Mileage versus rpm's

[kikkegek]

guys,

I have spent a lot time (and some money) optimizing my 350 to get the best mileage possible and still have a driveable vehicle.

see this topic on that:
http://67-72chevytrucks.com/vboard/s...d.php?t=429908

the thing I noticed during this proces is that whatever improvement I get my mileage for 100% highway (long stretches) and our regular 50/50% city/highway daily driving always get the same mileage.

We always get around 8.2-8.7 mpg....

Now whenever I read a mpg topic people always seem to have a 10-30% better highway mileage then during their 50/50 driving.

My setup is:
- Suburban K20
- TH400 with 4.1 end gears
- 350 small block (original from 1986)
- all the smog stuff has come of
- no more gasoline carb, just the throttle plate
- propane setup: IMPCO 425 mixer on the throttle plate
- IMPCO E-type evaporator
- electric fan for cooling
- MSD 6AL connected to HEI dizzy with light springs for fast and flat curve (for propane)

what ever we do, highway mileage doesnt seem to improve compared to the 50/50 driving. I installed a tach and I noticed we do 3,000rpm when driving 90 km/h (which is slow on Dutch highways).

could the 3,000rpm be the cause? or are there guys over here that also do 3,000rpm's and still get better mileage on the highway?

please let me know