39.8MPG Good/bad?

[ericsmith32]

Trying to figure out if anything else needs to be done to old yellar'. Got that reading going 75mph from Booneville to St. Louis and back to Columbia. Note that my auto got 34 on the same trip and I had the family with me that time. A few steep hills, 30 degree temps, winds were out of the north probably 10-15, unfortunately on that tank it was E10. Dad typically average 43mpg on his route from Nevada to Ft. Scott but since then I've redid the rear bearings and the dual stage thermostat. The only thing I can think of the spark plugs (dad did those 20k ago not sure what brand and gap) and the Bosch o2 sensor (has about 80k on it). If you let it idle and warms up it down right smells awful mine does the same thing... running rich?

Seems to run fine though on the road. The speedo according to the gps (didn't have it with me to verify mileage reading) when I'm going 75mph on the speedo it's actually 73. The reading itself to me isn't bad but comparing to the auto don't know... what do you say gentlemen?

[Aaronbrook37]

At that sort of speed, 40MPG is fantastic, Eric. I'm sure there's obviously some room for improvement with tires and aerodynamic modifications, but that's really, really fast. Your engine seems to be pretty healthy if it can pump out 40MPG at that sort of speed.

[ericsmith32]

It does have stock 12's on it. Forgot to mention. Yeah it does seem to like running fast. Compared to my dad's other Festiva it prefers 55-60 this one prefers 70-75 or faster!

[Pu241]

I'm will to bet your average speed was considerably below 75 MPH and the Festiva's ODO is not very accurate.
Mines off 16 miles out of a 400 mile run.
The GPS is the way to go, depending on model you can get average speed, peak speed, and distance.
Assume you recorded this in a log.
Next trip might use the GPS to compare.
If accurate, nearly 40 is great mileage, I'd have guessed closer to mid 30's at that speed.

[Darlo]

No matter what, you will get better gas mileage at 50mph than 70mph! If you are truly getting 40mpg at 70mph you should get 50mpg at 50mph. That is what I get when I am not running my studded snow tires...... Slow down and see a huge mpg increase.....

[hotfordcat]

That sounds really good for 70mph. I got 41mpg @75mph with my 93GL 5spd. Downhill from Billings, Mt to Rapid City. 368 miles, 9gals gas.

[blkfordsedan]

The 30 degree temps and E10 will cost you MPG. If it was 80+ degrees and you had straight 87 Unleaded, you would probably be in the mid-40's. I'd say you're doing good, all things considered.

[ericsmith32]

Thanks guys looks and sounds about average. I thought about bringing the gps but the wife needed it.. all I'll say on that. By mapquest it says 251 miles and by the odo it said 260 not sure how acurate mapquest would be. I would of slowed down but wanted to get home to see the boy. It does Autolite plugs yuck those will be coming out!

[econoaddict]

It does Autolite plugs yuck those will be coming out!

Why??
I have been running autolite standard plugs for many thousands of miles in my commuter festy and average 40-48mpg with no problems.
With the mileage your getting now I would leave them alone.
If your running rich, you should fix that first. Get that set up and you'll see even better mileage.

[Darlo]

Changing plugs = .25mpg if you are lucky
Slowing down = 5-10 mpg (no luck required)

[Lannis]

I run 36 PSI in my 12" tires, and short-shift up to 5th where I can. I average about 60 MPH on the highway part of my commute.

I get 43 MPG week in and week out. The engine has intergalactic mileage on it and uses a quart of oil every 1000 miles or so, so that probably doesn't help. Overall very happy with it.

Lannis

[Darlo]

I run 44psi in my 12's and usually dont exceed 53mph (85kmh), and I get 49mpg in the summer, and 43 in the winter (with 35psi in my studded 12's). I also burn/leak a bit of oil, but only 1 litre every 2500miles (4000kms)......

[ericsmith32]

Why??
I have been running autolite standard plugs for many thousands of miles in my commuter festy and average 40-48mpg with no problems.
With the mileage your getting now I would leave them alone.
If your running rich, you should fix that first. Get that set up and you'll see even better mileage.

Autolite plugs might be fine but I prefer the NGK's. The NGK's from my experience want to stick less over time compared others. The tires were pumped up to 35psi. It doesn't burn any oil.. did loose some oil when the valve cover bolts worked themselves loose. The mechanical issue I noticed was pulling to the left. The whole front end probably needs to rebuilt since it has 163k on it. That probably didn't affect mpg a extreme amount though.

[Kbrier]

I have 12" tires around 34 / 35 PSI with my 88L carb. 4 speed. Do 80% highway driving about 300 miles per week in central PA. Noticed that fuel mileage is down due to colder temps, but only ~ 6 mpg. Have cardboard in front of radiator and am trying a 3/4" valve in line with top rad hose to restrict coolant flow. Will see what happens. I really dig working with this car.