With stock injectors/fuel pump/fuel lines and tune, I'm wondering about the differences in fuel economy for a couple situations. (5spd trans)

I find that cruising at 40mph and watching the boost gauge (with vacuum) my gut feeling of which gear to choose might be wrong. I'll explain.

Cruising in vacuum I normally just use the highest gear without bog and enough throttle to maintain speed. I figured at 40mph 5th gear is better than 4th, but my vacuum readings are telling me otherwise.

At 40mph I'm at....
4th ~2500rpm
5th ~1900rpm

I find it takes more throttle in 5th to maintain the speed with vacuum readings similar to this...

4th gear: 20 inches vacuum to maintain 40mph at ~2500 rpm.
5th gear: 10 inches vacuum to maintain 40mph at ~1900 rpm.

Which is better on fuel economy? My gut said 5th, but with such a lower vacuum in 5th it appears the engine is working harder. So is working harder at 1900rpm worse than working less at 2500rpm? I'm not sure which side of that relationship hurts economy more.

I don't have a tactrix cable yet but am getting one in the next month or two so I can log injector duty cycles and such. I spend a lot of time at 40mph and with either gear providing enough acceleration without bog, I'm up in the air as to which gear to use before getting the cable.

I get decent mileage, I'm just somewhat bored and curious.

 

Higher vaccum = less throttle= better milage. I think in this case 5th gear is a little much at 40mph, you see that as less rpm's but slight bog and less vaccum, using a little more gas... could be a trade off, lower rpm + more gas= the same as more rpms less gas. Bottom line is the more vaccum, the less throttle, the better the milage.

 

So you're basically saying that if I controlled for one variable or the other, I'd find that higher throttle/vacuum equates to more fuel wasted even at lower rpm? I knew this but always figured that cutoff point was lower like 1700rpm or so for slower speeds with less wind resistance.

I'm simply interested in the math used to prove the underlying concept. I want to understand it thoroughly to the point where it can be a formula in a program or script. I am looking for something to do with my car computer. Think about a perfect mileage shift light for a given speed.

I was thinking that maybe less work at higher rpm would be worse because higher rpm results in more injection events. What I didn't know was how significant 600rpm of extra injection was compared to 10 less inches of throttle vacuum. With that math I should be able to find the exact perfect switchover points for a given throttle and speed.

For instance.... (besides the first numbers, I'm making this up to show my thought process)

4th: 40mph@2500rpm at 20 inches = 70 fictional units of gas
5th: 40mph@1900rpm at 10 inches = 90 units of gas

So 10 more inches compared to another 600 rpm ends with 10 units being worse. You want 4th gear despite higher revs...

4th: 55mph@3600rpm at 12.5 inches = 115?? units of gas
5th: 55mph@2500rpm at 7.5 inches = 110?? units of gas

At 55mph however you want to finally make a shift now to 5th gear where the rpm difference + wind resistance now makes 4th gear less beneficial and the same speed can be held with slightly less (5 units) of fuel...

So that's an example of the type of logic I'm shooting for. I need to do some more reading but I want to know if anyone else thinks about it this way? Obviously it's quite dependent on temp/tune/injection size/pump efficiency/altitude/etc. Is there a way to generate that data by simply logging and using math to calculate the optimal points? A friend with a Hybrid is only getting 35mpg and in good fun I'm trying to beat his car's mileage while having an Evo. Just passing the time between go fast purchases