Technical explanation of gear ratios and diff ratios

[Maxx-Waxx]

Can anyone offer a concise explanation as to how gear ratios and final drive ratios affect the acceleration of a car.

The reason i ask is i have Sagaris with standard box and 3.46 diff and am looking to upgrade to a Close Ratio box and either a 3.73 diff or 3.91 diff, as many people have told me thats its a big improvement, apparantly the CR box and 3.91 diff feels mental in 2nd 3rd and 4th compared to standard box and 3.46 diff

I downloaded a gearspeed calculator from here :-
http://www.gearboxman.co.uk/inforatios.html

Into which i inputted the rolling radius 6.55 feet and then a combination of either close ratio or standard box with the different diff ratios.

Standard Gearbox
2.95:1
1.95:1
1.34:1
1:1
0.8:1

Close Ratio Gearbox
2.43:1
1.56:1
1.24:1
1:1
0.8:1

What confuses me is the CR box with 3.73 diff ( which all the press cars had fitted ) is supposed to feel faster than the standard box with the 3.46.
I calculated the speed in gears to be:-


Std box/3.46diff
1st 55mph
2nd 83mph
3rd 120mph
4th 161mph

CR box/3.73diff
61mph
96mph
121mph
150mph

CR box/3.91
58mph
92mph
115mph
143mph


I cant see why especially 2nd and 3rd will feel any different in the CR/3.7diff to the standard box/3.4diff

Please help before i spend my hard earned

Thanks
N

[rev-erend]

Why not do a google search for 'Car Test' - it's a great bit of software that lets you see the effects of this kind of change of a car.

If you create you car and then a modified version and race them - on a chosen circuit or drag race then..

[VARLEYHYD]

Nick, Sorry no concise answer from me; try the calcs with a 3.73:1 with a standard box which is in my sagaris,see what you think/drive mine.
If its traffic lights take off you want this works well on mine but i find 2nd a bit short once moving, third is pretty good, with 4th maybe a little long.
Graham

[Maxx-Waxx]

rev-erend said:

Why not do a google search for 'Car Test' - it's a great bit of software that lets you see the effects of this kind of change of a car.

If you create you car and then a modified version and race them - on a chosen circuit or drag race then..

looks very impressive but it wont let me save the car i create.

[Maxx-Waxx]

VARLEYHYD said:

Nick, Sorry no concise answer from me; try the calcs with a 3.73:1 with a standard box which is in my sagaris,see what you think/drive mine.
If its traffic lights take off you want this works well on mine but i find 2nd a bit short once moving, third is pretty good, with 4th maybe a little long.
Graham

Cheers graham, the calc for 3.73 and std box comes out at

1st 51
2nd 83
3rd 120
4th 160

this assumes 7500 rpm

[JonathanT]

I think the actual final speeds are irrelevant to "how fast" the car will feel. What's probably more important is that because the CR box has the 2nd->3rd and 3rd->4th ratios set closer together which makes the difference. This probably means that you spend more time keeping the engine at peak torque (=acceleration).

1st gear looks to be longer which should mean that you are slower off the lights, but once going you'll fly!

[RichardD]

Maxx-Waxx said:

...looks very impressive but it wont let me save the car i create.

It works when purchased!

[RichardD]

JonathanT said:

...What's probably more important is that because the CR box has the 2nd->3rd and 3rd->4th ratios set ...

My Griff 500 with peak BHP at 5k revs and a SD1 gearbox means that 1st -> 3rd are too low really. Hence there are situations where on overtaking going 3rd is too slow and 4th a bit high with the ideal gear being inbetween.

The ZX6 bike I used to have was pretty much geared 1st -> 6th as 60mph, 80mph, 100mph, 120mph, 140mph, 160mph (well, 157 or something probably). No matter what the situation, the perfect gear could be selected

If you have a roadcar then you don't want 60mph in first as it won't be healthy for the clutch, and you'll want a car that can reach its Vmax, plus taller gearing is better for economy ().

Or, if your name is Daftlad and you like circuits then you want a car that will always offer the right gear at all times to make sure you and the car can reach their full potential

Having a car that can't reach its Vmax won't matter, if you hit the redline in top gear at the end of the longest straight you ever go to - that would be ideal !


I believe the CR box and 3.91 diff as used by Daftlad works out as

[Seasider]

RichardD said:

My Griff 500 with peak BHP at 5k revs and a SD1 gearbox means that 1st -> 3rd are too low really. Hence there are situations where on overtaking going 3rd is too slow and 4th a bit high with the ideal gear being inbetween.

What do you think the ideal diff ratio is for reasonably powerful 500

with
1st 2.95:1
2nd 1.95:1
3rd 1.34:1
4th 100:1
5th 73:1