Re: We need an archive...more
> Ok, so you've got a 240Z with a 280Z? 5spd
> and stock R180 diff.
> Manual (77-79)280Z 5spd ratios
> 1st 3.321:1
> 2nd 2.077:1
> 3rd 1.308:1
> 4th 1.00 :1
> 5th .864:1 (anything less than 1:1 is
> overdrive)
> Manual (79-81)280ZX 5spd ratios:
> 1st 3.062:1
> 2nd 1.858
> 3rd 1.308:1
> 4th 1.00:1
> 5th .745:1
> So, with your 3.364:1 diff, 1st gear is as
> follows:
> 240Z 4spd 3.549x3.364 = 11.94:1 overall
> 280Z 5spd 3.321x3.364 = 11.17:1 overall
> 280ZX 5spd 3.062x3.364 = 10.3:1 overall
> As you can see, the engine is turning much
> slower with the 280ZX, and this might be the
> transmission you have. There are casting
> numbers to help you identify this.
> Or, you can do it scientifically, using the
> following method.
> 1. Drive with a friend, who has an accurate
> speedometer, and have them drive at exactly
> 50mph.
> You stay right beside, and write down your
> rpm. (not while driving of course

)
> 2. Now, knowing your rpm @50mph, you need to
> measure the OVERALL diameter of your rim and
> tire.
> Measure from the ground, to the top of your
> tire. Record this number in inches as close
> as possible.
> 3. Use this formula, to determine which
> ratio diff you have.
> RPM = mph x overall gear ratio x 336/tire
> diameter
> So, for example:
> 280Z 5spd overall 5th gear w/ 3.36 R180 =
> 2.91:1
> 280Zx 5spd overall 5th gear w/ 3.36 R180 =
> 2.51:1
> Tire diameter of 23.2 (this is what
> mine is)
> RPM @50mph with 280Z 5spd=
> 50 x 2.91 x 336/23.2 = 2107rpm
> RPM @50mph with 280ZX 5spd=
> 50 x 2.51 x 336/23.2 = 1818rpm
> Obviously, you can notice a large difference
> with the rpm. This way you know what 5spd
> you have. If the numbers don't come out
> correctly, just swap in the different
> overall ratios for the rear ends, until the
> numbers match. Since everything else is a
> constant(or pretty close), the only part of
> the problem that can change is the overall
> ratios. You should be able to get a correct
> answer by swapping one of the ratios. Then
> you will know what type of transmission and
> what ratio rear end you have. Swapping a
> R200 into a 240Z, is a whole other story.
> You can check if you have a R180 just by
> visually looking at the rear end, and
> compare it to the diff from the 280Z. The
> R200 280Z should be physically much larger.
> I would assume the 3.364:1 ratio for the
> R180, since that is what most of them are,
> unless someone planted a R180 from a mid
> 80's 4wheel drive into your car, which is in
> my opinion, highly unlikely.
> If you do have the 280Z 5spd, then you need
> a 3.545 R200 or higher.
> If you have the 280ZX 5spd, then you need a
> 3.9 R200 or higher.
> The higher above the base figure will give
> you higher overall ratios, which will help
> in acceleration, but will raise your rpm
> while cruising on the highway. It's up to
> you to decide which way you want to go.
> By the way, I've noticed the manuals, even
> Haynes are somewhat vague in this area, and
> are not entirely accurate. Afterall, these
> manuals are British publications, and Europe
> spec cars often have different parts and
> requirements than US cars.
Dave,
The 77-79 5-spd's had the .864 OD you described. The '80 model had the newer 5-spd 1st & 2nd gearing but had a .777 OD. The 81-83's had the .745 OD.
Just to add a little more confusion to the equation.
Phantom