Measuring over gearing %

cornish rattler

Well-Known Member

Posts

6,432

Location

Manc chester

• May 19, 2021

• #1

Hi guys
I am trying to work out what my over gearing percentage would be if i was to fit a rover 4.1 diff a 4.3 diff or a 3.9 diff, at the moment I have 3.54 diffs fitted with 235 / 85 x 16's which is making pulling away in 2nd sometimes awkward unless I'm on a slight decent so normally have to use 1st so I'm wondering which diff to go for but can't work out what the percentage would be, I know 3.54 diffs give 28% over gearing and I know if I went back to std 4.7 diffs the gearing would be to low and will lose a lot off the top end of the scale but I just can't work the other diffs out, can anyone help please

 

R

rob1miles

Well-Known Member

Posts

3,565

Location

Slough and Bath

• May 19, 2021

• #2

Go on the Ashcroft gear calculator as you can put all these numbers in and it will plot a graph, very handy!
It will plot the gear changes once you put max revs in and you can put in wheel dia.
I run 235/85, 3.54 and OD and its fine with a Perkins 4203 but reverse is too high on the S2a box. With a trailer I pull away in low range. I would not change the 3.54 as the cruising is just right

 

cornish rattler

Well-Known Member

Posts

6,432

Location

Manc chester

• Jun 1, 2021

• #3

Sorry guys i forgot about this thread, anyway it looks like i need a 4.3:1 diff / diff's so on the look out for at least one 4.3:1 diff for now, anyone got one

 

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nickjaxe

Well-Known Member

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821

Location

North Cheshire/Merseyside

• Jun 4, 2021

• #4

Why use 2nd....first is your pulling away gear.
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C

Citizen Kane

Well-Known Member

Posts

322

Location

Hampshire

• Jun 4, 2021

• #5

Ashcroft do a whole range of different CWP ratios, including a 4.12 and 4.37, take your pick. They may even to a reverse cut one for the front.

 

cornish rattler

Well-Known Member

Posts

6,432

Location

Manc chester

• Jun 6, 2021

• #6

nickjaxe said:

Why use 2nd....first is your pulling away gear.
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Click to expand...

With a petrol or a tdi you don't normally need to use 1st but its just with the 3.5 4:'s in its to high for pulling away in 2nd, yes i can use 1st but when i have the trailer on and loaded then 1st is like 2nd if you know what i mean, i could just put the original diffs in which i have but then it would be to low geared for Mway work even with an o/d so i think 4.3:1 diffs will be right.