I was wondering if changing the gearing might result in a drop or shift from the set "power band" of the engine and result in poor performance overall? Would it need to be more severe of a change?

 

Think of it like a mountain bike. You change gears and it makes it easier to peddle and you can accelerate quicker, same thing. The trad-off is that you cant go as fast(or you have to peddle really fast)
It takes less work from the engine to accelerate with the higher(numerically) gearing than it does with the lower(numerically) gearing.
Quicker acceleration from a stop.

 

all i saw was 400 mpg!!!!!!!!!!!!!!!! idk much about anything else you guys are discussing but im getting like 300 max and i have alloy 18's bout 21lbs each on 40 series tires....
and thats about 80% freeway driving!!!!

 

400 MPG thats like 8000 miles per tank. I only get like 360 miles per tank.

 

fudge he got me i meant 400 miles per tank! LOL

 

yes speedometer/odometer is off but am fairly confident in my math as to the speed and distance I travel each day, still 65 to 70 mph and 80 miles each way, and it is very hilly in this area. Before the switch it felt like the engine was lugging in drive at 2300 -2400 rpm and so I switched to driving a lot in 4th, then switched the rear diff. Now rpm are up in drive but definately not bogged down, more responsive, especially in the city. I did not have access to any equipment to do a driving exhaust CO test but believe the engine was less efficient at the lower rpm, hence the mpg gain. Also should state that I quickly lose mpg if I push speeds up to 85 to 90 mph as rpm are quite a bit higher then before. Anyway - was just wondering if any one else that did a rear diff change noticed the same.

 

if your mpg went up from changing to a shorter final drive, all that means is that you were not down shifting when you needed to. If what you are saying is true being in a lower gear all the time would have resulted in the same MPG gain. Although I guess you probably have a 5 speed auto so now your 5th gear ratio has moved to some place between the old 4th and 5th ratios.

 

^To further that same point...

If you were driving in 4th a lot, and now you leave it in 5th, that would be the same effect as changing your diff in the other direction.



As for the comment about the Trans-Am... I feel the same way. Why won't the G get better MPG??! It's very irritating to me that my old TA was: Heaver, had wider tires, more aggressive gearing, waaaaaay more horsepower and torque, and a significantly larger displacement PUSHROD American V8, yet got only the same, if not better mileage

 

i think it has something to do with the fact that these cars were originally intended for japan where they don't do nearly as much highway driving. Domestic cars tend to have longer overdrive gears. I think they just don't really bother with these cars. that or they want you to have more power in the top gear for effortless passing because its more luxurious. Lexus's used to get awful mpg with the 2jz-ge in the IS300, SC300 and GS300, now the new 2.5 and 3.5 seem to do much better according to the ratings. I think if we had variable valve timing that would help too.

 

Yes, you gain much more low end acceleration, but lose your top end acceleration.

I'd messed around with replacing final drives to see it happen quite frequently. And yes, gaining fuel economy is typical as well, unless you rev TOO high on the freeway and then you waste gas by the fact you are spinning 4000 rpm or so. But with shorter gearing, engine load is decreased.

Its not all black and white though. You can't just toss shorter gears in and expect better fuel economy. Driving style plays a role as with anything.

 

Chica, I'm sure you meant 2500-3000 (not 25,000) but even so since your RPM at 70mph is not much more than 2500 to begin with, a 3000 RPM drop would mean going backwards!

 

Did they change this between years? My 2004 service manual thinks it is picked up from the transmission output shaft.

 

I had a 94 Z28 A4 with the 2.73 highway gears. That car would get 25mpg+ on the highway, however, around town, it was worse than my G. If it was cold out, that LT1 would drink gas until it was warm. In the dead of winter, I'd see 12-13mpg.

The reason those big V8s get great highway mpg is because they're only turning ~1400rpms at 70mph. My G35 with the 3.54 gearset is turning 2800rpms at 70mph, but is still achieving the same mpgs as my Z28. If you geared our cars like GM does their V8s, you'd likely see a significant increase in highway MPG. However, top gear acceleration would be non-existent and any sort of grade would be met with a mpg killing downshift.

Nissan geared these cars aggressively for performance with not a whole lot of regard for mpg. The aggressive gearing is a large reason why these cars perform so well, especially with the automatics. There aren't many sub-V8 cars out there where the auto verison is basically as quick as the manual verison. Look at the performance difference between the Acura TL 5AT and the 6MT. It's nearly 1 second and over 6mph only because Acura/Honda gears their autos very tall for MPG. I'd much rather than the performance. In a perfect world, our 1st gen G autos would have a taller 6th gear for stellar highway MPG. With that said, my 03 sedan 5AT with a full load and A/C going has seen as high as 29.8mph at 65mpg on a perfectly flat, no grade highway. No bad.

 

^Do you mean 29.8mph at 65mph?

 

It is picked up off the output shaft, but only for cruise control purposes. The speedometer gets it's true reading from the wheel speed sensors. For cruise control activiation, the ECU looks at both values. If the values become wildly different, the CC gets turned off. This is why some 03/04s can't use CC after a gear swap (usually 3.6+ gears). My 03 hasn't had an issue with the 3.54 swap and the use of CC at speeds below 80mph. I've never tried to engage CC at speeds greater than 80mph. As for speedometer accuracy with the 3.54 gears, it's basically spot on as confirmed with two different GPS systems.