poor mpg 91 PA

My 91 PA uses a lot of fuel. Measured over a couple of thousand of miles, it averages about 20,5 mpg, and this is 80-90% highway.
I already replaced the airfilter, spark plugs, plug wires, coils and o2 sensor. Also I didn't drive faster then 80.
When I calculate the mpg from the fuel gauge it is about 25, but when I actually fill up the tank it always goes down to 20.
How do the people who get good mpg measure this? I have noticed that when the pump stops when filling up the tank, there is still room for 4-5 gallon if you poor it in slowly. Could this difference account for the 30 mpg figures people are reporting? What else could cause the high fuel consumption?

What kind of highway travel are you talking about. Do you mean highway travel as in interstate travel? If your trips are around 15-20 miles you won't get 30mpg. You have to take into accound engine warm up time and inefficiency at the time. Long trips cancel this out.

When I use to take the interstate to work I only averaged around 20.5-22mpg in a 16 mile one way trip. I have seen as high as 34mpg and as low as 25mpg on long trips.

As far as gas pumping goes if the low fuel light comes on you have to pump at least 13.5 gallons (18/4= 4.5 gal = 1/4 tank). Usually i take my average at that time (miles driven/gallons used) and can roughly estimate my gas mileage before even going to the pump. I always reset the trip meter when filling up.

Also I have observed that the gas hand is about a gallons off. If you pump gas right at half a tank you only need 8 not 9 gallons and the if you pump it at the next mark below you need 9 gallons.

Also when pumping gas make sure that the fas tank is either flat or sloping away from the filler tube otherwise it won't fill up to the proper level. I have got inaccurate readings several times due this. Don't feel bad though I'm only averaging 19mpg

I have noticed that when the pump stops when filling up the tank, there is still room for 4-5 gallon if you poor it in slowly. Could this difference account for the 30 mpg figures people are reporting? What else could cause the high fuel consumption?You might need to find somewhere else to get gas, as there could be something wrong with this pump. If you notice this behavior everywhere, though, then maybe it's normal, in which case it won't affect your mileage unless it is leaking out somewhere when you fill it that much. Assuming you are calculating miles/gallons (which is what miles per gallon indicates), your numbers should be accurate, especially over multiple tanks. The only thing you can do to get your numbers more accurate is always go to the same pump and fill up in the same manner so as replace what was really burned. However, if you actually get more gas in than you burned, it's going to be there for you to burn on the next tank, so your numbers will seem low for the that tank, but not the newly filled one, in fact, they might even seem high if you don't fill it as much the next time. I drove a 93 PA once upon a time and never saw better than 24-26 mpg, but I wasn't doing any really long trips. However, I also wasn't going 80, as I'm not near any interstates. The previous owners said they got 29+, but that might have been primarily on trips to FL (from IN). Driving my 99 PAU from FL to IN after purchase at ~40,000 miles, I got 33 mpg on both tanks, I may have kept it under 70 then, though, I don't remember as it was yeras ago. After that, the shorter trips with multiple stops (45 miles to work with 3-4 stops each way once out of town), I generally got 28-29 in the summer. Now I'm down to 24-25 in the summer, and I don't know why. Additionally, the economy gauge in the car isn't accurate anymore, it was for a couple years, then it began to usually show higher than what I am actually getting, and the better I get, the farther it is high vs reality. In my experience, most economy gauges aren't accurate after some age and miles, but I don't know whether 1) this is due to sensors getting weak, wherein replacing them might kill two birds (accurate economy gauge & better mileage), 2) it is due to engine wear (that might not be cost effective to try to address), or 3) it is some combination of the two. Regardless, if this gauge behavior really is typical, that could account for the numbers people are reporting if they aren't actually manually calculating.

Thanks for your answers. I often do make 20 mile highway trips, and rarely more than 60 in one go...
When I made a 350 mile highway trip, I got about 22,5 mpg.
I mostly go to the same gas station. Note the 91 doesn't have an economy gauge...
It's good to know the milage is normal then.

I forgot about this thread (http://www.automotiveforums.com/vbulletin/showpost.php?p=5878220&postcount=19) where our milage is close to the "official" combined figures.

Just want to make sure I'm not forgetting anything what could cause the milage to be to high in the PA, gas overhere is expensive enough as it is !

My 91 PA uses a lot of fuel. Measured over a couple of thousand of miles, it averages about 20,5 mpg, and this is 80-90% highway.
I already replaced the airfilter, spark plugs, plug wires, coils and o2 sensor. Also I didn't drive faster then 80.
When I calculate the mpg from the fuel gauge it is about 25, but when I actually fill up the tank it always goes down to 20.
How do the people who get good mpg measure this? I have noticed that when the pump stops when filling up the tank, there is still room for 4-5 gallon if you poor it in slowly. Could this difference account for the 30 mpg figures people are reporting? What else could cause the high fuel consumption?

If you fuel up at a discount brand station, you might be getting more alcohol in the fuel. I have found that makes a difference on my car. Try fueling up at brand name station for a couple of tanks. Also gas in different parts of the country seem to be different quality.

That gas gauges aren't linear in their readings. I replaced the fuel pump in the tank on my 98 and have an AC/Delco replacement, but the gas gauge is closer to linear than any I've seen. If it's down the first quarter it will only take about 4 gallons (18 gallon tank). The way to compute is just keep a note book of the odometer reading and the number of gallons you put in. Over 3 or 4 fillups any error due to car position and not filling to the same amount of air bubble volume in the top of the tank will be averaged out.

You don't mention how many miles on the motor. It may be some age and mileage wear is affecting the mileage. If it were my car, I'd pick up a quart of Rislone oil additive (looks like a bright yellow quart of oil) and add half the quart about 500 miles before my oil change. Then repeat next time with the other half. It remove deposits on rings that might affect efficiency.

Also I'd use Techron Fuel System Cleaner. The 12 oz bottle is for up to 12 gallons of gas (our tanks are 18) or the 20 oz bottle is for up to 20 gallons. The 12 oz bottles are free with a mail in rebate at Autozone right now.
http://www.autozone.com/images/in_our_stores/store_rebates/chevron_0115_09.pdf
The rebate coupon is for purchases up through Feb 12. Note that Walmart and other stores have this also. There is a Techron Injector Cleaner that's cheaper, but I use the Fuel System Cleaner.

These are both things I use in my own Buicks about 3 times per year.

i want to figure out the mpg on my park ave also. i drive 50 miles total on the highway each day going and coming from work. i normally drive with the car in just the regular drive gear. is it better to use the drive gear with the overdrive circle around it, or just the regular drive gear to get the most fuel efficiency?

i want to figure out the mpg on my park ave also. i drive 50 miles total on the highway each day going and coming from work. i normally drive with the car in just the regular drive gear. is it better to use the drive gear with the overdrive circle around it, or just the regular drive gear to get the most fuel efficiency?At what speed (mph) do you drive? Anything above 50 mph, you should leave it in (OD) for best mpg.

At what speed (mph) do you drive? Anything above 50 mph, you should leave it in (OD) for best mpg.


i normally drive from 55mph to 75 mph(85 in the tunnel :grinno:) on my way to work. i will leave it on OD now. thanks.


also since i work late at nights, there are times when traffic is nonexistent and i don't have to brake for like ten minutes.

i normally drive from 55mph to 75 mph(85 in the tunnel :grinno:) on my way to work. i will leave it on OD now. thanks.
also since i work late at nights, there are times when traffic is nonexistent and i don't have to brake for like ten minutes.Wow, you should see a 4-5 mpg increase between D & OD at those speeds! :smokin: