[uneasyrider]

Here's an update. I went out today and got more accurate about the whole process and found I' getting 19-20mpg NOT 28mpg. I found that the point I filled my car to last time WASN'T all it could hould. This time I really made sure I got it topped off. I was also doing 70mph not 60-65mph but I don't think the higher speed was the major factor it was the 2gals of gas I found I could still cram in there. It will look like it's close to as full as it can get and couldn't hold more than a few cups more BUT it will take about two gallons more if you carfully fill it. The first time I filled up on that first run I must have been more careful than on the second fill. Not that 19 or 20 is horrible but it isn't 28-30. Ooops. At least that explains the error. It's a straight forward calc. but the data has to be correct.

[b-man]

Uh huh.

19-20 sounds realisitic, 28 no way.

Same pump.

Same station.

Same top-off technique.

Now it's time to calibrate the odometer.

[uneasyrider]

Yep, I had someone follow me this time and used that trip odometer. It was a Subaru Legacy wagon still on stock size tires and original rims and we checked it for 10miles against the mile markers just to be sure the factory wasn't off too much. A bit retentive but that way the error was only in the fill up which can still be a big one if your not careful. That dang fuel tank is strange. You'll fill it up into the neck and it will go back down and set there a few inches down in the neck so you think that's all it will do BUT NO. If you are real careful you can set and add about two more gallons after that and then it sets right in the top of the neck and DOESN'T go down. I got to thinking about it when I was filling up and I did that at the first fill up last time but not at the second thinking it couldn't hold more than a few cups more. I need to get my speed'o fixed, I need to figure out the right gear to put on there. It's 20% off on the button right now.

[marauder]

You guys are getting much better mileage than me even with the 700R4 overdrive tranny I recently installed.

I averaged 9 mpg in town over 8 tankfulls with the turbo 400 and now with the OD I'm seeing about 11.5 mpg for an improvement of around 25% however that mileage still sucks! I'd do backflips for 15mpg!

Now I've got a stock 400, Q-jet, super mild cam and 3:42 gears. I'm planning on going with the P-Dude roller cam and some ported 6X heads and intake however I can't imagine getting worse fuel economy. I know that Q-jet needs a rebuild and I'm hoping its the culprit.

[bobzdar]

Run lots of timing and lean primary jetting.

I get 19-20mpg highway in my '78 t/a. st-10 4 speed, 3.23 gears, 6x-8 headed 400, 744 cam, quadrajet on a Torker 1 intake, 1 5/8" 4 tubes through 2.5" dual exhaust. I run 38 total timing (gives best mph in 1/4) all in by 2400rpm with full manifold vacuum. At 2700rpm cruise (or around 70mph with 255/60/15 tires) I have over 50 degrees of timing and I barely have to press the gas to maintain speed. My carb has stock (lean) primary jetting with the secondary metering rods changed to give best mph in 1/4 (around .050" tip size, maybe CE rods? can't remember). This probably doesn't result in the best mixture distribution, but with the open plenum of the T1 it's probably decent under higher rpm WOT, which is all I really care about. I'm not running enough compression for the lean jetting or timing to cause any sort of heat/detonation problems, so probably get better mileage than what a 9.5 or 10:1 engine would be capable of using pump gas, and can do so on 87 octane.

If you're running an auto with a non-lockup torque converter, even with overdrive I'd say you'd be doing good to get 16-17mpg. With an o/d manual trans (or 2.73 gears) you could see 21-22mpg. Auto with lockup t/c and o/d could probably get similar 19-20mpg as my non o/d manual.

Keep in mind a 4.21" stroke engine will have more internal friction and therefore be less efficient than a 3.75" stroke engine. Probably a 1-2mpg difference even with lower gearing proportional to the displacement increase.