Gas mileage with taller gears

[Steve in VC]

There are a few of you that changed differential gear ratio's. Did you see any increase in gas mileage?

[Tom @ Fast Forward]

I changed from 3.6 to 3.9 on my 6 speed. Cruise RPM went from 4100 at 80 MPH to 3600. It might be a bit better gas mileage? Probably less than 1 MPG. Good idea for supercharged engines, not for turbos.

[Kyp J]

I changed from 3.6 to 3.9 on my 6 speed. Cruise RPM went from 4100 at 80 MPH to 3600. It might be a bit better gas mileage? Probably less than 1 MPG. Good idea for supercharged engines, not for turbos.

I thought you went the other way, to a higher ratio.

I also thought what I ended up with when I went from a stock open to a Torsen was 3.9 and what you did was more difficult to find parts for. Someone told me my original was a lower ratio but I forget what it was. I don't keep track of details like that.

I was trying to determine what my mileage was but since I only had reached 10K miles on the rebuild, it seems to be getting better with use. It was only about 22 when I tried to baby it and now is better than 25, just from getting broken in I think. In other words, too many variables, including not being able to go a whole tank without going nuts on the accelerator too many times.

[Tom @ Fast Forward]

My bad. I changed from 3.909 to 3.636. Another senior moment. Two more and I win a prize so I keep trying. ;)

Thanks for catching that .

[Scuba Steve]

I changed from 3.6 to 3.9 on my 6 speed. Cruise RPM went from 4100 at 80 MPH to 3600. It might be a bit better gas mileage? Probably less than 1 MPG. Good idea for supercharged engines, not for turbos.

Tom, Just curious why you feel the 3.63 is good for SC but not for turbo. I have a set of 3.63 gears coming for my turbo and I think they will be fine. I am fully spooled by 3400RPM so the only downside I see is possibly downshifting from 6 to 5 if I go in boost at cruise. That seldom happens.

[Tom @ Fast Forward]

Steve,

I was going from the fact that, with the supercharger, if you are running down the road at 3600 and cruise and decide to accelerate at part throttle, the SC builds instant torque but I wouldn't think there is enough air flow to spin the turbo to make similar torque. If you go foot to the floor, I would guess the turbo would be similar but at small part throttle conditions, I would think the Sc would make the torque and the turbo would not? That was my thinking but I could be wrong. The air flow at part throttle at 4000 RPM is a lot less than the air flow at WOT and 4000. Maybe with the turbo on cruise, the throttle will just open that much further to allow more air flow to make the needed torque? With my SC, it will make 10PSI with very little throttle change.

[Steve in VC]

Tom and Steve,

Don't worry as long as the gears aren't so tall as to effect "daily driving".

I am still NA (still collecting the $ for a coldside), and I went from 4.3:1 to a torsen 3.909:1, and couldn't be happier. I noted a little less off the line initially, but don't notice it now. I would have no problem going to a 3.63:1 even without boost because I do a lot of freeway driving and keep checking to see if I am in 5th (less now with the 3.9 rear end).



Please indulge me to wax technical (my wife is still trying to get a support group for spouses of engineers).

To provide 14lbs of boost in a Miata requires 18HP to 24 HP.

Close the bypass on a supercharger, the plenum pressurizes, and you create power. There is little delay because the engine / flywheel provide that HP immediately.

There still is a delay from plenum size, causing the difference in throttle response between "hotside" and "coldside".


A turbo relies on exhaust gas to spool, so there is a feedback mechanism with delay.

You are driving down the road at 60MPH (which requires less than 10HP in a Miata), the cylinders are being filled well below atmospheric pressure. This may spin a ball bearing turbo, but not with any significant power.

Hit the throttle, the cylinder fills at atmospheric and the engine creates as much HP as the NA dyno curve says it would (assume 70HP), and creates more exhaust gas, and starts to spool the turbo.

The turbo is a flywheel, so part of the energy goes to spooling it up, part of it goes to boost. As boost increases, power increases, exhaust volume increases, boost increases. All combined, this is called turbo lag. Smaller turbo's spool faster, bigger turbo's make more power.

Also, turbo's by definition are "hotside", so will take some time to pressurize the plenum.

Near the desired boost pressure, the wastegate starts to shunt exhaust gas around the turbo. It has to, otherwise the turbo will "run away", generating ever more power until the engine detonates. (I consider variable vanes sophisticated wastegates).


All of this takes more time than a supercharger.

Even the variable vane turbo goes through this sequence. So, even though it
is the most sophisticated turbo, it still doesn't duplicate a supercharger.

[Tom @ Fast Forward]

Steve,

When you say that you are fully spooled at 3400 RPM, is that at part throttle or foot to the floor?


Tom, Just curious why you feel the 3.63 is good for SC but not for turbo. I have a set of 3.63 gears coming for my turbo and I think they will be fine. I am fully spooled by 3400RPM so the only downside I see is possibly downshifting from 6 to 5 if I go in boost at cruise. That seldom happens.

[Scuba Steve]

Steve,

When you say that you are fully spooled at 3400 RPM, is that at part throttle or foot to the floor?

Foot to the floor. If I want boost, why would I do any less :biggrin: I do agree I will likely need to downshift when hitting boost in 6th gear. I just figure by the time I am in 6th, I really won't need boost.

[Tom @ Fast Forward]

that was my point. With the supercharger, you would just leave it in 6th climbing that hill because the torque occurs as soon as the bypass closes. Nothing against turbos, mind you. ;)

[jwalton]

Please indulge me to wax technical (my wife is still trying to get a support group for spouses of engineers)

I told my wife about your wife's support group and she feels the need to join...

I'm sure that Tom's wife would probably be a charter member!