need help final drive ratios vs diff ratios

[69royal]

as part of my hunter build i will be converting from 3 speed auto to 4speed manual with overdrive and will be running 185/60/13 tyres even tho seth and nomad have said that with od a 70 profile is better for more relaxed cruising at speed so what I'm wondering is if I'm better keeping my original rear axle or the od rear axle specs are as follows

3 speed auto final drive 3.7
rear diff ratio 2.094

4 spd o/d final drive 3.89
rear diff ratio 3.569
 
thanks

[gt6m]

totally lost on this,
a final drive, is your diff ratio.

so if you got a final drive { diff } of 3.89
then with an od box you would end up with a final drive figure of

3.89 share 25% {OD ratio } = 3.11

and a 3.70 would be 

3.70 / 25% =  2.96

So a 2.96 final overall ratio will be better for cruise / mpg

But, only if you got the power / torque to pull a higher gear.
as you may find that you will get a reduced rev cruise.
But, also less MPG, as your throttles are open moer, thus useing moer fuel.

But, you will only be looking at about 1/2 MPH between a 3.11 and a  2.96
so should be OK.

or am i reading it wrong,!!!

Markus

[marinacoupe]

I had a 25% overdrive driving the 3.63:1 rear axle on my tuned Marina 1800, which equated to a 2.72 axle ratio in overdrive fourth.

A 3.7:1 diff with a 25% overdrive is equivalent to 2.77:1. You'll need lots of torque in the engine to pull that uphill and probably dropping out of overdrive into fourth or overdrive third.

The 3.89:1 with 25% overdrive is equivalent to 2.92:1 would be a little better.

I changed over to a Sierra type 9 box with a fifth gear giving a 17% overdrive.

If it's a choice of one or the other axle I'd go for the 3.89:1 and overdrive and tune the engine for torque.

If you find it slightly overdriven, you can use a low profile tyre to take the edge off the gearing.

I can do all the comparative calcs for you including the tyre ratios if it helps.

Don't forget to use the correct speedo drive to get the speedo correct.

[gt6m]

MC, are you sure your figs are correct,!!

[marinacoupe]

Hi gt6m

I think so, but always happy to be corrected.

I recall the Super Minx / Hunter originally had an Overdrive as standard, a little research would uncover the original ratios which would be the best thing to target.

Anyone?

[marinacoupe]

Been doing some research on

www.automobile-catalog.com/make/hillman/hunter_minx/hunter_saloon/1967.html

It seems that the Laycock overdrive from a Hunter was 20% overdriven rather 25% (mine was ex-1500 Spitfire). So if you are using an ex-Hunter overdrive the final drive diff ratio would have been 3.89:1, with a 20% overdrive that comes down to an equivalent of 3.12:1, which I reckon would be lovely. The factories usually got the development of these things right given their budgets.

[69royal]

thanks for the help guys i thought od axle would be ideal just was trying to see if could compensate for smaller rolling radius of tyres all  that ratio stuff boggles me lol

[marinacoupe]

It's just maths, multiplication, division and percentages.

[nomad]

suggestions

0. find out what's on the car now. Jack up one wheel, lie down so you can see propshaft (mark propshaft if there's no unique marks on it). Rotate roadwheel exactly *2* full turns counting how many times prop turns. More than 4 = you have 4.11 diff, slightly less than 4 = 3.89, lots less than 4 = 3.7
Don't believe the manual (I found mine could have 3.89 or 4.11 fitted originally ), yours might now have a scrapyard *anything* ratio
1. fit o/d box as planned, suck it and see when driving for a while, change if needed.

"Plan B"
Strip out the Hillman axle, fit a Ford - then take your pick from the 97 ('ish) Ford ratios available

... or PM me in a couple of weeks, I have a complete 3.89 axle that might soon be spare (halfshafts not included )

[gt6m]

Here ye go Ole Boy, in this, can put any thing in any box,
there be a ratio there to suit your needs.

can mix match diff cars stuff to others too,
quite handy thingy, spen some time farting aboot, see what you come up with.

www.mintylamb.co.uk/gearspeed/

Markus

[69royal]

@gt6lm thanks for the link