3.9 Gears or 4.083 gears - rear Differentials

[Jacko3]

How much performance and gas mileage sacrifice would occur with the installation of the 3.9 gears or 4.083 gears? How much would they improve acceleration? And how much MPG would be sacrificed by these larger gears.

My mechanic has a 4.1 rear differential on his Mustang and he said he took off from a dead stop faster than a twin turboed Mustang. But eventually, the twin turboed Mustang did come by and pass him.

Any thoughts on this would be helpful.

[gwoods]

cheap way to find out it is to buy a set of shorter tires....

A 5% change in drive ratio via tire or gear is a 5% change in gas mileage, acceleration and top speed. Which is why I have 10 tires and wheel for my G

[all G'd up]

What is the stock gear ratios in the G...

[Jacko3]

I saw the 3.9 gear on everythingg35. They said it would produce 10% quicker acceleration. Does this mean the stock gear in the G-35 is a 3.5 0 3.6 gear? What size would you consider a shorter tire? I currrently have a 245X35R19 in front and a 275X35R19 in the rear. So what size would be shorter?

[SVTCOBRA]

What is the stock gear ratios in the G...

I believe that the 03 - 07 auto's have 3.36 and 6mt have 3.54....

[C-Kwik]

Short gears are generally not a great option for a street car. Not only doe gas mileage suffer, but you also end up buzzing along at higher speeds as engine RPM will be higher. And once you get beyond 1st gear, any acceleration advantages will be negligible as it will ultimately average back out.

[Jacko3]

C-Kwik:

Could you elaborate more on why it is not a good idea for a street car, if I can get good acceleration all the time? And by the acceleration averaging it out, do you mean that what I gain in the low gears, will be somewhat lost in the higher gears, since the higher gears are already small to begin with? Am I going to loose some of the torque I now have in higher gears like the 5th gear? I need to pick your brain on this one, as i have no idea about this type of stuff.

[rn79870]

3.69 for the 07 and 08 sedans.

[gwoods]

C-Kwik:

Could you elaborate more on why it is not a good idea for a street car, if I can get good acceleration all the time? And by the acceleration averaging it out, do you mean that what I gain in the low gears, will be somewhat lost in the higher gears, since the higher gears are already small to begin with? Am I going to loose some of the torque I now have in higher gears like the 5th gear? I need to pick your brain on this one, as i have no idea about this type of stuff.

I disagree with C kwik.

The hemi rams came with 3.55 gears and open diffs. I did a limited slip and 3.92 gears in mine when I started getting serious about racing. Many people did 4.10 gears and I rode in and drove several trucks with 4.56 gears.

Taller the gears the faster the acceleration in EVERY gear. Taller the gears the lower the max MPH in every gear.

For example with 3.55 gears you can go 45 mph in 1st gear, change for 3.92 and your max speed will be around 40 mph.

I agree with some of the gas mileage part of C-kwiks advise. If you are turning 2200 rpm at 60 mph with 3.55 gears you will turn about 2500 rpm with 3.92 gears. Higher RPM's means more gas consumption on the freeway. However on the street stop light to stop light taller gears can use less gas. Taller gears make your motors energy more efficient accelerating. It takes less gas to acclerate to 60 mph with 3.92 gears then with 3.55.

maybe oldnslow will chime in with his opinion? I'm sure he has made a few gear swaps in his day.

[Jacko3]

Gwoods:

So, why do the auto manufacturers not have the taller gears to begin with, since it improves acceleration? Apparently, according to RN79870, the 07 and 08 Sedans seem to have slightly taller gears, which probably accounts for their impressive acceleration, in addition to their improved weight, and somewhat not-so-good gas mileage, and impressive RPM range (7,000RPM +).

So, in essence, the taller the gear, the more torque you may be generating at lower speeds, and the lower the horsepower at the top end. Seems like a game of either increasing torque and sacrificing a bit of horsepower or sacrificing torque and generating more horsepower. This is truely a difficult task ands choice for any outo manufacturer to have to deal with for RWD.

I can see why AWD is preferred as it probably is the best of both worlds. Of course, gas mileage will suffer under AWD.

Would you also agree that 6MTs in the north are better off with taller gears than 6 MTs in the south, since there is more snow in the north than in the south?

What do you think?

[joe603]

Simple...the MPG would decrease on the highway.

[Sentientbydesign]

The taller gears won't produce more torque, you're just getting more torque multiplication.

The higher redline in the 07-08 has NOTHING to do with the final gear. It has to do with the serious overhaul that Nissan did to the VQ35HR.

Why would you want more useable torque in the snow?

Jeff,

I'm a little unclear as to how taller gears will improve fuel economy in the city. Are you saying that running an engine at a higher RPM with a smaller load is more efficient?

[rn79870]

All real quarter mile racers gear their cars so that they are a few hundred rpm short of redline in 4th gear. Not 3rd not 5th. Once they hit that magic gearing they screamed down the quarter producing the best times. Of course, they weren't concerned with rpm at freeway cruise speeds or MPG figures.

The bottom line is, if you want max performance in the quarter mile, configure your car so that you cross the end trap just shy of redline in 4th gear. This allows your engine to spend more time in the power band, or as they say, higher in the envelope.

[C-Kwik]

The hemi rams came with 3.55 gears and open diffs. I did a limited slip and 3.92 gears in mine when I started getting serious about racing. Many people did 4.10 gears and I rode in and drove several trucks with 4.56 gears.

Taller the gears the faster the acceleration in EVERY gear. Taller the gears the lower the max MPH in every gear.

Yes, it will accelerate faster for each given gear, but in the long run, it still averages out. Let me provide an example:

Lets say we have 2 similar cars with the same motor, but different rear ends. The transmission gear ratios for both cars are:

1st - 16:12nd - 8:13rd - 4:14th - 2:15th - 1:1

Car A has a rear end gear with a 1:1 ratio.

Car B has a 2:1 ratio rear end.

Car A will have the same overall gearing as the transmission's gearing:

1st - 16:12nd - 8:13rd - 4:14th - 2:15th - 1:1

Car B will double the torque multiplication such that the overall ratio will end up being:

1st - 32:12nd - 16:13rd - 8:14th - 4:15th 2:1

Notice a pattern? The overall ratio of Car A is the exact same as Car B from second gear on. That means for a given speed, both cars will accelerate the same once Car B is out of first gear. The difference is Car B will have gained a small lead while in 1st gear, and end up at the finish line sooner. This allows Car B to grow the lead by the finish line as well as it will be at a higher speed at all times. But the instantaneous acceleration rate will remain the same from 2nd gear on.

The gear ratios I used in my example are of course, unrealistic, but even with realistic ratios, the average after 1st gear, possibly partly into second is going to be about the same. It is an effective method for increasing 1/4 mile acceleration, but I wouldn't want to take the hit in gas mileage in a daily driver. Personally, I don't do much drag racing (I prefer the twisties) so a rear end gear swap is likely not going to do much for me.

I agree with some of the gas mileage part of C-kwiks advise. If you are turning 2200 rpm at 60 mph with 3.55 gears you will turn about 2500 rpm with 3.92 gears. Higher RPM's means more gas consumption on the freeway. However on the street stop light to stop light taller gears can use less gas. Taller gears make your motors energy more efficient accelerating. It takes less gas to acclerate to 60 mph with 3.92 gears then with 3.55.

Doubtful...for the same reasons I stated above. I'd agree if the gear ratios themselves were spaced more closely together, but that isn't necessarily the case here.

So, why do the auto manufacturers not have the taller gears to begin with, since it improves acceleration? Apparently, according to RN79870, the 07 and 08 Sedans seem to have slightly taller gears, which probably accounts for their impressive acceleration, in addition to their improved weight, and somewhat not-so-good gas mileage, and impressive RPM range (7,000RPM +).

Manufacturers have to balance many things. Different engines have very different characteristics. Engines with peaky powerbands tend to benefit from close ratio trannies coupled to a numerically low final gear. This allows them to obtain a higher Average HP without necessitating a ton of transmisson speeds and low speed changes between each shiftpoint. Another consideration could be strength of gears and such as the size of gears are affected by their ratios.

So, in essence, the taller the gear, the more torque you may be generating at lower speeds, and the lower the horsepower at the top end. Seems like a game of either increasing torque and sacrificing a bit of horsepower or sacrificing torque and generating more horsepower. This is truely a difficult task ands choice for any outo manufacturer to have to deal with for RWD.

Horsepower doesnt change ever. An engine that makes X number of HP at XXXX RPM will always make that much HP at that RPM. In fact, if you calculated HP based on wheelspeed and torque at the wheels, it would be the same as the HP output at the motor at its corresponding engine RPM (ignoring drivetrain losses). FWD cars and AWD cars don't have anything different to contend with in this regard. They both still have final drive gears.

I can see why AWD is preferred as it probably is the best of both worlds. Of course, gas mileage will suffer under AWD.

Would you also agree that 6MTs in the north are better off with taller gears than 6 MTs in the south, since there is more snow in the north than in the south?

As I stated, AWD has nothing to do with this. Generally, manufacturers use the same differential gearing in the front and rear. If they didn't, the center diff would be under a lot of stress all the time as it would be "slipping" all the time from differing front and rear wheelspeeds.

In inclement weather, like snow, a numerically lower gear ratio would be ideal as there would be less torque at the wheels. I a number of instances, I've started a car off in 2nd gear when traction was too low to keep the wheels from spinning.

Just so everyone is clear, a taller gear is a numerically lower gear (4.09 is shorter than a 3.55 gear). This seems backwards when you think about gear ratio numbers, but the tallness and shortness of a gear refers tot eh resulting vehicle speed. That is, a taller gear will cause a vehicle to run at a higher speed for a given gear and engine RPM vs a shorter gear.

All real quarter mile racers gear their cars so that they are a few hundred rpm short of redline in 4th gear. Not 3rd not 5th. Once they hit that magic gearing they screamed down the quarter producing the best times. Of course, they weren't concerned with rpm at freeway cruise speeds or MPG figures.

The bottom line is, if you want max performance in the quarter mile, configure your car so that you cross the end trap just shy of redline in 4th gear. This allows your engine to spend more time in the power band, or as they say, higher in the envelope.

There are all kinds of things that are done with gear ratios to try and maximize acceleration in any kind of race car. Higher HP vehicle generally need less gears as the acceleration is so fast that a typical 1st gear would be utterly useless. Note how many high HP exotics can hit 60 mph in 1st gear. Some high HP drag racers use 2 speed transmissions. In fact, one of the guys that helped bring a lot of attention to import drag racing was Adam Surawatari in his RX-7. The last set-up I remember reading about was that he ran a 2 speed powerglide. It is likely that he did gear the rear end to hit somewhere between the peak HP point and redline as he crossed the 1/4 mile mark though. Top speed race cars generally have widely spaced lower gears with closely spaced upper gears that optimize torque to the wheels as they near their projected top speed. High speed oval track gearing tends to has some similarities to this as well.

[gwoods]

Gwoods:

So, why do the auto manufacturers not have the taller gears to begin with, since it improves acceleration? Apparently, according to RN79870, the 07 and 08 Sedans seem to have slightly taller gears, which probably accounts for their impressive acceleration, in addition to their improved weight, and somewhat not-so-good gas mileage, and impressive RPM range (7,000RPM +).

IMO the manufacture picks the best comprimise gear ratio. 3.69 is a great overall gear. 4.56 gears are useless without wide enough tires.

My G broke sideways on the 1-2 shift with the factory tires. I went to 235's and now it just chirps the tires on the 1-2 shift. I might do 245's soon.

Yes, it will accelerate faster for each given gear, but in the long run, it still averages out. Let me provide an example:

Lets say we have 2 similar cars with the same motor, but different rear ends. The transmission gear ratios for both cars are:

1st - 16:12nd - 8:13rd - 4:14th - 2:15th - 1:1

Car A has a rear end gear with a 1:1 ratio.

Car B has a 2:1 ratio rear end.

Car A will have the same overall gearing as the transmission's gearing:

1st - 16:12nd - 8:13rd - 4:14th - 2:15th - 1:1

Car B will double the torque multiplication such that the overall ratio will end up being:

1st - 32:12nd - 16:13rd - 8:14th - 4:15th 2:1

Notice a pattern? The overall ratio of Car A is the exact same as Car B from second gear on. That means for a given speed, both cars will accelerate the same once Car B is out of first gear. The difference is Car B will have gained a small lead while in 1st gear, and end up at the finish line sooner. This allows Car B to grow the lead by the finish line as well as it will be at a higher speed at all times. But the instantaneous acceleration rate will remain the same from 2nd gear on.

The gear ratios I used in my example are of course, unrealistic, but even with realistic ratios, the average after 1st gear, possibly partly into second is going to be about the same. It is an effective method for increasing 1/4 mile acceleration, but I wouldn't want to take the hit in gas mileage in a daily driver.

I agree that you trade acceleration for max speed.

Here is an in car video with the 23" tall tires, notice I'm in 4th gear for a while in the back 1/8th



Here is a video with the 25" tall tires, I shift into 4th right before I trip the beams

[SVTCOBRA]

I have always heard that gears are the cheapest way to 'go fast'. Of course, I'm NOT talking about top end speed!

My poor Cobra is saddled with 3.27 gears.I've been told that going to even 4.10 gears in my Cobra will not effect the MPG but by 1 on the HW at around 65.Gears was on my short list for the Cobra....before she got For Sale signs posted in the windows.

[C-Kwik]

I agree that you trade acceleration for max speed.

Well, maximum speed is not really much of an issue in a drag race. Few cars will be coming close to their terminal velocity in that short of a distance. And the max speed for a given gear is not an issue unless you lower the gearing so much that you redline 6th (Unlikely). The point I'm making is simply that the acceleration after first gear is not going to be any faster on average. Your videos only show that indeed, you are finishing the race higher up in 4th gear. I'd be much more interested in the data off the timeslips for those runs.

[Tampa G35 Sedan 6MT]

The Gears do make a big difference at the Hwy speed!

Take for instance my Jeep I went from stock 29" tires to 33" tires..

The Jeep has 3.55 gears

Before When I would be going 80 I could nail the gas and never have the transmission shift into 3rd but now with the taller tires I can make it shift into 3rd. That don't mean that it is any faster!

If the gears are taller (In some cars.. .Not our G's) you can get better mpg... I saw an article that someone posted on Nico about making mustangs get better MPG by tuning! It stated that it went from a 3.27 to a 3.65 or something like that and got 1 mpg more on the highway and they also gained in the City because the motor had to work less to get the car from a stand still!

Our G's are very aerodynamic and I feel that if we could get the motor to rev at about 2500-2700 rpm's at 80 we could get over 30 mpg...

But to have it rev 2000 rpms would only use more gas as the motor would be under more stress at that RPM because of wind resistance!

DJ

[TeflonG35]

These are the 3.917 rear differential gears from Central 20. They are designed for a 6MT rear differential. These will not bolt up to a 5AT differential.

The new 3.9 final drive setup from Central 20 Japan is an economical way of improving acceleration in the 350Z and G35. Increases available power by 10% per gear, resulting in faster acceleration in every gear. These gears really begin to shine with the larger staggered tire setups prevelant in the Z/G world. Professional installation required. Only for use on 6 speed manual transmission models.We also offer an install kit which includes the basic parts needed for installation. This kit is all genuine Nissan parts, and includes pinion Nut, pinion crush sleeve, and pinion oil seal.Please note that in addition to the parts shown, there are also several sizes of side lash adjustment shims that Nissan offers. There is no way for us to determine what size(s) your car might need. The ONLY way to determine the proper size lash adjusters is during the installation by a competant installer. For those with higher mileage cars or who want additional peace of mind during the install, we also can offer the bearings as well as the bolts used internally. Again, these are all genuine Nissan parts. For the ultimate in performance, we also recommend the NISMO finned rear differential cover.

http://www.g35parts.com/9083/o...d=601

Why only the 6mt? Why no AUTO love? I want to accelerate fast too....

[SVTCOBRA]

Pretty sure that if I went from 3.27 to 3.73 (a popular choice amoung stang owners) in my cobra, my MPG would go down instead of up, but that's just what I have read over the years on Furd forums. I never got around to doing the mod.

And, if you don't believe me, then buy my cobra (it is for sale!) and prove me wrong!!!

And, sorry Jacko for bringing up mustangs in your thread, but I've been

[gwoods]

gained in the City because the motor had to work less to get the car from a stand still!DJ

I can only speak from experience.

When I changed from 3.55 to 3.92 gears I gained city MPG and lost HWY mpg. I gained acceleration and lost top speed.

I'd be much more interested in the data off the timeslips for those runs.

I didn't get a slip for the run this Saturday night, it was my last run I exited the strip and went straight home. It would be impossible to find my slip from the other run. I have a shoebox of time slips LOL

I am interested too so maybe if speedworld has a Sunday test n tune day I can go do back to back runs with both sets of tires.

[Kendahl]

If your differential ratio now is R, changing to 4.1 gears will change torque at the rear wheels by the ratio 4.1/R. If R<4.1, the change will increase torque and, therefore, acceleration. Acceleration is proportional to the difference between the linear force at the rear wheels (torque X tire radius) and drag. Increasing torque doesn't increase drag. Therefore, the improvement in acceleration should be greater than 4.1/R.

It is well known that the slower the engine speed, the better the gas mileage. (That's the reason for overdrive.) I suspect that the increase in fuel consumption would be less than the increase in torque. You can offset the effect of a lower differential ratio by shifting at a lower road speed, but only until you reach top gear.

The effect on top speed depends on how the car is geared. The ideal is to find a differential ratio such that the car reaches top speed at the engine speed where horsepower is highest. (This may be a lower rpm than red line.) If the differential ratio is numerically too low, the car will top out at less than maximum horse power. Shorter gears would actually increase top speed. However, you need to be careful not to get so short that the car runs out of rpm before reaching top speed.

[Tampa G35 Sedan 6MT]

well said

[TeflonG35]

I still want to know why the gear upgrade doesn't work for the auto.

[Tampa G35 Sedan 6MT]

I bet it has to do with LSD

[TeflonG35]

I bet it has to do with LSD

Now you're sounding like a hippy.

[Beancooker]

It will work with the auto, but you will *could* end up with transmission problems. The autotragic is set up to have a 3.35 (I think) final drive gear. When you change out the final drive gear, it changes everything forward, and *could* cause problems.

[TeflonG35]

Could that be fixed with a tune of some sort?

[gwoods]

It will work with the auto, but you will *could* end up with transmission problems. The autotragic is set up to have a 3.35 (I think) final drive gear. When you change out the final drive gear, it changes everything forward, and *could* cause problems.

Get this

With the 23" tall tires if VDC is on the car WILL NOT move. the tire pressure sensor light stays on, the ABS light stays on and the BRAKE light stays on. Turn VDC off and the car appears to run okay but all those lights stay on.

I think having the 23" tires all the time would ruin the transmission for sure.

[Beancooker]

Could that be fixed with a tune of some sort?

^ What Jeff said.