: another MPG post

03-19-06, 01:12 PM
is 13-15 mpg combined city and freeway driving something to bitch about? i have heard these 307 broughams are capable of 20!:eek:

03-19-06, 02:32 PM
They are rated at 17/24, and those ratings are always a bit optimistic. I get about 14-16 MPG average in my '92 deVille with a PFI 4.9, so 15 MPG sounds pretty reasonable for a 17 year old car with a 4bbl carb, which could be out of tune.

How do you drive it? Aggressively?

03-19-06, 06:59 PM
I would say if properly tuned as GM intended yes, my Cutlass w/307 got 27 on the highway often. Granted, that is driving the speed limit (65) to do it, but it did 27.3 several times. A friend of mine's parents got 27.3 several times driving faster, but had a 2.56 geared Cutlass with OD, mine was a THM200C w/2.14's.

My mom got 23-25 often with the 85 Fleetwood Brougham and the HT4100. Her 84 98 w/307 got 24-25 often highway as well. Those trips were driving 75 most of the time. We easily got >500 miles to a tank often highway.

03-21-06, 10:57 AM
Don't complain, my last tank of gas was 303 miles, used up 21.6 gallons. 14.02 mpg avg that tank. 96 Fleetwood, LT1, 112k miles on it. I've had tanks not even go that far lately too.

DopeStar 156
03-21-06, 12:32 PM
I would say that's a problem. I can go a whole week on one tank of gas and I end up filling from a quarter. I have an '89 Fleetwood Brougham just like you with the same 307.

Edit: Well, almost the same..... :bighead:

03-21-06, 01:16 PM
Driving city/highway, I am solid at 16.5-17.5 mpg when below 32F, above typically closer to 18-18.5, peaking in the summer w/AC on around 19.

Now that I am able to retune my PCM anytime, I am experimenting with programs to get me better mileage with 87 octane and abiltiy to use 105 octane E85. I just figured out that I can use the new pencil style LS6 injectors, which are rated at 28.5#/hr.

I hope they won't hurt my 87 octane mileage, but cruise rpms I am seeing around 150-155 or so LTFT's to maintain stoch A/F with around 50-60% E85. with E85 @ $1.999 a gal, and 87 octane at $2.589, it is hard to NOT look into using it.

In my calculations, with an average 2.5-3 mpg drop in fuel mileage, and a current $0.59 a gallon difference, I STILL can go 30-40 miles FARTHER on a tank with E85. With less E85, mileage is better, and the average cost still pays out pretty well, with around 20-30 miles more to a tank.

I am hoping the newer design LS6 style injectors have a better spray pattern, and might actually help mileage too.

03-22-06, 09:50 AM
I get, on average, 14-15mpg driving my 200k+ mile FWB with the stock 307. Most of my driving is stop and go in traffic, since it's my daily driver car to work. My carburetor is also kinda wacky.

03-22-06, 12:31 PM
A friend of mine has an 86 that I might be able to get, if he wants to sell it. It has been parked for 1.5 years now and not cranked. (it was his dad's and he died, so the car sits and sits outside).

I would love to work on one again and see what I can do with it.

03-22-06, 03:07 PM
wow this sounds like it sucks . what i am going to be expecting for mpg on my soon to be funning 500 cid 76 caddy deville

03-22-06, 08:54 PM
2-3 Mpg

03-22-06, 10:35 PM
Tuned well, I would expect around 11-12 city, and around 16-17 highway, keeping speeds down to around 65 tops. Those figures are reasonable guesses with 55 mph speed limits.

You might get a little better by slowing some to 55, and do some old speed tricks like recurve igntion, even my HEI mod (look in tech forum) and get the initial timing as high as you can tolerate and do some more conservative mods on the timing curves.

Much emissions on the older cars was crude at best, and modernizing it to be more efficient will help. Heck, if you can get an EFI manifold and a modern EFI computer, that would be ideal. Do what is needed to get to 9.5:1 compression, EFI, OD (major beefed up THM200 4R) and some mild gears like 3.08, 2.93 or 2.73 will help make a killer 500 and get decent mileage....