Final drive calculation.

[Aram]

Hi guys,

Since I put the Altezza hardware in my car the speedometer has quite an indication gap.
When driving 100km/h on the gps the speedometer indicates 130km/h.

I don't want to change my tyre size (205/40/16) so I will change the ring gear.

But how do I calculate this??

Altezza (SXE10):
1: 3.874
2: 2.175
3: 1.484
4: 1.223
5: 1.000
6: 0.869
Final Gear Ratio: 4.100

AE86: (for comparison)
1: 3.587
2: 2.022
3: 1.384
4: 1.000
5: 0.861
Final Gear Ratio: 4.300

Thanks!

Aram.

[ditn]

or you could install an Abbott Cable X 3012 - 3/4 (GM Snap On) and have the exact km/h you want with whatever you want
Thats what im gonna do.

more info here:
http://www.gearboxconversion.com/Cable_X.html

[The Doctor]

well, your speedo is 30% over reading, so a final drive that is upto 30% taller would make the gap smaller, but will also have a HUGE effect on the drive itsself. I went from final ratio 4.3 to 3.3 (which is less then 30%) and it killed the speed/acceleration i had, but gave me comfort and better fuel economy in return.

If you like the way it drives, i would do what ditn recommends, there are different companies making stuff like that. Otherwise go for a bit taller gearing and live with less then 30% overreading on the dash (a stock 80s Toyota will overread by 10-20% anyway, highly inaccurate stuff).

Have a play with a calculator like this:
http://www.cargister.com/calculator-gear-ratio

[Aram]

Well, I would like to get a better ratio, when I do 100kmh on the gps the engine runs around 4k rpm, that's not really for longer drives on the highway.

I thought from 4.3 to 3.3 was about what I needed?

That Abott cable does sound nice though.

[The Doctor]

Can you find a 3.3 for a T-series diff? On my car i can use Starlet KP61 R&P, so i could use an automatic diff which has the 3.3 ratio, dunno about the T-series.

[Bean]

4.3 to 3.3 is a huge step - ask oldeskewltoy I'm sure he can help out.

EDIT: are you sure you currently have a 4.3? as 100km/h @ 4000rpm in 6th sounds more like a 4.77

did run the numbers through a calculator and with a 4.3 you should see about 120km/h - with a 3.9 100km/h would then be just over 3000

according to the same calculator a 4.3 would result in top speed of about 250 @ 8500rpm which sounds about right and should still retain enough torque... personaly I wouldn't go below 3.9.

for highway comfort you might be able to replace the 6th gear with one from an S15 (not sure it fits) - this would give you a considerable taller gear (they are 0.7something)

[Aram]

I have the standard 1987 rear axle, I thought that was 4.3?
But my current tyre size (205/40-16) might not have correct rolling circumference in relation to the standard wheel.
That could also make a difference in speed I think.

[Uzelac]

4.3 is standard for AE86, 4.1 for AE86.

You can use this sheet i created some years ago to test everything:

https://docs.google.com/spreadsheets/d/1...y=CNqEs-cI

[Bean]

I took the tire size into account... something is a bit off

[oldeskewltoy]

The CableX is the simplest solution, albeit not the least expensive.

Tire diameter, as well as an in-accurate speedometer will cause havoc on accurate speed readings.

As far as running a 3.3 final drive.... I had a 3.58 final in my E7 Corolla, and with that I barely had a need for 5th gear..... I've since moved to a 3.90.