Diff/gearbox

stuart barrie

Club Member
I have a 1973 240z it seams to be very low geard is this normal and what can I do to improve the setup.
At 3000RPM a speed of 55MPH
At 4000RPM a speed of 74MPH
At 5000RPM a speed of 93MPH
All in 5th gear
Diff seams to be 3.9-1
 

zfarm

Active Forum User
1) have you checked that the speedo is accurate ? That is, about as accurate as it can be expected to be ( 10% variation or less ).
2) Have you got the standard size wheel/tyre combination fitted, or at least, something close, like a 195/70X14 on the rear.

If the answer is Yes to both the above, then suspect that the diff is from either a UK 260Z ( 3.7) or even from a US 4-speed model ( 3.36).

Somewhere else in the technical questions section relates how to measure the revolutions of the rear wheel and do the sums to ascertain exactly which one you have fitted to your vehicle.
 

Black Bug

Well-Known Forum User
There's a really cunning little toy here you can play with to figure out gearbox/dif/speed relationships. Apparently I've got something like a 5:1 diff, either that or my speedo is up the spout, hmmm I wonder?!!! ;)

Cheers,
Rob
 

Carl Beck

Well-Known Forum User
Zfarm wrote:
>1) have you checked that the speedo is accurate ? That is, about as
>accurate as it can be expected to be ( 10% variation or less ).
>2) Have you got the standard size wheel/tyre combination fitted, or
>at least, something close, like a 195/70X14 on the rear.
>
>If the answer is Yes to both the above, then suspect that the diff is >from either a UK 260Z ( 3.7) or even from a US 4-speed model (
>3.36).

Hi Zfarm (everyone):
Hummm... let's think about this one. ;-)
The speedometer is driven off 4th gear in all the stock transmissions and they are all 1:1 in forth gear.

So if you change rear gears - to no matter what ratios... the speedo will still give the same "indicated speed" at any given RPM. Only your "actual speed" will change. Likewise changing tire diameter will effect your actual speed, not your indicated speed.

On the other hand - if you swap transmissions, without changing the speedometer gear to match the rear end ratio that you have - you will get a different set of "indicated speeds" holding your RPM the same.

For example, if someone pulled the original 240-Z five speed out and installed a 260-Z five speed, without swapping the speedo gears (to account for the change from 3.9 to 3.7)...you would get a different set of indicated speeds at the same RPM levels.

So first thing to do is figure out which rear gear you have, then the second thing to do is look to see if the transmission has the correct speedo gear for that rear end.

FYI:
3.364 : Yellow 16 teeth - #32703-78100 Assy-Pinion Speedometer(5spd.)

3.545 : Black - 17 teeth - #32703-78101 Assembly Pinion Speedometer(5spd)

3.7 : Blue - 18 teeth #32703-78102 ( 5spd.)

3.9 : White - 19 teeth #32703-78103 ( 5spd.)

4.11 Red 20 teeth (don't know the part number off hand)

Stuart:
The Euro Spec. 240-Z's were "pretty low" geared, came with the 3.9 rear gear and an overdrive 5th gear to offset that at cruise as I recall. Off the top of my head, 4000 RPM at 74mph doesn't seem too far out of line to me. The L24 is very happy turning that speed and the 3.9 certainly gets if off the line in a hurry.

If you are driving at an indicated speed of 74mph - is everyone passing you, or are you passing everyone else? What do you guess your "actual" speed is? If tire diameter is other than stock, you can adjust your indicated speed to more closely match your actual speed by changing the speedometer gears in the transmission. Actual speed can be calculated using your odometer over a measured mile. Well we use measured miles here in the US.

As I recall - adding or subtracting a tooth from the speedo gear tooth count - changes your speedo reading by plus or minus 6 percent at any certain RPM point.

kind regards,
Carl


Carl Beck
Clearwater,FL USA
 

Rob Gaskin

Treasurer
Staff member
Site Administrator
The easiest and most accurate way of determining speed is to use a device like my 'Road Angel' and it will also help to protect your licence!
 

stuart barrie

Club Member
Thanks for the help.
The tyre size is 225/60/14
I supose the diff will be a 3.9 as I rotated the wheel counted revolutions and came up with 3.9 before I knew the options.
So to reduce RPM at cruse speed I will need Diff and speedo gear to match.
 

Carl Beck

Well-Known Forum User
Originally posted by stuart barrie
...snipped..cjb...
So to reduce RPM at cruse speed I will need Diff and speedo gear to match.

Hi Stuart (everyone):
You can play with the numbers (all on ZHome.com), but it looks like you have a few options.

1. Change the rear gear from 3.9 to 3.364 (stock in the US) and that should drop your RPM (holding constant actual speed)... by about 14%. So instead of 4000RPM at an actual speed of 71mph, you would be turning 3440 RPM. (if I'm thinking about this the right way).

However this might feel a little doggy around town in first and second gear with your present 5spd.

2. Change the 5spd from the Euro Spec. with its .864 overdrive, to the US Spec. 81/83 280ZX 5spd. with its 0.745 overdrive 5th gear. Again that's about a 14% reduction. The advantage to this approach is that it leaves your Z feeling a little more peppy around town because of its slightly lower first and second gears (numerically higher ratios).

3. You could do both #1 and #2 if lowering your highway cruise RPM is the main goal. That's what I have in one of my 1972 240-Z's. It's the car I take on longer road trips, and it's set up for mainly that purpose. I wanted to be able to hear the stereo at cruise, without the engine noise. I wanted to be able to hear my passenger if she was speaking to me, without the roar of the engine.

With the Euro 5spd. you have fairly tall first and second gears. (lower numerical ratios -2.906 & 1.902).. with the 81/3 US Spec. 5spd you have a bit lower first and second gear (higher numerical ratios 3.062 and 1.858), which helps off-set the taller rear gear at lower speeds.

I would guess, and it's only a guess - that if you rebuild your 30+ year old 5spd., you could swap in the 0.745 overdrive 5th without any problems. As I recall all the stock 5spd.s from 72 Model Year forward through 83 were the FS5C71B with only different gear ratios.

just some thoughts...
good luck,
Carl


Carl Beck
Clearwater,FL USA
http://ZHome.com
 
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