Gear ratio formula maths

[Dum-Dum]

Hi all, Can someone please write me the formula for working out road speed if i have gear ratio, final drive ratio, engine revs and rolling radius of a wheel as im having mega trouble with the maths.


Try a gear ratio of 0.86, final drive of 3.6, engine revs of 4500 and rolling radius of 187cm as i know what these should make.

[Niall]

God what are you doing now?

[n0v0s]

try this

http://www.gearboxman.co.uk/inforatios.html

Have to download the app

[Poodle]

If i understand correctly it should be as simple as multiplying the engine speed by rolling radius, then dividing by the total gear ratio, so:

(rpm x rolling radius) / (gear ratio x fd)
(4500 x 187) / (3.6 x 0.86)
841,500 / 3.096
271,802.33

This ^^^ is in cm/minute, so multiply by 60 to get the speed in cm per hour:

271,802.33 x 60 = 16,081,140

Then divide by 100,000 to get km/h

=160.8km/h, or near-as-dammit 100mph...


Ta-da!

[mark_airey]

If i understand correctly it should be as simple as multiplying the engine speed by rolling radius, then dividing by the total gear ratio, so:

(rpm x rolling radius) / (gear ratio x fd)
(4500 x 187) / (3.6 x 0.86)
841,500 / 3.096
271,802.33

This ^^^ is in cm/minute, so multiply by 60 to get the speed in cm per hour:

271,802.33 x 60 = 16,081,140

Then divide by 100,000 to get km/h

=160.8km/h, or near-as-dammit 100mph...


Ta-da!

Get you and the clever maths ness

[Dum-Dum]

Great but it should be 130 mph so your maths is a little out

[Dum-Dum]

God what are you doing now?

May of slightly bought a 1.5D engine, was wondering what a HDi box would do to it.

[Dum-Dum]

Poodle I think my HDi ratios are wrong but i cant work out where.

[samass]

how did you work out the HDi ratios?

[Dum-Dum]

service box, 1st and 2nd line gear train. states them in the parts and i took the top ratio off each number.

[ginge191]

[samass]

can you print screen it so i can have a look? did compound ggear ratios at uni recently, ill see if i can help

[Grant]

May of slightly bought a 1.5D engine, was wondering what a HDi box would do to it.

And why on earth have you done this? hock: lol

[samass]

1.5 > 1.4

powaaaahhhh

[Toby]

1.5 > 1.4

powaaaahhhh

1.5 - 58bhp

1.4 - 51bhp.

The logic is sound.

Shoulda got a 1.9 XUD though

[samass]

that and the 1.5 is just a better engine

[Dum-Dum]

that and the 1.5 is just a better engine

Actually its not "just a better engine"

Its an iron block which is heavier and the inlet manifold is still a shit design.

I just want something to rebuild, add a bosch and maybe boost.

[Grant]

But i thought you werent doing anything to this car? Plus its about to fall apart? lol

[Dum-Dum]

Shhhhh

[Curt]

NAD's are to save money Chris?!

You're just a bit backwards aren't you?

[Dum-Dum]

Nope the NAD is so slow it feels like its going backwards.

With boost and bosch itll be going forwards very very quickly.



Im really only doing it so when ive cleared out the garage ive got something to play with.

[Poodle]

Only just seen this, damn! Thought i'd cracked it when the calculator flashed up 100.3mph lol.

[Dum-Dum]

TBH i think the numbers are wrong somewhere cos id done the maths before i posted here and got about 100 too

[Poodle]

Fair enough. Be nice to know either way for sure, so at least we would know where to make changes.

[Ruan]

If you boost the 1.4 it'll die instantly... They're wet liner for christs sakes!

They were renound for shagging liners and gaskets from day one at 51hp...

[Diggers]

Nope the NAD is so slow it feels like its going backwards.

With boost and bosch itll be going forwards very very quickly.



Im really only doing it so when ive cleared out the garage ive got something to play with.

the 1.5s a bosch anyway aint it? ma old mans and the 106 GTID was a bosch but ovsly a non turbo one

[Ruan]

Depends, some are CAV some are Bosch.