Gear Ratio

[firedime]

I have a 71 510 with a stock L16 and a z five speed and a stock diff. I have read articles on subi lsd's and they are hard to find so far. My concern is gear ratio. I would like to be able to cruise at 65 or 70 on highway at about 1800 to 23 or 2500 RPM. Any suggestions? Thanks

[datzenmike]

First, the biggest difficulty with going highway speeds is pushing through the air resistance, To go from 30MPH to 60MPH or double the speed requires not twice the HP but 4 times as much. So as you can see it takes more and more power to go just a little faster.

 

Second, cars are powered and geared to go at least the posted highway speed and enough to pass other cars quickly so lets for argument sake say 80MPH top speed. This is the point where the engine makes just enough power to push against the air and hold a steady speed. A cruise speed of 70 MPH requires less power and the engine RPMs could be dropped slightly with an overdrive gear or rear end ratio change but only if the engine RPM are not dropped so low that the motor isn't making enough power to push aside the air at 70 MPH.

 

Assuming stock 5.60x 13 tires (185/70R13) and 3.90 rear end ratio and a Z 5 speed which has an overdrive ratio of 0.864 then an engine RPM of 2500 would give a speed of 53 MPH. At 70MPH the engine will be at 3,300 RPMs.

 

Automatic Z (not zx) and Maximas have R-1800 differentials and swapping one of these would drop the 70 MPH RPMs to only 2,850. In fact you would need a differential of just under 3.0 to get down to your 2,500 RPM target at 70 MPH.

 

I have some serious doubts that the tiny L16 would have a prayer of pulling a 510 through the 70 MPH air while only revving 2,500 RPMs. A VG30 motor might do it. Also, lower speed acceleration would definitely be destroyed. 1st gear take offs would be a total pain when in-town driving.

 

You have an over drive 5 speed that should be enough. It drops the 70 MPH RPM from 3,800 to 3,300.

 

For argument sake you could try a later zx transmission with a monster 0.745 over drive 5th gear. This would give you a 2,850 RPM at 70. But again I doubt the L16 will be able to pull it.

[ggzilla]

The L16 will pull it, it doesn't take much HP to do a steady state. As you said there is the air drag, but there is no other substantial load it needs to pull (not even the weight of the car) on a level highway.

 

You can always shift down to 3rd when you want to overtake another vehicle or go up a mountain pass. The L16 won't mind a bit running at 5000 RPM -- that's what the factory designed it for.

 

For a stock 510, won't you need to change the diff from 3.90 to 3.70 to get the revs down that low? Z revs lower because the tires are larger diameter.

[firedime]

wow, you guys are on it and quick. I appreciate the info. I did forget a huge factor, that I will be running either 14x6 w/ 195 60 14 or 15x6, 15x7 wheels with 195 50 15 tires, have not made up my mind which would be better outside of diameter being less to turn and weight. I have 13 libres but I am going to get rid of them as 13 tires are scarce. When you said that the off the line will be diminished, I do not want that. So, what would be a good diff for the happy medium of around town and decent highway speeds with relatively low revs with the two tire and wheel combo's. Any help is appreciated. Thanks

[ggzilla]

The happy medium is the stock diff ratio of 3.90. Revs a little more on the freeway than you wanted, but doesn't reduce quickness off the line. The overdrive gear on your 5-speed will lower the revs significantly from stock even if you don't change the diff ratio.

 

Keep in mind that

* 155/80R13 - stock - is 22.76 in diameter

* 195/60R14 23.21 in diameter - 2% more

* 195/50R15 22.67 in diameter - 0.4% more

 

195/50R15 will be virtually no change from stock

 

The 14s would be like having a 3.8 diff ratio.

 

195/60R15 tires would be like having a 3.66 gear ratio ... and ride softer than the 50-series.

[ggzilla]

Also note that the Z 5-speed may have a higher first gear than the stock 510 first gear. If so, you've already lost a little off the line.

 

510 sedan: 3.382

510 wagon: 3.657

 

Z 5-speeds first gear ratio:

1977-1983 3.500

1984-1989 3.321 non-turbo

 

turbos have a steeper first gear, which is great for off-the-line takeoff.

Non-USA 5-speeds and Nissan competition gearboxes generally have a much higher first gear except for the Rally option boxes.

 

All these factors work together: Tire diameter, Transmission ratios, Final ratio.

[datzenmike]

Assuming 195/60R14.... 23.21" and 3.90 rear end ratio and a Z 5 speed which has an overdrive ratio of 0.864 then an engine RPM of 2500 would give a speed of 51.25 MPH. At 70MPH the engine will be at 3,414 RPMs.

 

Assuming 195 50 15 .... 22.68" and 3.90 rear end ratio and a Z 5 speed which has an overdrive ratio of 0.864 then an engine RPM of 2500 would give a speed of 50 MPH. At 70MPH the engine will be at 3,494 RPMs.

 

 

 

Also note that the Z 5-speed may have a higher first gear than the stock 510 first gear. If so, you've already lost a little off the line.

 

510 sedan: 3.382

510 wagon: 3.657

 

Z 5-speeds first gear ratio:

1977-1983 3.500

1984-1989 3.321 non-turbo

 

'77-'78 280z and '79 280zx were 3.321 first gear

'80-'83 280zx (non turbo) were 3.064 first gear

 

Only the 82.5-'83 T-5 turbo 5 speeds were 3.5 first gear and anything '84 and after had VG30 motors.

 

 

 

turbos have a steeper first gear, which is great for off-the-line takeoff.

Non-USA 5-speeds and Nissan competition gearboxes generally have a much higher first gear except for the Rally option boxes.

 

All these factors work together: Tire diameter, Transmission ratios, Final ratio.

 

The '79 620 and '80 720 (both with L motors had a 3.592 first gear. This will preserve your take off performance but the over drive isn't any better than what you have.

 

Lowering your rear end ratio will hurt performance. A transmission with a much taller overdrive would be best. The '80 (0.773) or the '81-'82 (0.745) non turbo 5 speeds are the best compromize.

[Laecaon]

My 93 Thunderbird did 70mph at 2000rpm dead on. 3.8L V6 145hp not alot of torque. Achieved 27-28mpg.

My 01 Jetta does 70mph at 3000rpm roughly. 2.0L inline4 115hp 120ft-lb. Gets 30mpg.

 

71 510 with L18, 3.9 diff, dogleg tranny, 185/60/14 tires, 70mph is easy to stay at even go up hills, no idea on RPM, still get 30mph freeway easily. Have had no problem getting my speedometer to 95...

 

What am I saying? These motors can keep up with traffic, even with higher rpms, but they still get decent gas mileage.

 

I do want to switch to 3.7 gears though... I care alot more about freeway mileage than takeoff performance.

[datzenmike]

Coming home from '10 Canby I hit 90 in my 710 L18 automatic. So yes they will rev to the sky and when revved will make more than enough power to do this. If limited to just 2,500 RPM they may barely do 70. If you add a head wind or sight grade it means downshifting. The other thing is you will have to step down on the gas more to cruise comfortably.

 

My 620 has 33" tires and 4.11 gears. Normally the tires stock are 6.00 X 14 or only 24.33" tall and the rear end is 4.375. This works out to a 3.03 rear end ratio and at 70 I would be revving 2,500 RPMs. I know this is not an ideal comparison but I hardly go over 50 MPH as there's nothing there. To go 3,000 RPMs I'm almost to the floor with the gas and there is a long long wait to get up there. On windy days I can't and if there is a slight grade I have to grab 4th or I slow down. Too much overall reduction in gearing will (or can) cause something called power limiting where you can actually go faster in 4th than in 5th.

[firedime]

awesome stuff, thank you. I will stick with the stock diff and check my trans to see what year it is and see what the ratios are as I bought the car with it already in. I really appreciate it, have a good one!

[datzenmike]

Post a picture of the speedometer cable area of the tailstock. They are hard to identify exactly but often you can tell what it isn't.

[oldschool90]

When I rebuilt my trans, I swapped a couple of gears on it and now it cruises at 75MPH @ 2600 RPMs. Has enough power to get up hills too. Course, 1st-3rd are lower, but I can deal with that.

[datzenmike]

When I rebuilt my trans, I swapped a couple of gears on it and now it cruises at 75MPH @ 2600 RPMs. Has enough power to get up hills too. Course, 1st-3rd are lower, but I can deal with that.

 

Did you send me a list of gears and teeth numbers for that???

[oldschool90]

 

[oldschool90]

Did you send me a list of gears and teeth numbers for that???

 

I don't know what you're talking about. I did put up a wanted ad because I needed the drive gear/input shaft for my trans and you were explaining some things to me, Maybe that's what you're talking about?

[datzenmike]

Sent you a p/m for info on this.

[leespipedreams]

With out starting new topic... I just put in a 3.7lsd in my 71 510. I have a L20b motor /dog leg / and 185-60-14tires... What rpm will I do at 70mph? Lee

[datzenmike]

3268 RPMs.(in 5th)

3826 RPMs in 4th.

5020 RPMs in 3rd.

[Laecaon]

Mike what are the gear ratios of the Dogleg?

 

I think I read from you that 1-4 are the same as a standard 510 tranny, so I guess more importantly what is 5th?

[datzenmike]

The 200sx dogleg and the B-210 dogleg have the same ratios as the old 510 4 speed plus a 0.854 overdrive 5th. The dogleg used in the A-10 (HL510) with the Z series motor was different.