Diff Ratio

[Guest shaggy]

I'm sure its been asked and answered before, but how do you tell what the diff ratio is? Not sure if mine is right for the revs as 4000RPM gives me a speed of around 75MPH and would ideally like this to be a little lower rev range, but without knowing the ratio, I don't know what to look for as a replacement.

 

Thanks

Si

[Guest shaggy]

Just had a reply from Dave Castle. When I checked it last night, I was sure I was doing something wrong as the prop turned on 2.2 revs to one rev of the wheel. Is this possible for a granada diff?

[Guest MacMustang]

Just had a reply from Dave Castle. When I checked it last night, I was sure I was doing something wrong as the prop turned on 2.2 revs to one rev of the wheel. Is this possible for a granada diff?

That seems very low. I would have expected at least 3? If you have got a mad 2.2 diff in there it will have a high top speed but low acceleration. Mind you I didn't realise there was such a thing as a 2.2?

[Longboarder]

Mark the prop flange. Align with some sort of pointer. Raise one wheel by jacking. Mark the wheel and arrange pointer. Rotate wheel (easier than rotating prop) while counting prop turns. Accurately once is OK if you have a protractor on the prop. As you don't then give the wheel several turns. Ten is a good number as it keeps the maths easy. Work out number of turns of prop for one turn of the wheel. Double and that is the diff ratio. Likely numbers are 3.38, 3.62 and 3.92 but there are others.

 

Nigel

[Guest shaggy]

Cheers Nigel. Just checked and its a 3.64.

As its a bit worn anyway, I could really do with lowering the ratio, so, if anyone out there has a 3.38 or a 3.14 stuck in the back of their garage, needing a good home, let me know.

Si

[Mitch]

Simon, is this the hood or the fake snake?, cant see the hood with pinto needing to be lower, and if anyones got a 3,14 diff lieing at the back of there garage, they dont know how much its worth!!!

 

 

Mitch

[Guest chris brown]

Simon. If you are talking about the Hood then something is wrong with your calcs as a type 9 box (or MT75) with standard ratios matched to a 3.62 diff and 15" wheels gives 71 mph at 3000 rpm (give or take one or two mph). Come to think of it, it should be about the same in the "fake snake" as 5th is always higher than 1 to 1 regardless of the box.

 

This is the maths for 1000 RPM in TOP gear using a 3.82 diff. Found it on the RS forum wrong diff but it gives you the idea.

 

(A): 60 x 1000(RPM) =60,000 - (60 is because were looking for miles per hour which is 60 minutes). 60,000 is your first number.

 

(: 913(revolutions per mile of the tyre/wheel,195/50/15 in this equation)x3.82(diff ratio)=3487.66, This then needs to be multiplied by the gear ratio(top gear=0.85), so 3487.66 x 0.85= 2964.511.

2964.511 is your second number.

 

©: Divide the first and second numbers; 60,000/2964.511 = 20.239425mph.

So your speed per 1000rpm is - 20.239425mph.

 

If you want to know any other speed/rpm then just change the gear ratio to the one you want.

 

MPH @ 1000RPM in each gear is:

 

1st = 5.326165 mph/1000 rpm.

2nd = 8.0390242 mph/1000 rpm.

3rd = 11.623994 mph/1000 rpm.

4th = 15.498659 mph/1000 rpm.

5th = 20.239425 mph/1000 rpm.

[Bob Tucker]

I'd start by checking the speedo.

Use a GPS to get an accurate speed & see what the speedo says.

 

If you calculated accurately & got 3.64, that sounds like an LSD IIRC.

 

Bob

[Guest shaggy]

In answer, its on the fake snake, and speed was calculated against another car doing 70mph, which showed up on my speedo as 85, but revs were just over 4000.

Not sure if its a LSD or not

[Mitch]

Si, Pauls just come down to a 3,62 from a 3,9. should make crusing much more fuel efficiant, and being a v8 wont effect 0-60 times too much,

 

if its against another car, who says there speedos correct anyhow, you need to check

A your revs are correct

B you speedo against a GPS system (satnav) dont forget you now have large wheels and tyres on, your speed will need calebrating to suit, diff, tyres, and gearing

 

C you may be worried about nowt,

 

are you sure its 3,64. you could take off the plate on diff and check there, as its written on the gears inside.

 

 

Mitch

[Bob Tucker]

to check if its an LSD

 

jack up both rear wheels

turn one wheel forwards

if the other one turns the same way (ie forwards) its an LSD

if the other wheel turns backwards its not.

 

As Nigel says, 10 turns to be accurate, but a quicker way(for a non LSD) is..

jack up one wheel, mark the prop flange & the wheel.

Turn the wheel TWO full and accurate turns & count the turns of the prop flange.

Most likley ratios are 3.62 or 3.92.

Its easy to tell if the prop flange has tuirned just over 3 and half (3.62) or nearly 4 (3.92)

 

Not sure if this will work if its an LSD????

Bob

[Guest Paul Morgan]

Hi Simon,

 

As Mitch said I've just swapped mine before Le Mans from 3.92 to 3.62. It now does 70mph at 2900rpm using LT77 (SD1) rover gearbox. Confirmed by my sat nav.

 

I wouldn't go lower than this though, don't want to loose acceleration.

 

Paul

[Mitch]

spoke with Andy windows today, his Cobra is doing about 2700rpm at 70, dont know ratio, but we think its a XR4 doner,

 

 

Mitch

[Guest shaggy]

OK, so following another road test today 70mph against sat nav and another car showed tacho at 3500 RPM.

 

Diff is a bit clunky, but not so bad as to worry too much yet, although a lower ratio would be better just to keep the RPM down.

 

Double checked the wheel vs prop turning, and is deffo a 3.64.

 

Thanks for the advice, and simple calculations (Chris), much appreciated.

 

Si

[Guest salty_monk]

Yep, 3.62. A 3.92 would be closer to 4,000 from memory.

 

Dan