TOPIC: 1998 GMC Sierra 15% increase post groove

305 v8 vortec engine. approx. 18 -19 mpg pre groove on highway. Last test was 21.1 mpg highway driving into 10mph wind. This has been repeatable. The next mod will be rerouting the idle air to pcv input with a shutoff valve, when off idle.
I will update with photos soon.

I wholeheartedly advise you to regap the spark plugs BEFORE you make any additional modifications, Jim.

Regap to at LEAST 20% over stock gap setting. This will quite probably cause you a sharp intake of breath when you do your first runs! Also, be sure to do a pump-down vacuum test of all lines, and finding and FIXING all bad connections and diaphragms.

The vacuum system MUST be dealt with first. The smallest of leaks can cause dramatic reductions in gains possible with The Groove. Just ask Nick (Gadgetman Fingerlakes). He'll tell you from first-hand experience! (See the documents section under Gadgetman Land for more on vac testing, etcetera).

Thanks for the report, and mostly for the attitude. You are most DEFINITELY one of "US"!

Ron