Bought a Mazda3 S Sport sedan with manual transmission Saturday. The car is really fun to drive. Love the engine's response and the transmission is sweet.
My question/comment is about the net gear/drive ratios chosen by the designers for 5th and 6th gear. The 2.5L engine has a very nice a broad torque curve, with plenty of torque at the low end, and with low RPM pull-aways in the upper gears being no problem at all. With that in mind, and given the 4.11:1 final drive ratio, I cannot help but think they could have lowered the gear ratios a hair on 5th gear and even more on 6th gear and not compromised the Mazda "zoom" in any way.

Here are some numbers comparing the net gear ratios (net of gear ratio and final drive) for the automatic's 4th and 5th with the manual's 5th and 6th:

Automatic 3.65:1 final drive
Gear's ratio Net ratio
4th 0.93 3.39
5th 0.69 2.52

6 spd Manual 4.1:1 final drive
Gear's ratio Net ratio
5th 0.84 3.45
6th 0.72 2.95

This results in the 2.5L engine being at 2000 RPMs at 50 mph in 6th gear on the manual, versus only 1700 RPMs or so (by my calculations) with 5th on the auto. 75 mph equates to 3000 RPMs and 2550 RPMs, respectively, in the manual's and automatic's top gears.

I did a plot of the whole ranges of all the gears for each transmission, and the manual is of course "sportier" across the whole range. That said, I think it is needlessly so in 5th and 6th, and a 5th gear ratio of .78 to .80 and a 6th gear ratio of .60 to.63 would have been more natural targets without compromising in any way the sportiness the manual provides to the vehicle. Having an extra gear over the automatic, in my opinion, should allow the manual to end up in the top gear with no higher RPMs than the automatic affords.

Comments/thoughts?

 

kinda understand where you coming from but just to clarify are you complaining that the 5th and 6th gears are too close together or too far apart, or is this just opinion based question about the ratios. I think they applied the same ratios the spd3 has (didnt look it up) if thats the case then you have to realize that standard 6spd trans are not suited for top end but more geared for tourqe since they are more better for lets say circuit rather than drag, make since? no? cant really give an accurate response unless i know what exactly what your asking.

 

What I am saying is that 5th and 6th are too short and too tightly grouped for top end gears. Not enough RPM drop going 4th to 5th or 5th to 6th resulting in too many RPMs in 6th at highway speed.

I crunched the numbers on them and I discovered what my gut was telling me, that upshifting to 5th and 6th is not giving me a continuation of the trend I get wih the 1st 4 gears, or that you see in most other manual set ups or automatics, that is each higher gear should give you more change in speed (delta V) per change in RPM than does the prior gear. A concave upward curve that has the effect of the delta V per delta RPM compounding with each higher gear, if you will. Instead, we see that 5th gives less delta V than does 4th, and 6th even less than 5th. A diminishing returns set up that I have not encountered before.

 

i have a little theory on that, my guess is instead of spending money on configuring a new 6spd trans to match the 2.5 non turbo, they just placed the same spd3 trans in it, what alot of people tend to forget with newer transmissions is that very last gear is considered an overdrive gear or passing gear, knowing that you should also know that your really supposed to be cruising in 5th gear and instead of dropping down a gear upshift to 6th which the ratio is closer but gives the top end tq to pass. you should also realize that econobox 6spds are not ment for top end vs a 5spd, the basic principle behind that is the more gears the better gas milage becasue you dont need to rev out so long so the engine wont bog, 6spd trannies are really close together vs 5spd. If i confused you or did not answer your question im sorry...im not that good at explaining things over the net haha i do better in person.

 

The biggest reason for the Gear Ratio's being so close is the 2.5 is still a 4 cylinder, if they where farther apart it would hurt the drivability in 6th gear. Basically you are still moving a bunch of inert mass down the road and getting to low in the RPM range would make the engine work harder causing a loss of MPG. The Automatic can make up for it with the fact that even while locked in the torque convert will still slip a bit to save the MPG.

 

So, what's wrong with being in 5th gear (instead of 6th) at 50 MPH?

As to needing to have close spacing so as to not have to rev out the engine, a 400 or 500 RPM drop from 5th to 6th is hardly a cause of having to rev out the engine in 5th......nor would a 800 RPM drop be.


I would like to see the SFC curves for the engine at different RPM. Perhaps it is more fuel efficient around 2500-3500 and it is at lower RPMs?

 

I know with my 08 the difference although better than the 4-500 is still a lot closer than the first 4 gears, in fact when getting on the freeway in 3rd to match traffic I sometimes will skip 4th and just go to 5th.
Sometimes the 4-500 is all the RPM it needs to get back into the optimum torque range for the best economy.
If you are cruising at 70 MPH at 3000 RPM it sounds like 5th is a passing gear or for better use in the mountains on steep grades, since I am running about the same RPM at 70.