Rolling diameter for modified cars

[FeeBee]

I was comparing my 19 inch wheels with the original 13 inch wheels. What happens when you use a commodore gearbox, diff, etc. How does that effect the speedo in regards to the rolling diameter. From memory the rolling diameter needs to be within an inch of the original, but do you use the commodore original rolling diameter? Or do you just get your speedo re-calibrated ? The 19s did look very good under the rear

[Blacky]

you can get a corrector box for your speedo cable, it depends on what gearbox you end up using - if its electronic I dont know how it works .
You can buy a speedo that operates off a GPS unit these days as well.

[bumper]

I'm buying a unit from the states, as I'm using the t700 box and want to run the orig speedo, this box changes from electronic to cable, also in the box there are allot of switches so you can use a GPS to set it to the correct speed to suit all combinations

[Harv]

The speedo reading will change depending on the tyre diameter you choose, but also on the diff ratio. For some gearboxes (for example the TH350), you can buy a myriad of both speedo drive and driven gears (often colour coded). This lets you put together a drive/driven combination that delivers the correct speedo cable speed for a given vehicle speed. I need to go through this some time soon, as the speedo in the V8 project wagon runs, but is being driven too fast (runs the speedo off the dial ).

Does anyone have an old original cross-ply 13" tyre (not a new radial replacement) that they can measure the circumference of (with a measuring tape) please? If I can get my hands on this, I'll post up an example calculation of how to estimate a correct drive/driven set.

Cheers,
Harv

[jack_fc]

Harv, I measured one a while back - approx. 605 to 610 mm diameter...

cheers, jack

[Blacky]

Heres a link to the one I use - http://www.tciauto.com/tc/transmission- ... gears.html

[Harv]

As promised, attached below a set of calculations for choosing speedo gears for a modified FB/EK.

Take a bog stock standard FB/EK Holden, and imagine it moving such that the back wheels turn around once:
a) The back wheels turn once, and the car travels a distance equal to the circumference of the old 13” tyres. This distance is equal to (π x diameter of tyre). The original tyres were 605-610mm diameter (we’ll say 607mm). This means the car has travelled (π x 0.607 = ) 1.907m
b) Whilst the back wheels (and axles) spin once, the differential gearing means that the driveshaft has had to spin much more. The standard driveshaft spins at ratio of 35:9, or 3.889:1 (i.e. for one revolution of the rear wheels, the driveshaft has spun 35 ÷ 9 = 3.889 times).
c) The driveshaft spins at the same speed as the gearbox output shaft. The gearbox output shaft has the speedo drive gear on it, which has seven teeth. This means that the speedo drive gear has turned over (7 x 3.889 =) 27.222 teeth.
d) The speedo drive gear turns the speedo driven gear. The speedo driven gear has 23 teeth (assuming it’s an FB/EK gearbox... the FX/FJ gearbox had 22 teeth). This means the speedo driven gear (and hence the speedo cable) has turned around (27.222 ÷ 22 =) 1.237 revolutions.
e) Finally, lets convert back to a set distance. For every 1m the car travels, the speedo cable will spin (1.237 ÷ 1.907 =) 0.6489 revolutions.

So what we are looking for in our freshly modified FB/EK is a tyre/diff/speedo gear combination that turns the speedo 0.6489 times for every 1m the car travels forwards.

f) Now let’s assume we have modified our back wheels to be Commodore 15” wheels (with 205/65/15 tyres). These have a diameter of 645mm. For one revolution of the rear wheels, the car moves (π x 0.645 =) 2.026m.
g) Assume we have also fitted a VN-VP Commodore differential, with a ratio of 3.08:1. Our driveshaft (and hence gearbox output shaft) will spin 3.08 times.
h) Assume our speedo drive gear has “X” teeth, and our driven gear has “Y” teeth. The speedo cable will spin (3.08 x X ÷ Y) times.
i) For our car moving one metre, the cable will spin 3.08 x X ÷ Y ÷ 2.026) = 1.520 x (X÷Y) revolutions.

Now what we want for our speedo to work correctly is 1.520 x (X÷Y) to equal 0.6489, or in other words X/Y = (0.6489 ÷ 1.520 =) 0.427 Assuming we have a Turbo 350 gear box, we can look use the site Blacky has posted above to get the number of teeth for different speedo gears. I then put them into Excel, and calculated X/Y for all the different combinations. I then organised them so that they are listed below in order of increasing X/Y:

Drive Gear “X” Driven Gear “Y” Ratio (X/Y)
Black 8 Light Blue 45 0.1778
Black 8 Grey 44 0.1818
Black 8 Purple 43 0.1860
Black 8 Green 42 0.1905
Black 8 Yellow 41 0.1951
Black 8 Black 40 0.2000
Black 8 Brown 39 0.2051
Black 8 Blue 38 0.2105
Black 8 Red 37 0.2162
Black 8 Natural 36 0.2222
Purple 10 Light Blue 45 0.2222
Purple 10 Grey 44 0.2273
Black 8 Orange 35 0.2286
Purple 10 Purple 43 0.2326
Black 8 Green 34 0.2353
Purple 10 Green 42 0.2381
Purple 10 Yellow 41 0.2439
Purple 10 Black 40 0.2500
Purple 10 Brown 39 0.2564
Purple 10 Blue 38 0.2632
Purple 10 Red 37 0.2703
Purple 10 Natural 36 0.2778
Purple 10 Orange 35 0.2857
Purple 10 Green 34 0.2941
Grey 15 Light Blue 45 0.3333
Grey 15 Grey 44 0.3409
Black 8 Black 23 0.3478
Grey 15 Purple 43 0.3488
Grey 15 Green 42 0.3571
Black 8 Grey 22 0.3636
Grey 15 Yellow 41 0.3659
Grey 15 Black 40 0.3750
Red 17 Light Blue 45 0.3778
Black 8 Red 21 0.3810
Grey 15 Brown 39 0.3846
Red 17 Grey 44 0.3864
Grey 15 Blue 38 0.3947
Red 17 Purple 43 0.3953
Black 8 Blue 20 0.4000
Green 18 Light Blue 45 0.4000
Red 17 Green 42 0.4048
Grey 15 Red 37 0.4054
Green 18 Grey 44 0.4091
Red 17 Yellow 41 0.4146
Grey 15 Natural 36 0.4167
Green 18 Purple 43 0.4186
Black 8 Natural 19 0.4211
Natural 19 Light Blue 45 0.4222
Red 17 Black 40 0.4250
Green 18 Green 42 0.4286
Grey 15 Orange 35 0.4286
Natural 19 Grey 44 0.4318
Purple 10 Black 23 0.4348
Red 17 Brown 39 0.4359
Green 18 Yellow 41 0.4390
Grey 15 Green 34 0.4412
Natural 19 Purple 43 0.4419
Black 8 Brown 18 0.4444
Red 17 Blue 38 0.4474
Green 18 Black 40 0.4500
Natural 19 Green 42 0.4524
Purple 10 Grey 22 0.4545
Red 17 Red 37 0.4595
Green 18 Brown 39 0.4615
Natural 19 Yellow 41 0.4634
Red 17 Natural 36 0.4722
Green 18 Blue 38 0.4737
Natural 19 Black 40 0.4750
Purple 10 Red 21 0.4762
Red 17 Orange 35 0.4857
Green 18 Red 37 0.4865
Natural 19 Brown 39 0.4872
Purple 10 Blue 20 0.5000
Green 18 Natural 36 0.5000
Natural 19 Blue 38 0.5000
Red 17 Green 34 0.5000
Natural 19 Red 37 0.5135
Green 18 Orange 35 0.5143
Purple 10 Natural 19 0.5263
Natural 19 Natural 36 0.5278
Green 18 Green 34 0.5294
Natural 19 Orange 35 0.5429
Purple 10 Brown 18 0.5556
Natural 19 Green 34 0.5588
Grey 15 Black 23 0.6522
Grey 15 Grey 22 0.6818
Grey 15 Red 21 0.7143
Red 17 Black 23 0.7391
Grey 15 Blue 20 0.7500
Red 17 Grey 22 0.7727
Green 18 Black 23 0.7826
Grey 15 Natural 19 0.7895
Red 17 Red 21 0.8095
Green 18 Grey 22 0.8182
Natural 19 Black 23 0.8261
Grey 15 Brown 18 0.8333
Red 17 Blue 20 0.8500
Green 18 Red 21 0.8571
Natural 19 Grey 22 0.8636
Red 17 Natural 19 0.8947
Green 18 Blue 20 0.9000
Natural 19 Red 21 0.9048
Red 17 Brown 18 0.9444
Green 18 Natural 19 0.9474
Natural 19 Blue 20 0.9500
Green 18 Brown 18 1.0000
Natural 19 Natural 19 1.0000
Natural 19 Brown 18 1.0556

From the table, it looks like the combination of red drive gear and black driven gear will come close to what we want (X/Y = 0.4250), as will green/green (0.4286) or grey/orange (0.4286).

Cheers,
Harv

[Blacky]

Wow Harv, you have got WAAAAAY more patience than I have ............................