Gas Mileage

With the price of fuel rising by the day, I thought I'd check to see what my 1999 Regal GS was getting for mileage. I checked the system and it's telling me that I'm averaging 22.2 MPG. Is that typical, or does that seem low? I know there are a lot of variables. I don't do much highway, always only me in the car, and the average temperature here over the past couple of weeks has been between 5-10 degrees above the freezing mark. Also curious as to what speed these cars get their best mileage. I'd be interested to see what others are getting in their cars as well.

Do you reset the DIC every time you fill up? Definitely a lot of variables. Weather being one factor, traffic conditions and how your drive. Have you run a manual calculation? If you are get around 20-22 mpg in city driving only that sounds normal to me. I get an average of 21 mpg in city. Much higher figures highway which can go as high as 30 mpg with speeds ranging from 55-70 mph.

Also, are you using premium gas (at least 91 octane)?

I actually don't reset everytime I fill up. Is that a better way to get a more accurate reading? Never done a manual calculation. I won't consider my driving city driving, but not highway either. I drive about 25 minutes one way to work. Paved roads, but not highway. Average speed of around 90 kmph. (55 mph?) I'm not terribly aggesive, but not super great either. Never done a manual calculation either. Is the computer not that accurate? I don't always use premium fuel. Would that make much of a difference? I'm probably due for new plugs in the car, so I know that should help also.

Thanks for the input.

When I had my 99 GS, I would average around 25 mpg. If I were on a trip using cruise, I could get up to 29 mpg. Traded it in a couple of years ago with over 100K miles on the clock. The engine ran like a clock, but the AC was a PITA. Using 93 octane would have gotten real pricey in todays market. Over the road, on cruise I'd drop down to 89 octane with no loss in gas milage. Around town driving, it would get into the low 20s mpg. In the winter, it would drop to sub 20s.

Tune ups are important. I replaced plugs wires etc around 60K, the only time I did that in seven years of ownership.

If you are not resetting the DIC then that is not an accurate representation of your current mileage only cumulative average mileage over a period of time since last reset. The DIC is nice but you cannot depend on it. Just fill her up and reset the DIC and then see what you get. Manual fuel mileage calculation is the same old method most drivers use that do not have a DIC/trip computer. Basic math.

You can get away with running gas such as mid-grade but you should go to a one range colder spark plug just to be safe and make sure there is no knock retard (KR) especially when supercharger boost kicks in. Keep off hard acceleration and pedal to the metal. Regular unleaded (87 octane) can be used in an emergency situation only but should not be used for any lengthy period of time.

As far as new plugs and wires, fuel filter, etc. that will help but don't look for any miracle mile per gallon increase. :grinno:

Since you're talking about 90 kmh, I assume your 22.2 mpg is based on imperial gallons. Translates to 18.5 mpg in USA? I wouldn't do anything outside of usual maintenance items.

Since you're talking about 90 kmh, I assume your 22.2 mpg is based on imperial gallons. Translates to 18.5 mpg in USA? I wouldn't do anything outside of usual maintenance items.

I'm not sure if the car calculated the mpg based on US or imperial gallons. I think on the liter side of the scale, it was telling me 10.?? Don't remember exactly.

You can get away with running gas such as mid-grade but you should go to a one range colder spark plug just to be safe and make sure there is no knock retard (KR) especially when supercharger boost kicks in.

Don't get any knocking when I'm on it. I've listened for that.

Do these engines prefer a particular brand of plug? And you mentioned a colder plug. Am I pushing my luck asking for a part number? :sunglasse

The way it is supposed to work is if in Metric mode then you will get the kilo stuff. If in English mode then all calculations I believe should be based on the US gallon and in mpg. There is a E/M (English/Metric) switch at the DIC where you can switch it on the fly.

I've used the Autolite APP 605 plugs (double platinum) in my Regal GS with no issues. This is one range colder than the stock plugs. Autolites have an easy numbering system to see plug heat ranges and cheaper than the AC Delco equivalents.

On the knock retard issue you cannot tell by sound or feel. You have to run a scan tool capable of reading out knock retard and in real time under typical driving conditions. I used Autotap running on a laptop when I did my testing just to be sure. On my GTP I have an Aeroforce scan gauge for KR.

I was reading another thread where someone had an early 90's Regal, and to change the plugs, he needed to undo the engine mount, and rock the engine forward. Is this also true for my 99, or is it not that bad on this car?

Really not necessary although you could do it. Not much extra clearance to be gained. All you need is the right spark plug common hand tools and patience. The rear bank plugs will be the most difficult. Mainly removing the plug boots. Also, there is a metal shield over the boot so be careful not to damage them. Better to replace the ignition wires at the same time. Be sure to use silicone dielectric grease on the inside of the boots which will make future service easier.

Just thought I'd post some mileage results, now that I know to re-set the trip every tank.

When I left work yesterday, I re-set the trip. I drove until I got to the gas station (almost home), and with regular cheap gas, I got about 22.5 mpg. I filled up with the good stuff, and continued home. (also re-set again) Drove to work this morning, and when I pulled into the parking lot I was reading 25.6 mpg. I realise a short trip is not a good exaplme, and there are a few variables, but I'm going to do a few different tests, and post results here. (including after tune up results)

IMO your testing really isn't conclusive but in the end I am quite sure you will find that you will actually get better fuel economy running the premium grade. Unless you run the existing fuel of whatever grade out of the tank all you will be doing is blending the fuel. For example regular and premium will probably net the equivalent of mid-grade.

We have this thread posted at one of our other forums concerning the issue of why you should run premium gas in any L67 SII 3800 engine and the effects of KR. Good read, if nothing else. See link below.

Click here (http://www.automotiveforums.com/vbulletin/showthread.php?t=664475)

Hi all,

I, or rather my wife, have a '96 3.8L N/A Regal. I ussually drive my '94 Plymouth Sundance to work 22 miles of often slow freeway each way and average 27-28 mpg. I have a theory that I can do just as well in the Regal. I hope to drive it for a couple of weeks to test my theory soon. I can cruise at 65 mph at just under 2000 rpm. I suspect if I keep my RPM to no more than that, even in slow and go, I will prove myself right. If I do, I am considering buying a 2007 or 08 3.8 L rental return Grand Prix to replace my Plymouth. We shall see.

Don
Union City Ca.

Well after a couple of tanks, I'm up to 26.4 mpg. I don't think with my current driving situation, that I will will do much better than that. Definately an improvement switching fuels. Now I just have to do a tune up. (Long over due) As BNaylor stated, it probably won't make much of a differance, but every little bit helps.

Thats pretty good overall if you are doing a lot of in city or combination mixed highway. The best test for mileage is when you take a long trip. The DIC reading will pop above 30 mpg and the average will be much better. On regular trips up to 500 miles one way and 1000 miles round trip I average around 28-30 mpg in my GS but it will vary, of course.

Every little bit helps so it is good to keep the car in tune. Not just plugs but new fuel filter, PCV valve and air filter. On the issue of air filters I get the best mileage with the OE AC Delco drop in air filter versus any other aftermarket brands.

Ya, I thought that was pretty good also. I wouldn't consider it city or highway driving. Mostly short runs on secondary roads. When I was on a longer stretch, I switched to the instant mileage, and it was going as high as 36mpg. I figure on straight highway, 30 mpg is not unreasonable to expect.

Thanks for the tip on using the OE filter. I wouldn't have expected that.

Hi all,

I have run the mpg test on my wifes '96 Regal to see if it can match my '94 Plymouth Sundance which gets 27-28 mpg commuting between Union City and Sunnyvale in the San Francisco South Bay area.
IT DOES! One week of commuting, 216 miles, got me 30.3 mpg. ;>
It is mostly freeway with allot of slow and go. some times stop and slow. The go is usually less than the 65mph speed limit. I suspect I get the best mpg at ~50mph.
So if I can come up with $15,000 I may buy a '07 or '08 Pontiac Grand Prix with N/A 3.8L with about 15,000 miles on it. Fortunately most folks are looking for 4 cylinder compacts andd consider Grand Prixs as gas hogs so they are rather cheap.

Don

Just thought I would give some new info. I drove from Toronto to Troy Michigan last week, and here is the mileage I got. Going down, I got a little better than 27 mpg, but the first part of the tank was not highway driving. On the way back I got almost to 28.5 mpg, and the first hour of the trip, I was stuck in traffic, so overall I don't think I did that bad. This is with the car not yet having a tune up.

So if I can come up with $15,000 I may buy a '07 or '08 Pontiac Grand Prix with N/A 3.8L with about 15,000 miles on it. Fortunately most folks are looking for 4 cylinder compacts andd consider Grand Prixs as gas hogs so they are rather cheap.

Don

IMO not that cheap. :grinno: I'll keep my '97 Grand Prix GTP until it falls apart. Also, FYI the '04 and up with the Series III 3800 engines do not appear to get as good fuel economy as the earlier generation with the Series II 3800 engine.