Transmission ratios and suggested rear gears

[2ManyZs]

Since there have been numerous questions on the subject I thought this would be a big help to anyone planning on swapping transmissions or rear ends in the future.

This is an excerpt from an answer to a tech question from the old Z Car magazine. If anyone has the magazines it is in the Nov/Dec 1997 issue on page 10 and 11.

The stock transmission gears are as follows:

240Z 1st 2nd 3rd 4th 5th

4 speed type A 3.549 2.197 1.420 1.00

4 speed type B 3.592 2.246 1.415 1.00

280Z

5 speed 3.321 2.077 1.308 1.00 .864

280ZX 5 speed

1980 3.062 1.858 1.308 1.00 .773

1981-on 3.062 1.858 1.308 1.00 .745

Now the author went on to compare the final drive ratios with different ring and pinion combinations as a way to compare the 280 5 speed to the 280 Zx 5 speed. This is the overall drive ratio, which is the combined mutiplication of the transmission and the rear end gear.

Recommended rear diff ratios for the early 280 (77-78) 5 speed:

Either a 3.70 or 3.90

5 speed w/3.70 12.29 7.68 4.84 3.70 3.20

5 speed w/3.90 12.95 8.10 5.10 3.90 3.37

Recommended rear diff ratios for the 280 Zx 5 speed:

Either a 3.90 or a 4.11

5 speed w/3.90 11.94 7.25 5.10 3.90 3.01

5 speed w/4.11 12.58 7.64 5.38 4.11 3.18

Now if you are running the later ZX 5 speed 81-on the first four ratios are the same but the overdrive 5th will give you a final drive of 2.90 with the 3.90 rear, and it will be 3.06 with the 4.11

Take note that by running a 5 speed of any type, if you don't change the rear end gear you will really have a tall overdrive, notice that all the 5th gear final drive ratios are all under 3.36 except for one. Which means that you will turn less RPM's on the highway while cruising but you will substantially increase your acceleration in the lower 4 gears.

IMO with the later ZX 5 speed you could also go to a 4.38 ratio in the rear and you will still not see an increase in the highway RPM's and you will gain a slight improvement in the lower gears as well.

Remember, if you change the rear end ratio you will need to re-calibrate the speedometer drive pinion whether you change transmissions or not. The drive pinion is based on the rear end ratio, regardless of what transmission you have in the car.

You will need the following pinions to calibrate your speedometer:

Yellow, 16 tooth for the 3.36 ratio

Black, 17 tooth for the 3.54

Blue, 18 tooth for the 3.70

White, 19 tooth for the 3.90

Red, 20 tooth for the 4.11

Purple, 21 tooth for the 4.38

[mperdue]

I was just looking through this post for about the 5th time and I'm still not sure about one thing. Is the final drive ratio (listed in the article) mean the number of engine revolutions per wheel revolution? If so that means the 4.11 diff in a ZX 5-sp requires 3.18 engine cycles per wheel cycle but the 3.9 is only 3.01? Seems like a significant improvement in cruising RPM with the 3.9 but I don't really have a grasp on how that really affects the milage.

I'm trying to decide what rear diff to put in my car and I like the idea of greater acceleration but I don't want poor milage on the highway. Does anyone know how much REAL difference the milage would be for these 2 w/ 81ZX tranny. Is it a difference of 1mpg or 4-5mpg (for highway driving)? Thanks for the help, and thanks 2MZ for the tech article

Michael

[Royce]

Yes, the final drive ratio represents engine revolution to wheel revolutions. You would probably experience a 1 or maybe 2 MPG difference. Look at it another way- with 205/60R15 tires, and a '81-'83 280ZX 5-speed you would have the following RPM @ 75 MPH...

3.9 - 2967

4.11 - 3127

A difference of less than 200RPM, not very significant. The gap is smaller if you drive slower and higher if you drive faster! If you could get it to 6500RPM the difference in top speed is about 8 MPH, 164 compared to 156. So I wouldn't change to 4.11 from a 3.9 but if you have a 3.3 or 3.5 with a ZX trans, maybe!?

And don't forget about your tires size, you could make a the same RPM/MPH relationship change with tire selection. For example going from 215/60R14 to 205/50R15 would net almost the same results as going from 3.9 to 4.11 with 215/60R14 and give you potentially better handling at the same time. You are not changing the final drive ratio just the distance the car will travel given the same number of wheel revolutions.

Corrections, questions, and comments are welcome!

Have fun!

[Nigel Mulvey]

FYI The gear ratios on the Aus and UK five speeds which were available from 1970 were:

1st - 2.96

2nd - 1.86

3rd - 1.31

4th - 1

5th - 0.85

Diff ratio 3.70:1

Source Road and Track Magazine, January 1970.

[mperdue]

Thanks Royce! I was hoping the difference in gas milage wouldn't be too much. This car will eventually be a "daily driver" except that I'll probably continue to take the train/bus about half the time, so maybe I can deal with losing a little milage. The car still has the stock 240Z rear diff for a '73 (3.54?) so I think I will switch to the 4.11 - when I get the money that is - and when I get to the point where I need it again. Right now my top speed is 0mph!

Michael

[Giedrius]

Mperdue, here is one explanation I got on Zcar.com forum:

<quote>

Assume that the circumference of the rear tire is 4 feet.

If you had a differential ratio of 1:1 then every time the drive shaft rotated once the rear wheel would rotate once and the wheel would move 4 feet. So at 1000 RPM the wheel would move 4000 feet per minute.

If you had a differential ratio of 2:1 then every time the drive shaft rotated once the rear wheel would move 2 feet. So at 1000 RPM the wheel would move 2000 feet per minute.

If you had a differential ratio of 3.9:1 then every time the drive shaft rotated once the rear wheel would move 1.02 feet. So at 1000 RPM the wheel would move 1020 feet per minute.

If you had a differential ratio of 4.11:1 then every time the drive shaft rotated once the rear wheel would move .97 feet. So at 1000 RPM the wheel would move 970 feet per minute.

Wayne Monteath

Masham, Quebec.

<end quote>

I really like reading all the differential/transmission related posts, since I have Nissan Motorsports 4.444 LSD and I am debating if I should put it back in my car or not. Also, spedometer gear that's matched for it is green with 23 teeth. I never seen that one listed anywhere.

Giedrius

[m240]

I'm a little dense. I am currently putting a 4 speed from a 72 z in my 73 that had an automatic tranny. Will I have to change the speedometer drive pinion to correct for this?

Thanks, Mark

[V8-240Z]

Here are a couple car math caculators

http://www.smokemup.com/auto_math/index.php

http://www.prestage.com/carmath

[Royce]

m240,

The stock 240Zs in US market had a 3.36 rear diff ratio. The stock 240Z auto had a 3.54 rear diff ratio. So if your trans has the stock speedo gear than yes it will need to be changed. I don't know if the auto trans speed gear is interchangeable with the speedo gears in the manual trans, but it very well might be! I have a friend with a 240Z auto trans in his parts car, maybe I'll take a look one of these days. Anyone else know for sure? Also, another thing to consider when looking for the right speedo gear is that you have to compensate for tire size changes too! How many people are running a tire that is stock height!

My favorite RPM/MPG calculators is http://www.geocities.com/z_design_studio/ which has most the stock Z trans and diff ratios in it and provides an interactive graph so you can look at different speeds or RPM very easily.

[Giedrius]

Good Tire Size Calculator and Comparison

You can compare the size of the tires and speedometer errors.

[m240]

Thanks for the info. I know that I should have posted this in some other area than technical articles but I saw this subject I thought I'd jump in.

Thanks Again, Mark

[Hallaian42]

I have a 72 240z with the 79-83 280 L28 engine with 5spd. I believe that the differential is still the stock 240z. Right now it has no speedometer (or cable or that matter). Thats the way i bought it. Which conversion do i need to make my speedo work? Do i need to swap the speedo gears? Thanks

P.S. Where is a good place to find a speedocable?