why do 3400's get better mpg than 3100's?

[robertisaar]

those 2.3 4 bangers were horrible... not enough even to move something as light/aerodynamic as those years of mustangs. what kind of gearing though?

as for the bump in economy you got switching from auto to 5 speed: while manual shifting can help, most often, its the steeper gears and fact that manual trannies lose less power compared to an auto.

recently, some autos are losing less power than manuals...

[Raffaelli]

My moms 4cly 89 Mustang auto = 17~ mpg
My own 5.0 86 Mustang GT auto= 22~ mpg
later with a 5 speed swap = 24/25mpg

The bigger engines cruise with much less throttle position and load on the engine. A pitifully small mill doesnt have the balls to get any power or mpg out of. (not sayin a 3.1 is small)

Plus manually shifting the gears helps quite a bit.

[robertisaar]

no, just the blend of fuel that gets delivered.

technically, yes your computer does richen the mixture based on IAT values, so you'll always lose economy when the air is colder...

something to remember: on a stock tune, except for when its warming up or in PE mode: the ECU is stuck with a 14.7:1 AFR, no higher, no lower. thank the EPA for that one...

[no_doz]

whys that-does the stock tune go rich as hell with the addition of the computer sensing incoming cold air? if so... that sucks!

[robertisaar]

then we get to factor in summer and winter blends of fuel....

summer=better MPG...

[IsaacHayes]

^^ Yup, Everything here is now E10. Before it was in full affect there was one station who was last to convert. I went out of my way to go there because a tank would last quite a few more days.

[3400-95-Modified]

^ which is a good reason to go through multiple tanks of gas at the same pump using the same grade of fuel to get a realistic number...

Another thing you need to realize with MPG numbers... is the fact that some states are mandated to only sell E10% fuel... so those that don't have 10% Ethanol in their gas will get better mileage. I noticed a decent drop when that was mandated in my area.

[pocket-rocket]

I got 32-35mpg in the 92 on a long trip once a few years ago. Had the cruise set to 65 and was in a lot of nice wide open highway without much traffic or hills. I like lean cruise

[nixtux]

My 95' Cutlass completely stock managed around 29MPG on the interstate (cruise at 70MPH.) Winter months I got around 27-28MPG. With a 16.5 Gallon tank in the summer
with A/C I could cruise 450 miles before I'd have to fill up. There wasn't much fuel left though.

Purely city, 30-40MPH I'd average around 20-22MPG.

I haven't had a chance to calculate the gas mileage on a 3400 since it started over heating after about 100 miles. But the tank was about 3/4 and in the end it was about a quarter tank. Seems to use more fuel but I don't know because I don't have any experience with it yet.

I will say the 3400 has a nice snap to it. But I hate the way the 4t40-e's shift.

[Rhedalert]

The 3100 in my sister's '02 Grand Prix is mated to a 3.29 FDR tranny. Any 3400 in the later W-bodies were 2.84 or maybe 2.93 or 3.08? This is probably the biggest contributing factor. The 3100 is not underpowered enough in this application to mess with fuel economy.

[robertisaar]

^ which is a good reason to go through multiple tanks of gas at the same pump using the same grade of fuel to get a realistic number...

[PCGUY112887]

It seems that the gas station you use may set off your MPG's (if your calculating it that way) because different pumps don't seem to automatically click off at the same time, some fill your tank a bit fuller than others which will throw things off.

[3400-95-Modified]

It's all in the power to weight ratio... Those 3100 equipped cars were underpowered for their weight so they got worse mileage.

I have been getting an average of 25-27 mpg on my .030 over 3400 in my Beretta that has a FD of 3.08 with a 2000 stall converter. That's combined highway and back roads, 45-50 speed limits.

I also got 27mpg with my 96 and at that time it was a 3.42 ratio, 3k stall converter and the big cam... Granted that was highway mileage but that was at a constant 75mph.

As other say there are too many variables to determine exactly why they get better mileage with a larger motor.

[CNCguy]

Weight and drag have a huge impact on mileage. On Monday, I was talking to one of our customers with a LX9 MGB that had just returned from his fourth trip from Dayton to Vegas. He is a pilot and maintains excellent records about fuel consumption. Mack claims 35.8mpg average for the entire trip and a best of 41.9mpg per tank on flat open roads out west. The car is running a Ford T5 with a .63 5th gear, 3.89:1 rear axle and 215/60-14 tires.

I know it is comparing apples to oranges but they are some pretty decent mileage numbers.

[SofaKingWeToddDid]

sounds good to me. ill trade you for my lumina... haha