Honda desperately needs to raise third gear AT from about 1 to 1.18 or so. The Fit bogs down when its shifted into third at economy rpm. There is no reason to have a Fit go more than 90 mph in third. 80 would be just fine.
I'm getting desperate enough to rebuild my own transmission. I would if I could find the right gear.

 

90 in 3rd? I know the GD3 will do not 90 in 3rd gear, just right around 80. In my opinion, the final drive ratio needs to be slightly higher. I'm guessing 4.6 - 4.7 gearing opposed to the 4.56 gears.

 

I believe you are talking about 5 spd manual which has a 1.321 third and a 4.129 final. In third you should only get 80.

We're talking about third in AT which has 1.06 third and 4.56 final. At 6500 ropm the calculated speeds are 30/60/90/130/180. Way overgeared in third.
And on the back and front straights at VIR I get at least 90 mph in third before upshifting to fourth. Every lap.

 

No, I was talking about the AT. I've played around with the paddles in a past freeway sprint to see roughly at what speed does *insert gear* run out. In the process of doing that I was unable to hit 90 MPH in 3rd gear, at least not without hitting the redline.

 

Whats wrong with shifting at redline?
At redline, 6500 rpm, the 23.44" diameter tire with 1.067 third and 4.56 final calculates 93 mph. So the max speed in third is 93 mph. When you go as fast as you can, redline is the upshift. Except in vettes; you're better off shifting about a thousand under redline.

I'd be a lot better off if it were a true 80 at 6500 rpm. Third then would have a gear of 1.200 : 1, not 1.067, and my engine would not be casually bogging at 2500 rpm.

 

Nothing, so long as you don't hit the rev limiter. I understand that in a racing situation it is important to run the engine to redline and shifting at that point so that the RPMs still fall in the motor's powerband.

Now, I just looked at my original response and will admit that me knowing that hitting 90 MPH was somewhat of a false statement. I did believe that ATs could do 80ish because the trans will not downshift into 3rd gear at around 75 MPH. So, you got me on the highest speed in that gear.

However, the final drive gears should be shortened in my opinion. Having 5th run up to 180 is just way too steep, 4th still pulls rather nicely at 70 as is; but could be a little higher by about .1-.2.

 

93mph is only theoretical. You have to consider air drag, whose only variable is velocity. The faster you go, the stronger the force of air resistance. The transmission may be geared for 93 in 3rd, but that doesn't mean it'll go 93 in reality. If anything, it's geared like it is to go < 90mph, maybe less. The engineers probably factored drag into the gearing. (example, an Insight going 75mph has to overcome over 2,200lbf due to air drag alone)

 

At VIR I see 93 in third regularly every lap.

PS my calculation says you only have to move 165 lb of air about 3 ft every second at 75 mph and 45 degree angle of attack acceleration. Thats a long way from 2200 lbf. If that were the case I rather doubt an Impulse would break 45 mph.
Perhaps you used the air volume of 2200 ft3 per second but air only weighs 0.075 lb/ft3.

 

I used ρ = 0.072lbs/ft³ instead of 0.075lbs/ft³

Fd = ½ρv²CdA
ρ = 0.072lbs/ft³
v = 75mph = 110ft/s
CdA of a '99 Insight = 5.1ft²

0.5(0.072 lbs/ft³)(110 ft/s)²(5.1 ft²)
=
0.5(0.072 lbs)(12100 ft/s²)(5.1)
=
2221.56 lb-ft/s² > 2200 lbf

lb ≠ lbf

 

Fascinating equation; one I'm not familiar with.
If the force is 2200 lb and its covering 110 ft/sec that's 2200x110/550 or 440 hp using the old W=FxD and 1 hp = 550lbft/sec. Where did that equation come from?

 

It's the equation for air drag.
I'm hoping I didn't go wrong somewhere, but I'm afraid I did

 

I wanted to know where the equation comes from, not what it is. A littlr research if you will. I suspect the p is not density but a pressure of some kind. The dimensions don't work as written. Whenever you use calculations from equations make sure the dimensions work too.
In this case they turn out to be lbft/sec2 but I don't know the dimensions of the constant 0.5 either.

 

The Drag Equation

Looking a bit more into it, it may be a mistake on my part in not converting units. Stupid poundals.

EDIT: Looks like I need to convert from lb(mass) to lb(force) =\ so the force is actually about 69.2lbf and a quick calculation tells me that the Insight needs 13hp at 75mph to overcome drag.

Classtime soon!

 

This thread is a mess. First of all 3rd gear is not drag limited so why are you calculating horsepower and drag at 90mph? A fit can redline 3rd gear so you can use the formula for gear ratios, redline and tire diameter and ignore all the other factors. If you wanted to figure out top speed in 5th then you would need to find hp to overcome drag and use that since the fit can not redline 5th and do 180+mph

 

Good points. My Fit cant redline in fourth either. And only barely in third. My Fit gains at most 2-3 mph shifting 3rd to 4th at VIR.
First the equation represented has density halved while the write up does not.
Second, dimensional analysis says lb/ft3 xft2/sec2 x ft2 leaves lbft/sec2. Thats hardly a drag dimension.
Third, where is air viscosity and compressibility considered, as well as 'inclination' and ...
It is a mess.
And max speed isn't limited by drag alone; mechanical and hydraulic losses must be included.

 

max speed in the fit is more limited by drag than anything else, due to the complete lack of power.

As for the gearing, IMO it's perfect. I shift the a/t at 3krpm or so and acceleration is fine. If anything, increase torque in the midrange...........but I sort of like having the "top end" that the Fit has, compared to other cars in its class that take massive dumps in the powerband after 4000rpm or so.

 

I too usually shift from 2nd to 3rd at 3000 rpm but my engine struggles to maintain speed much less accelerate. If I wait til 3800 as the auto does I have no such problems. Since third isn't used that much I don't understand why the gear jumps from 1.679 to 1.067. That 37% 'drop' is far too great. About 1.2 (28%) would be more appropriate. And lead to better mpg. The power requirement adds gas feed.
And yes increasing dispacement to get more torque would be a blessing.
But I have no idea what massive dumps you speak of; the Fit Hp /T curves I see look just like others in shape. Only the magnitudes change with displacement.

 

.........

Mahout: I think that your criticism of the ratio choice for 3rd gear on the automatic, while valid, needs to be tempered by the fact that there are only five ratios to apply. If the automatic were a six-speed, then you could shorten third gear and perhaps fourth. I suppose the choice to avoid the 6 speed automatic is due to cost considerations, and perhaps as well,considerations of weight and space. Keep in mind that competing vehicles, such as Yaris, make do with 4 speed automatics. I have road tested the Yaris and Scion XD 4 speed automatics, and the ratio choices are very undesirable, with a huge gap between 2nd and 3rd.

 

I appreciate your point but the standard gear ratios for the Fit automatic are like 30/60/90/130 and 180. When your driving is done on a course (street or track) there is a need for short-shifting in the most often used range. For Fits, its the 40 to 70 mph range. The rpm range is 2800 to 5000; thats makes the Fit 'lug' at the lower end, 2800 rpm, due to shortage of torque. I believe the Fit would benefit with a 1.201 third gear which gives a top speed of 80 mph at 6500 rpm. And between 40 and 70 the rpm range is 3200 to 5600 rpm, a far more desireable working range considering the torque curve. And passing much improved as well as the relaxed driving leading to better mpg. And yes that means fourth is a bit too high as well hut not as noticeable nor does it really matter.. Fifth at 180 mph max ( falling off a cliff I guess) appears to be barely right for interstate driving - 2700 rpm - on level ground. The 'gradient' program comes into action a bit more than I'd like on minor hills.
PS I've test driven Yaris and the Versa. The Versa automatic, being a CVT, is much better. And considering how little the Fit is driven over 80 mph there is no need for a 6 speed. The offshore CVT would be very nice though. Honda's 'gradient downshift' program just isn't as good as the Sentra SER CVT transmission (which is why it equals the Fit mpg.
Not only does the Nissan transmission upshift automatically at redline it stays in the chosen until you literally get down to stall speed, which allows you to select downshift whenever you want. Honda should have that program in their CVT or otherwise.

 

how about you man up and get a manual transmission? better fuel economy, faster, more fun to drive, the list goes on...