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Just wondering if my mileage of 19.7 is decent?
I've been doing Fuelly for a while and it seems to be right around this number. Didn't really get a chance to know what mileage should be before mods and a motor swap.

Heres the variables for the question:
-99 SE bonneville with a L26
-Intense FWI w/K&N filter
-180* therastat
-Napa Belden wires w/NGK spark plugs
-New AC Delco O2 sensor
-3.05 gears
-Stock tire size
-Mostly short in town drives with some longer drives
-Some heavy foot time in there once in a while.
-No PCM tune done yet to compensate for the 180* thermostat.

Is there something I'm missing or forgetting to help up that number?
Just wondering what kind of mileage you guys are getting out of your cars.

My average mileage with the Impala was around 24.5 with many short trips, and I had most of your mods as well (although I had an Intense PCM and a ZZP downpipe). Have you checked your tire pressure lately?

Mostly in town is going to kill your mileage. That sounds like a decent number. My number is mostly highway though, which is annoying me. I guess being 19 and driving as such is having an impact, lol.

Tire pressure is right where it should be. Being N/A I thought it might be a little better. Maybe my right foot needs to listen to the mpg's instead of the air getting sucked into the motor.

00Beast wrote:Mostly in town is going to kill your mileage. That sounds like a decent number. My number is mostly highway though, which is annoying me. I guess being 19 and driving as such is having an impact, lol.

Now imagine what being 17 and driving as such does to your gas millage...

For mostly in-town that sounds about right.

These cars are highway cruisers. They're meant to eat up the miles, not sit at a traffic light.

That sounds about right. My '95 with an L36 under the hood routinely delivers 19 or 20 mpg around town and upper 20s or better on the highway. Gotta love it. A comfortable size car with plenty of power and the fuel economy of a compact.

im at 14 around town. lol 19 is good

99BonnevilleSE wrote:

00Beast wrote:Mostly in town is going to kill your mileage. That sounds like a decent number. My number is mostly highway though, which is annoying me. I guess being 19 and driving as such is having an impact, lol.

Now imagine what being 17 and driving as such does to your gas millage...

I remember that like it was two years ago...
My 94 LeSabre would average 8-9mpg in town only , but it was in DESPERATE need of a major tune-up, but the body was too far gone to be worth it. I have calmed down a lot since then, I now get anywhere form 17-20 in my SSEi around town with some highway.

When you say L26, you must mean you're running the L26 intake and fly-by-wire TB? If not, you have an L36 which just happens to have an L26 long block under it. Your mileage would be no different than any other L36.

Your FWI will net you a slight increase in mileage if you keep your foot out of it.
Your thermostat MIGHT make a small change to the negative depending on how cold your mornings are.

Your 'almost 20 combined' seems about right for several in-town short trips combined with some heavy footing, but there's no way to know without seeing the roads, stops, speed limits, etc.

Yes L26 bottom end, thought the heads breathed a little better than L36. I kind of thought that the thermostat might affect it, but its worth it for the lower operating temps. I know I should get a PCM tune for fan turn on temps.
I know that this is just a general mileage average, just as long as it's not to far off.

I average around 19-20mpg with mixed driving and a somewhat heavy foot

Bing wrote:Yes L26 bottom end, thought the heads breathed a little better than L36. I kind of thought that the thermostat might affect it, but its worth it for the lower operating temps. I know I should get a PCM tune for fan turn on temps.
I know that this is just a general mileage average, just as long as it's not to far off.

180° thermostat will hurt mileage in the winter, but not this time of year. L36 heads aren't the restriction. The LIM, manifolds and downpipe are, so I don't expect a tiny difference in valve diameter will affect you any noticeable amount. In fact, I suspect the better L26 LIM casting has more to do with the slightly higher output than the head design. Any ported L36 should be equal.

I did notice that when I had initially done the swap and started driving the car that mileage was around 16-17 mpg. This time frame was around late winter early spring with temps in the 30's to 40's.

A down pipe and high flow cat might be next years project, now that I know I have a good motor.

Will these 2 mods hurt or help my mileage out of curiosity.

Anything that let's your engine breath easier will help mileage, as long as you don't use the loud pedal more.

You should be getting 22mpg avg with out trying or less if you try, but you should see about 25-27mpg on the highway. That is the millage i was getting on my 93 Bonneville, 93 LeSaber, 2000 & 2003 grandprix. On my new 98 Bonneville I am getting 24mpg avg look at my signature. Inflate your tires above the suggested and make it closer to sidewall max, also try to drive for better MPGs and after a few tanks you will be doing better.

As stated by many others, 20-ish for mostly in town driving is on par. My '98 L36 Monte Carlo got about that in similar driving scenarios. Until you get about 50% highway driving per tank, low 20's is the best you'll see. If you get 50-75% highway driving, expect to see 23-26mpg, and 25-30mpg in 75-100% highway driving. This is all assuming you drive with a steady foot, without multiple WOT runs per fillup. Right now, my commute changed last year with a new job, so with the '91 Olds 3800, I get 15-20mpg, because my commute is 3.5mi., 50-60mph with 5 stop lights, on a cold engine. KILLS gas mileage! The 15mpg tanks of gas are really depressing when the same car will return over 29mpg on a little road trip.