what kind of mileage your car gets.

I have an 85 Euro with the 4.7L "S2" (M28/21) high compression engine. I use 93 when I can find it, 91 otherwise.

oldfrat

Many thanks for the responses. I am truly appreciating being able to tap into the wisdom this board offers!

I'll give 87 a try and see how it goes. Looks like the range should be no problem. I'll have longer trip test drive for third coast before the CA trip.

Agree via Flagstaff would be more fun, but the last time I went that route I ran into heavy snow. While I've lived most of my life in snow country and know how to drive in it, I doubt this car has ever been driven in it before, nor the various melt-inducing stuff they put on roads. I'd just as soon keep it that way. I don't mind adding miles to the odometer, but am averse to adding rust. I may cut up from Phoenix to Lake Havasu, though, to avoid LA traffic.

Brad

OTR18WHEELER

FWIW, '85 auto, I am safe at figuring 20 MPG HWY @ 60 MPH +/- , I run 100 mile trips twice a week, but have never gotten low enough to light the low fuel warning. I usually run 93 oct gas, but once in a while I will use 89, I figure I save a couple of bucks at the pump once in a while. 93 octaine @ $3.55 vs 89 oct @ $3.40/ gal. It's all tree food anyway right?, A couple dollars a week ain't worth cheap gas.

linderpat

for 16 valve US cars, regular period; everything else is a waste of money. Lots written about this here, but it's been awhile.