FE-FC Holden Discussion Forum

                G'day all, i have a really really winey diff and as i am about put an aussie four speed in shortley i was wondering what is the best ratio diff for cruising? , i have a HR rear end in now .
                                            Cheers Rusty

depends on the cruizing-
cruising for chicks on main street you should go a 3.89, its the loudest, makes your motor seem beefier and luach better at the lights as you race upto 15mph, best one to pull attention, makes a bigger scene and snaps back around the block quick when you want to take a second look at that hottie on the sidewalk.

cruising to work inthe city go a 3.55, it's nice and low for zipping through the traffic in the morning and you get more use out of all four geers, good for raing the opposition commuters of the lights and for quick merges.
cruising to work in outer suburbs 3.36, 3.55 might be too low if you are frequently hitting 70 or 80 and you will get better fuel economy with the slightly higher diff.

cruising to work in the country,  stretch your legs a bit and wind er out without revving the t*ts off the motor, 3.08 is a nice 100 kph diff, you cant feel the accelorator pedal work and still have a relaxed trip.

cruising outer the state- go a 2:78, break some land speed records good for long trip 100kph or higher best travelling economy, just dont go expecting it to wheel stand at the lights, you have to take the good with the bad.

cruising all round, if you work in the city, country, cruise for chicks, and do long distance trips a 3.08 is the best all rounder but is dearer than the 336 and 355.
also remember you can go a little higher with the diff(thats lower numbers) with the extra gear becuase of the ratios are closer than a 3 speed meaning it wont load up as much when starting off in the next gear.
also remember 4th is the same as 3rd in the 3 speed and not a overdrive so your top speed is the same as a 3 speed.

Your gearing is also effected by your rear wheel diameter. Taller tyres mean taller ratio ie 2.78s, smaller tyres make a smaller ratio ie 3.55s.

Rusty
The other FC (commonly termed  as f'ing C) ::) had a depress four speed with 14" wheels and 3.55 ratio, just perfect around town and on the highway, it was mounted behind a grey.

Effie C

     I'd say ditto to FC Cool's reply.Whilst we are on a diff ratio subject I'd like to ask a question.Years ago Street Machine mag had an article on calculating engine RPM with various diff ratios.The formula for calculating took tyre circumference and other things into the equation.I used this for calculating RPM and diff ratios on various projects of mine over the years with good results very close to actual revs when the vehicle was dynoed.        My problem is now I can't find the article or the calculations to use and I can't remember either.I want to calculate theoretical RPM at various MPH readings useing a number of ratio/tyre combinations.
  Would anyone have a copy of this article or their own proven calculations to help me???  Thanks Terry.

Thanks for the info guys , cheers Rusty

I've got a Supra box in mine, and have had 3.55, 3.36 and now a 3.08. It's pretty much spot on. Could possibly go higher, though I think 2.78?? would be too high....maybe without 5th gear it would work, though I suspect first may be too low.  ??? ???(motor is a very mild 186 with 300 holley)

hey
just a quick question.. where is the best place to source another diff ratio from? can any diff just be adjusted somehow or get another one?
thanks

You can't adjust a diff ratio specifically.  You replace the whole diff centre with another that has taller or shorter gearing to suit your application.  (Taller = higher number, = quicker take-off = less suitable for highway.  Shorter = lower number = longer "legs".)

The diff centres from banjo diffs (FE-HG) are more or less interchangeable.  Later ones as well, probably.

cheers
RET