Mate got 4/11's an me 3.7 yet rpms same?

Hello all. First off my mate is confusing to talk to thats why im on here asking the experts.

We both went to 2 different mechanics, i got 3.7's an he got 4.11's trutrac diff, yet at 80kms on the highway they both rev the same, 2000rpms.

My mate says thats how a trutrac diff works but that just sounds funny to me. My only conclusion is that he mechanic insists they are 4.11s but
the diff place he sent it off to actually put 3.7's in it. Also he didn't have his speedo calibrated at this point. any ideas?

Also they are both 5.7 litre utes he has the vu maloo an i have the vz ss. I have 20 inch wheels an he has 19s.'

Also just to let you know, i have try-y's with 3.5 sureflo exhaust an a growler intake, he has twin 3 inch with 4 into1s and mine beats his, you'd think
would flog mine. Still doesn't make sense how they can rev the same tho.

Didnt get the speedo calibrated.. or you didn't.

get them side by side on the road at the quoted 2000 rpm see the difference !!!

Its either different overall diameter of the tyres. Or re calibration or wrong gear put in.

One wouldn't be an auto and the other a manual would it?

Converter locking up in one and not the other?

Without knowing exact tyre sizes we are guessing, you may have different rolling diameters offsetting the diff ratio.

Are they manual or auto?

80km/h @ 2,000rpm seems high for both cars
Based on a 235/45/17 which most tyre combinations should be close to the same rolling diameter to be legal, and an auto with the stall convertor locked up (which is should be at that speed unless an aftermarket one is fitted) 3.7=1723rpm and 4.11=1898rpm.

On my website qldkev.net I have a spreadsheet where you can plug in tyre combinations and see the revs with different gear ratios

If the speedo isn't re-calibrated then the car is not actually doing 80km/h. Remember, the RPM is that of the motor, the rpm sender doesn't know or care about the diff and gear ratios. Speedo however, does need to know.
If you have a GPS speedo (plenty of phone apps) you'll soon find out.

Otherwise there's simply not enough information. IE. manuals/autos, different autos (is one 4 speed and the other 5/6?) etc.

Quote:

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Originally Posted by Crazykent

Hello all. First off my mate is confusing to talk to thats why im on here asking the experts.

We both went to 2 different mechanics, i got 3.7's an he got 4.11's trutrac diff, yet at 80kms on the highway they both rev the same, 2000rpms.

My mate says thats how a trutrac diff works but that just sounds funny to me. My only conclusion is that he mechanic insists they are 4.11s but
the diff place he sent it off to actually put 3.7's in it. Also he didn't have his speedo calibrated at this point. any ideas?

Also they are both 5.7 litre utes he has the vu maloo an i have the vz ss. I have 20 inch wheels an he has 19s.'

Also just to let you know, i have try-y's with 3.5 sureflo exhaust an a growler intake, he has twin 3 inch with 4 into1s and mine beats his, you'd think
would flog mine. Still doesn't make sense how they can rev the same tho.

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OlMate is a Muppet.....

get him to recal his speedo, he would be doing 60km/h

Have a read of this thread....

http://www.ls1.com.au/forum/showthre...iff-Gearing-A4

Should give you some answers

As someone else suggested, the 80kmh speedo readings you both claim are most likely incorrect, so one way to at least eliminate one unknown is to read the speeds using a GPS. That way you know you're at least working from a correct base.

You can confirm speedo accuracy with a simple test.
Line up at either end of a longish road and drive towards each other at an indicated 80km/h.
As you pass each other, reach out and do a "high 5".
The person who's hand hurts most is going slower.

It's the only way to be sure.


HTH

Just jack up the rear end, turn the tyres around and count how many times the tailshaft rotates for each rotation. . Then you'll know for sure what ratio is in it.

Quote:

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Originally Posted by darcy

Just jack up the rear end, turn the tyres around and count how many times the tailshaft rotates for each rotation. . Then you'll know for sure what ratio is in it.

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Now thats a good idea!

Gonna do the gps thing too. Thanks guys. Gonna be awkward if they've put something different in haha

Dont you guys think its weird that my car still beats his quite easily? An also his flogs mine from a rolling start like mine does to his from a standing start. Cheers.