When I drive on the FL Turnpike I usually do around 80mph every day. At that speed and in 6th gear I have noticed that I am doing about 3.3k rpms. Last night, for a change I was driving my wife's Bimmer at the same speed and I noticed that her car was in 5th gear (auto) at about 2.6k rpms. WHAT IS UP WITH THAT??? How come my car runs the rpms so much higher at the same speed? Is there a way that I could change the 6th gear ratio so I can save some gas? That would be good, specially now with premium gas being over $4 in South Florida.

 

Simple. It's all about the gearing ratios:
(taken from cars.com)
Infiniti G37S 6MT:
(1st) 3.79
(2nd) 2.32
(3rd) 1.62
(4th) 1.27
(5th) 1.00
(6th) .79
(reverse) 3.45
Axle ratio 3.69

BMW 328i/335i: (I assume you have a E90/E92 2006+ 3 series)
(1st) 4.07
(2nd) 2.37
(3rd) 1.55
(4th) 1.16
(5th) .85
(6th) .67
(reverse) 3.20
Axle ratio 3.23

A lower OD ratio and a lower axle ratio equates to lower rpm at a certain speed. The question is of course why would Infiniti have the VQ37VHR revving at such high rpms.....

Even the new 7A coming out in a couple of years may not help too much-
(taken from http://www.wheels.ca/newVehicles/profile/218989):
First Gear Ratio (:1) 4.89
Second Gear Ratio (:1) 3.17
Third Gear Ratio (:1) 2.03
Fourth Gear Rato (:1) 1.41
Fifth Gear Ratio (:1) 1.00
Sixth Gear Ratio (:1) 0.86
Reverse Ratio (:1) 4.04
Seventh Gear Ratio (:1) 0.76
Final Drive Axle Ratio (:1) 3.54
(even though it's in the 2009 FX, most likely Infiniti will use this across the entire line)

I may be wrong about this and I hope that the new 7A will improve gas mileage drastically, but right now I'm not gonna get my hopes up. Of course; time will tell.....

 

Expect different ratios in different cars with the 7A. They won't gear the G like an SUV.

 

Do you possibly have the comparison between the 1st Gen G35 5AT and the G37?

 

Yup....
(again using cars.com as a reference):
2003 G35 sedan/coupe 5AT:
(1st) 3.54
(2nd) 2.26
(3rd) 1.47
(4th) 1.00
(5th) .83
(reverse) 2.37
Axle ratio 3.36

2008 G37 Coupe 5AT:
(1st) 3.84
(2nd) 2.35
(3rd) 1.53
(4th) 1.00
(5th) .84
(reverse) 2.76
Axle ratio 3.69

 

Wonder if we can find Gear Ratio for the G37 Euro ver? Does Sedan/Coupe share the same ratio?

 

Each change equates to a new and different powertrain for Federal standards. Each separate powertrain requires its own set of compliance tests. These tests are fairly expensive. So, most manufacturers no longer change drivetrain components to suit their whims.

 

Lower rpms doesn't save you a drastic amount of gas... turning the crank really fast on a sip of fuel isn't much worse than turning it slow on a sip of fuel... what hurts your fuel economy is very quickly increasing how fast the crank is turning by injecting as much air and fuel as you can.

Unless we're talking about the difference between 2500rpm and 6800rpm (when the ECU starts playing with the air/fuel ratio to protect the engine), it doesn't really matter what the tach says when it comes to fuel economy while cruising.

What matters is how much air is going in (ie, the throttle position) because the ECU will (usually) keep the fuel/air ratio the same and thus if your throttle is not open very far you are not using any more gas.

In simpler terms -- cruising is cheap, accelerating is expensive. Accelerate really slowly like grandma and you will get better mileage regardless of if you shift at 2000 rpm or 4000 rpm.

Now a question for you -- what is the redline on your wife's Bimmer?

 

6.8k

 

Interesting. The equivalent rpm in the engine's rev range for the Bimmer is 2992. The G is a bit more aggressively geared (obviously your particular model also has 6 gears). This should make the G more peppy while cruising at 80mph than the bimmer if both cars are held in their top gear.

But as I said before, this should have minimal effect on your mpg.. one quick start will have much more impact than 300 rpms cruising.

 

The 2003 G is the same transmission as my 2003 FX.
Same ratios-