Quote:
Originally Posted by sarguy01
At 30,000 miles, an increase of 10% from 15 to 16.5 mpg will save $636 at $3.50 a gallon. We are also assuming that a good tune can get 10% more economy. I'd bet that figure would probably be more realistic at less than 5%, if there is any gain at all.
But, remember, not everyone can and should tune their own car. A bad tune can destroy an engine quickly. Tuning and dyno time cost money, so you can probably add another $500-$1000 to the $800-$900 tuning hardware. Now we are up to a $1,300-$1,900+ payback of almost 100,000 miles using the above numbers.
The manufacturers do a very good job of tuning and keeping engines safe. Leaning them out and changing the timing gives them less of a safety margin if there are weather changes, bad gas (lower octane), etc.
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You can use conservative "canned" tunes that will slightly lean out the highest load cells, which is extremely helpful to fuel economy for aggressive drivers because the factory tune is usually way too rich.
Leaning out the low load cells is done pretty frequently to save gas while cruising, and on less efficiently geared cars like mine you can definitely get 10% back just from the reduced pumping loss on the freeway. My engine runs at 30% indicated load on the freeway! The torque calculations work out to the engine putting out something like 13% of its available torque, and so it's gotta be running at like 400+ g/kWh BSFC. Highway cruising my car only does like 37mpg, when it has similar drag and RR to a Prius. Most economy cars are doing a bit better but any passenger car with a V6 or V8 is probably doing even worse.
Personally, a tune/piggyback would be one of the first purchases for me on my next car, because it both saves fuel and gets you more power (for relatively cheap compared to an exhaust or something too!), plus you get to fiddle with stuff like cam timing (the cam timing on some cars is set for more "response" and less efficiency).