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Mustang Technical Discussion > Pre 1973 > gear ratios > Community Forums > Mustang Forum Australia - Mustang Tech

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123abc
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I have a 351 FMX with 2.75 ratio in the rear. I want to change these as the orig ones were 3.00. Would 3.25 be a better all round choice?


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nassi
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What are you using the car for? At the end of the day everything is a compromise if it isnt a single purpose build.
cheers


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boofhead
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The 2.75 is a great gear for pure cruising on the freeway. For example at 2000 RPM your going 92 Km/H.

A 3.00 is a common diff ratio for the early performance oriented models So at 2000 RPM your going 84 Km/H.

A 3.25 is a good upgrade to be a gear for cruising and improved acceleration. So at 2000 RPM your going 78 Km/H.

A 3.55 is a ratio oriented more for acceleration though great cruising if you have an over drive. So at 2000 RPM your going 71 Km/H.

A 3.80 is perfect for the performance guy that still go on occasional highway trips. So at 2000 RPM your going 67 Km/H.

A 4.11 is as high as I would go for a non-over drive street car. Main use is performance driving. Highway driving not very pleasant. So at 2000 RPM your going 61 km/H. Good acceleration - traction can be an issue in the wet for an aggressive driver.

You just need to choose what your likely to be using the car for and pick your ratio to suite. The above calcs are based on 26.5 " wheel and final ration of 1 (aka No over drive).

Note: Best combination is high gear with an overdrive - best of both worlds.


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123abc
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nassi bit of everything really. It just seems really crap off the mark with 2.75's. It came factory with 3.00 according to the Marti. Just looking to improve my bottom range but still have all round drive ability.


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nassi
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boofhead (I hate calling a man I haven't even met something like that), has pretty much nailed your options. Its down to pick and choose.


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ozbilt
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Mrsozbilt's 69 Mach is a 351 2bbl FMX with a 3.00 open rear end. Car accelerates really well (carb has been modded, but that is all) & cruises nicely on the open road with decent mileage .....


Kerry

It is easier to get older than it it is to get wiser

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boofhead
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You will have no problem once you have ....

123abc - If you changing your ratio you might consider what center to run... not that I am trying to make this to complicated.


EDIT: I re-read my first sentence and I worded it incorrectly... silly boofhead...


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Last edited by boofhead on Sun Jun 09, 2013 10:51 am; edited 1 time in total

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123abc
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I'm getting the center, I have, done. Thought I would take the opportunity to change it while out. 3.00's sound good.


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123abc
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Next question. Does any one know the correct speedo drive to go with 3.00s and FMX ?


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boofhead
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Your choices are (tooth count and color code): 16 (blue for AT, orange for MT), 17 (green AT, purple MT), 18 (grey AT, green MT), 19 (tan AT, pink MT), 20 (orange AT, blue MT), 21 (purple AT, red MT)

Toploader
16/6 = 2.567-2.767
17/6 = 2.728-2.940
18/6 = 2.888-3.113
19/6 = 3.048-3.286
20/6 = 3.209-3.459
21/6 = 3.369-3.631

C4 (assuming same for FMX).
16/7 = 2.200-2.372
17/7 = 2.338-2.520
18/7 = 2.475-2.667
19/7 = 2.610-2.814
20/7 = 2.750-2.964
21/7 = 2.888-3.113 <- this combination


You need to find out what gear is in the gearbox then find the cable version to suite your ratio.

Note: Had a quick search and found this which might help

www.transmissioncenter..._Gears.htm


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Exhausted
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Thanks for the info boofhead. Mine has a 3.25, truetrac centre & still lets go far to easy under light acceleration at 100 kph in the wet. Back end is always trying to beat the front end.


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nassi
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Exhausted wrote
Thanks for the info boofhead. Mine has a 3.25, truetrac centre & still lets go far to easy under light acceleration at 100 kph in the wet. Back end is always trying to beat the front end.


Have you tried left foot braking?


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boofhead
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Exhausted - Remove the lead foot and put the single spinner back in. Poke


BTW: I am running a 3.0 in the Cougar A code 4 speed top loader. I current have 3.55 in my 65 FB (C4 though currently going T5 - hence off the road at the moment).


I will someday think of something clever to say.

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Ausjacko
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This is a great diff ration thread that would be a good reference. Is there somewhere to copy this to?

Good work fellas.
Jacko


'68 J-code GT Fastback
'67 S-code GT coupe, 'Pink Bitz' formerly known as 'Hookin' up a brother'
'69 M_____ GTS Fastback 'Blasted'

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123abc
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checked my build sheet it has green listed as the drive color so if i follow that it should be right green 17 tooth for 3.00s behind a fmx


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