Sudden Jump in MPG? What?

[MrSony]

Ever since I built this engine, I've gotten 10mpg. High, city, light driving, heavy driving, 10. T.E.N. 16 gallon tank, filled twice on my last DSM, IA to Omaha, NE trip. 135ish miles each way.

But tonight, I filled up like any normal time. Didn't have a lot on me until payday, so I put 17.01 in. Got me 5.386 gallons of 91. Basically a 1/3 tank. The mileage was 96,610.

Drive around with one of my buddies, rack up 101 miles by the time I'm back in my driveway. I look at the gas gauge (obscured by my tach, kinda layin on the tilt lever). I have an 1/8th tank left.

WTF?

How did I get 18mpg all of a sudden? This was with some highway and city and "fun time" driving as well. Maybe the 2.41s made me use more throttle to get around, and the 3.42s due to the torque multiplication require me to use less throttle to move at the same rate...

This weekend I'm going to put 4 gallons in my 5 gallon jug, drive around until it dies and won't kick off, put 4 gallons in (1/4 tank), drive to another station (refill jug to 4 gallons again) and see if I get the same MPG. If I do, I could really get used to it.

 

[GP403]

Maybe the 2.41s made me use more throttle to get around, and the 3.42s due to the torque multiplication require me to use less throttle to move at the same rate...

I'm thinking you probably answered your own question.

 

[MrSony]

I'm thinking you probably answered your own question.

 

[fleming442]

Funny how that works. I had a similar experience with my project, only I was starting with much less torque.

 

[olds307 and 403]

The first time after installing 3.42 gears in place of 2.56's were, what is that burning smell? Oh yeah, it is the tires. I lost mpg mainly because the car was so much more fun.

 

[pontiacgp]

In the city your mpg should be better with the 3.24 but with the 2.41 on the highway if you were getting 10 mpg there is definitely something wrong.

 

[Michael Watts]

You probably are getting better now going from 2:41`s to 3:42`s .Is that you found your "sweet spot" for your engine combination. And maybe your driving style. The 2:41 will give you more "top end" but it will take you longer to get up there.

 

[TURNA]

Downhill both ways?













😛

 

[Bonnewagon]

I found the same thing. My '81 Malibu wagon had the POS Chevy 229 with a 3 speed manual trans. It should have gotten great mileage with the 2:41 gears. It totally sucked except on the interstate going 80 mph. I swapped in a Pontiac 350 and 4 speed with 3:23 gears and the mileage doubled. It was also night and day performance wise. I feel it is a matter of volumetric efficiency. A bad combination struggling to move a heavy car is doomed to be a dog. The bigger motor with good gears doesn't have to work as hard and you stay off the gas pedal (somewhat).

 

[MrSony]

Little addendum to this story, my odometer is off because of the gear change, I drove 1 mile in my dad's saturn, and drove the same distance in my car. My car read 1.6 miles. I think that's why my mpg seemed like it nearly doubled. I will report back once I have installed the correct speedo gears for my application.