Quote:
Originally Posted by cal30_sniper
My best so far is 14.7mpg from Johnson City to College Station at 70 mph. When I kick it up to 80 out west of Fredericksburg, it drops to about 14 flat.
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That's 10 mph difference and only a 5% drop in fuel economy. The rule of thumb is 1% decrease in fuel economy for each mph increase. It seems like your engine is not very sensitive to speed / load.
For comparison, my 82 Suburban with the 6.2L gets 29 mpg at 58 mpg but drops to 24 mpg at 70 mph. That's a 12 mph difference and 21% fuel economy difference. Basically, it loses nearly 2% for each mph increase.