The scanguage actually, by giving you the results in real time, lets you see things you may have never realized. I found that I rest my foot on the gas pedal at times (instead of 400mpg + coasting it was like 125), forget to go to neutral on the coast (same kind of info), it even reminds me to find the best line in highway traffic to see which lane is drafting better with REAL results. If I had picked the default slow lane, I would have had to brake, if I had picked the fast lane I would have been doing 70mph +. The other 2 lanes I found the data was the best indicator of where to be.
I am still breaking in the Yaris and the guage has been a godsend.
No way a human checklists catches all these nuances on my repetitive 120 mile round trip commute. Without out it I would just be a zombie at times in the car following the leader.
One thing to add, I have read and seen some people going for LED's for certain ranges coming off the scanguage almost to tag them like Pavlov's dog. Humans react very well to Ambient devices that give them 1 data point in a range to react to. IE (Red light on phone you have voicemail) From the scanguage if out of spec limit for mpg red light. Keep the red light off, much like the hybrid owners are doing with the ICE indicator.
A Scangauge is going to help you more than LRR tires. Why? It affects HOW you drive. Driving habits account for a larger percentage of your fuel economy gains than any other mod.
A scangauge will pay for itself faster than LRR tires will. LRR tires will also wear out faster than a scangauge will.
Making a blanket statement that you don't need a scangauge because you already know everything that needs to be done is also silly. If you already know everything then I don't know why you post here. We're a group of people searching and experimenting. We're not a group of know-it-alls.
I agree with what SVOBoys said about getting more information in 2 weeks with instrumentation than in a year with just receipts.
No amount of knowledge is going to get your better fuel economy. Knowledge is only part of the equation. When you figure out how to implement the knowledge, then you will be king. The only way to properly hone your skills is to get instantaneous feedback to know when you're doing something wrong.
Zpiloto has a car with a 2.0 engine. I had 2.4
He also screwed with his transmission, I didn't. Picking up another 3-5mpg was too much risk for the transmission.
Well I don't know about screwed, it started to complain I stopped fishing and in 2 weeks it's back to normal so go figure. Instrumentation is the single most important function for forming FE driving habits. Some of the things that you can do to increase FE are not even noticable unless you have feed back. I look at it this way I'm over 60% over EPA and 40% of that probobaly driving techniques which with out instrumentation is really a tough nut to crack. Don't step over a dollar to pick up a nickel if you truley want to bust the FE open.