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+--- Thread: Gear ratio formula maths (/showthread.php?tid=2874)
Gear ratio formula maths - Dum-Dum - 18-04-2012
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.
Re: Gear ratio formula maths - Niall - 18-04-2012
God what are you doing now?
Re: Gear ratio formula maths - n0v0s - 18-04-2012
try this
http://www.gearboxman.co.uk/inforatios.html
Have to download the app
Re: Gear ratio formula maths - Poodle - 19-04-2012
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!
Re: Gear ratio formula maths - mark_airey - 19-04-2012
Poodle Wrote: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
Re: Gear ratio formula maths - Dum-Dum - 19-04-2012
Great but it should be 130 mph so your maths is a little out
Re: Gear ratio formula maths - Dum-Dum - 19-04-2012
Niall Wrote:God what are you doing now?
May of slightly bought a 1.5D engine, was wondering what a HDi box would do to it.
Re: Gear ratio formula maths - Dum-Dum - 19-04-2012
Poodle I think my HDi ratios are wrong but i cant work out where.
Re: Gear ratio formula maths - samass - 19-04-2012
how did you work out the HDi ratios?
Re: Gear ratio formula maths - Dum-Dum - 19-04-2012
service box, 1st and 2nd line gear train. states them in the parts and i took the top ratio off each number.
Re: Gear ratio formula maths - ginge191 - 19-04-2012
![[Image: my-head-hurts.jpg]](http://damonday.com/wp-content/uploads/my-head-hurts.jpg)
Re: Gear ratio formula maths - samass - 19-04-2012
can you print screen it so i can have a look? did compound ggear ratios at uni recently, ill see if i can help
Re: Gear ratio formula maths - Grant - 19-04-2012
Dum-Dum Wrote: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
Re: Gear ratio formula maths - samass - 19-04-2012
1.5 > 1.4
powaaaahhhh
Re: Gear ratio formula maths - Toby - 19-04-2012
samass Wrote:1.5 > 1.4
powaaaahhhh
1.5 - 58bhp
1.4 - 51bhp.
The logic is sound.
Shoulda got a 1.9 XUD though
Re: Gear ratio formula maths - samass - 19-04-2012
that and the 1.5 is just a better engine
Re: Gear ratio formula maths - Dum-Dum - 19-04-2012
samass Wrote: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.
Re: Gear ratio formula maths - Grant - 19-04-2012
But i thought you werent doing anything to this car? Plus its about to fall apart? lol
Re: Gear ratio formula maths - Dum-Dum - 19-04-2012
Shhhhh
Re: Gear ratio formula maths - Curt - 19-04-2012
NAD's are to save money Chris?!
You're just a bit backwards aren't you?
Re: Gear ratio formula maths - Dum-Dum - 19-04-2012
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.
Re: Gear ratio formula maths - Poodle - 22-04-2012
Only just seen this, damn! Thought i'd cracked it when the calculator flashed up 100.3mph lol.
Re: Gear ratio formula maths - Dum-Dum - 22-04-2012
TBH i think the numbers are wrong somewhere cos id done the maths before i posted here and got about 100 too
Re: Gear ratio formula maths - Poodle - 22-04-2012
Fair enough. Be nice to know either way for sure, so at least we would know where to make changes.
Re: Gear ratio formula maths - Ruan - 23-04-2012
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...
Re: Gear ratio formula maths - Diggers - 23-04-2012
Dum-Dum Wrote:Nope the NAD is so slow it feels like its going backwards.the 1.5s a bosch anyway aint it? ma old mans and the 106 GTID was a bosch but ovsly a non turbo one
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.
Re: Gear ratio formula maths - Ruan - 23-04-2012
Depends, some are CAV some are Bosch.