Alright guys ive seen several people complain about how high the Fits' rpm's are on the highway for the MT. Alot of people were also talking about adding a longer ratio tranny to lower the rpms and supposedly raise mpg. Now if my thinking is correct this would only lower mpg.

I know rpm and throttle BOTH play a role in mpg, but in my opinion its mostly throttle. If your car was at a lower rpm you would need to give it alot more throttle since the Fit is such a low displacement engine and at highway speeds the drag starts to slow down the fit big time. I mean im just going by what the fuel gauge on the dash says. Another example, try cruising on the highway in 4th. Youll be spinning at least 5,000 but youll need almost no throttle and youll be getting 40+ mpg. Of course this will cause some accelerated wear but I think Honda nailed the tranny ratio to give it enough power to keep up on the highway but not sacrifice gas mileage.

Im not telling people to go around redlining their car for gas mileage. But IMO higher rpms = less throttle required to maintain speed/accelerate = better mpg?

 

Interesting. I always thought it was RPMS, but I'll keep this in mind during my next fillup to see if there's any difference. Definitely makes sense to me.

 

I honestly wouldnt recommend it, I was just trying to make a point across. Dont wanna be responsible for someones engine exploding cause they cruised at 6,500 on the highway for half hour... Lol Although I probably shift higher than what most people are used too. I shift at around 4,000 and get about 36mpg, without all those fancy hypermilling tricks.

 

It's going to be extremely difficult doing an accurate comparison on the road because all parameters have to be identical for all of the tests, i.e., ambient temperature, humidity level, wind velocity and direction, tire pressure, traffic flow and direction, etc., which is a highly improbable, if not impossible, feat to accomplish. The best method for getting accurate results is to use a dyno. But that's expensive for this type of test.
IMHO, the Honda Fit engineers did their homework when determining the optimum auto and manual gear ratios for maximizing mpg for combined city and highway driving.
That said, one can possibly increase the mpg for either type of driving(city or highway) by changing the final gear ratio, but only at the expense of the other.

 

I agee with you to some extent... i see it like this if u have to keep the pedal depressed half way to keep your revs at 2500ish, your probably wasting more gas than having the pedal depressed 1/4 the way or less and running just over 3000 revs. of course thats just how i think, no idea if theres any truth to it.... i think if honda could have gotten significatnly better mileage with a higher gear they would have.....

 

I am not an engineer, OK? But this doesn't make sense to me. When you're in a higher gear, the engine is running at lower RPMs at a given land speed. It doesn't take more throttle (more fuel) to achieve equal engine speed (RPM) in one gear than another. Therefore, higher gears = less throttle = less fuel consumed.
If cars got better fuel mileage in lower gears, there would be no need for additional gears.
Maybe I am misunderstanding your thread. Surely I must be. The way I see it, throttle is RPMs.

 

Its not about gears, its about throttle and rpm. Let's say they raise the gear ratio and your at about 2500 instead of what your usually at in 5th. Your making waaaay less power which is going to require you to give the car more throttle to overcome the wind and drag resistance to maintain speed. Thus applying more throttle and lowering mpg. I think this would apply more to MT since autos have torque converters.

 

Well I drive an AT and for example, if I'm going up a long steep hill and the car is in 4th gear doing 40-45mpg @ 3.7K RPMS, in order to achieve the best gas mileage do I:

a. Do nothing, let the car do it's thing.
b. Intervene and upshift to 5th gear, thus reducing RPM's and what you're saying is increasing throttle.

 

Well if your in 5th gear you would have less power and have to apply more throttle to get up the hill. Lower rpms yes but you will be lowering your mpgs. Stay in 4th and apply as little throttle as possible.

 

Alright, there's three variables here:
-engine speed
-throttle / manifold vac (interrelated)
-air fuel mixture

First consider that two of these directly affect mileage:
-any manifold vacuum is a pumping loss, meaning the engine has to work against it to maintain it. Less throttle, more vac, more pumping loss.
-best mileage is at stoich (14.7:1) or leaner, so all fuel is effectively burned; think limiting reagent in chemist terms. This car's leanest mode is stoichiometric. As dynamic compression in the cylinders increase (less manifold vac) at some point the fuel mix will have to be richened to prevent both preignition/detonation and overheating.

Friction in the engine is relatively constant, though the rings will have slightly more under more compression.

Assume to cruise the same speed the motor requires the same power output in any gear. Difference is gear friction is negligible.

So the point of best mileage would be where these two variable conditions cross- the lowest manifold vacuum you can maintain without needing to richen the fuel mixture.

Here's where the fun comes in. You have to make it run stoich at all daily driveable cruise speeds, otherwise the customer's gonna be mad from changing spark plugs every oil change. If you make it perfect at 60 mph, say 8 inHg vac and the car can still run stoich, great. Wait until they go 70 mph, and to fight wind resistance they give it enough throttle to run 6 inHg. Uh oh, it was found in Arizona weather it would ping at that compression so with the factory tune now it's running 13:1 A/F. That car just managed to lose a LOT of mileage. It doesn't take a lot of throttle to lose vacuum-- anyone who's ever driven with a vac gauge knows it disappears fast in the first 1/8 or so throttle opening.

So, cars are made with higher final drives so they don't richen up until well over highway speeds.

The MT Fit can still use a 6th gear though.