Best rpm at 100km/h

Submitted: Thursday, Jan 06, 2005 at 22:28
ThreadID: 19132 Views:3884 Replies:3 FollowUps:9
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Hi all,

I have done my conversion on the GU from RD28T to TD42T and now need to work out the optimal gearing for the diesel. Currently with 285's and 4.88 ratio's it sits on 2624rpm at 100km/h in 5th. I have reworked a RPM calculator that I have and if I can work out what this type of engine should be reving at 100km/h then I can work out my ratio's. So far I have worked out (running 285's):

4.88's - 2624rpm
4.66's - 2488rpm
4.3's - 2312rpm
4.11's - 2210rpm
3.9's - 2097rpm

I have heard rumours that diesels should sit around the 2100-2200rpm mark so am figuring on 4.11's. Any help or feedback would be appreciated. If you could provide ratio, tyre size, speed and rpm it would be great.

Regards
David
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Reply By: Eric Experience. - Thursday, Jan 06, 2005 at 22:37

Thursday, Jan 06, 2005 at 22:37
David.
A lot depends on the aplication, ie towing or steep roads etc, but that vehicle with that motor is fitted with the 3.9 in the factory so thats what I would use. Eric.
AnswerID: 91682

Follow Up By: Member - David C (QLD) - Friday, Jan 07, 2005 at 07:46

Friday, Jan 07, 2005 at 07:46
Hi Eric,

Many thanks, I will be doing a bit of towing with it (1500kg camper) but want to get it set up for optimal revs and fuel consumption first then have it tuned for extra torque.

The (new) TD6 runs 3.9's but with 275/65/17 tyres (31.07 inch) which equates to 2228rpm at 100km/hr
The TD42T ran 4.111's with 265/70/16 tyres (30.6 inch) which equates to 2384rpm at 100km/hr

As I have the taller tyres, I am thinking of 4.111's to match it to the standard TD6's figures ie 4.111's with 285/75/16 tyres (32.83 inch) which equates to 2222rpm at 100km/h (plus the 4.111's are more common so easier/cheaper to obtain)

I have the intercooler from the old RD28T so when it is all set up it will be the same as a TD6 anyway.

Regards
David
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Follow Up By: Geoff- Friday, Jan 07, 2005 at 08:09

Friday, Jan 07, 2005 at 08:09
David,

GU1 TD42T Wagon - 3.9's with 265/70/16
GU1 TD42T Ute - 4.11's with 265/75/16

Cheers
Geoff
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Follow Up By: Michael ( Moss Vale NSW) - Friday, Jan 07, 2005 at 08:36

Friday, Jan 07, 2005 at 08:36
Hi Eric, My GU 4.2tdi at 120kph on the speedo, sits on 2700 rpm, similar revs to your 100kph. I do 1200 km's per week on the freeway so im happy with that, But if i was doing less freeway and more weekend out 'n' about, those current gears would be great. It would bring Low range first gear to where it should be.... Michael
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Follow Up By: Michael ( Moss Vale NSW) - Friday, Jan 07, 2005 at 08:37

Friday, Jan 07, 2005 at 08:37
OOoppps, i meant david...
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Follow Up By: Member - David C (QLD) - Friday, Jan 07, 2005 at 09:00

Friday, Jan 07, 2005 at 09:00
Hi Michael,

Just checked the calculator and yes it matches your figures almost exactly - 2714rpm at 120km/h. Mine (in current configuration ) would be 3165rpm at 120km/h which is almost an gear difference.

Regards
David
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Follow Up By: Member - David C (QLD) - Friday, Jan 07, 2005 at 09:10

Friday, Jan 07, 2005 at 09:10
Hi Geoff,

My figures come from the Nissan brochures that I have downloaded over the years (this one was 22 June 1999). It has the 4.111's listed as standard for the TD42T wagon. Not saying that I am right or you are wrong or anything but just what Nissan has presented.

As indicated in the brochure, the only wagon to be released in the GU series 1 that had 3.9's was TB45E. Unfortunately it does not cover the utes.

I have uploaded the brochure to my site if anyone is interested - Patrol.pdf.

Thanks
David
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Reply By: Geoff- Friday, Jan 07, 2005 at 09:24

Friday, Jan 07, 2005 at 09:24
Hello David,

My GU1 TD42T wagon has 3.9's.
So does every other GU TD42T wagon that I know of..........
It sits on 2100rpm at 100k (accurate) with 285/75's.

Regards
Geoff
AnswerID: 91728

Follow Up By: Michael ( Moss Vale NSW) - Friday, Jan 07, 2005 at 10:32

Friday, Jan 07, 2005 at 10:32
Geoff, the 3 litre tdi has a slightly lower diff than the 4.2tdi and makes a fair difference to the revs in 5th gear, it must be 4.11... The 4,88 is really low but would make ease of steep up and down hill. mmmmmm
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Follow Up By: Geoff- Friday, Jan 07, 2005 at 19:26

Friday, Jan 07, 2005 at 19:26
Michael,

I think you'll find the 3L has something like 4.3's (4.375-ish from memory). They're also a heavier duty rear unit (h260 vs h233 in the 4.2's) in the earlier manual version of the 3L. Same type as 4.8L.

My TD42T with 3.9's and 285/75's handles the load (TrakShak camper and effectively 4 adults plus gear) pretty well on the highway.
Lower gearing would be better for tight stuff, but I don't do a lot of that.
The 3.9's suit my use, but may be a bit tall for others.

Cheers
Geoff
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Reply By: Michael ( Moss Vale NSW) - Friday, Jan 07, 2005 at 10:36

Friday, Jan 07, 2005 at 10:36
David, did you buy a complete donor car, A conversion in a modern car is a big job these days if you have to chase all the required parts..??!!!!!
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Follow Up By: Member - David C (QLD) - Friday, Jan 07, 2005 at 19:02

Friday, Jan 07, 2005 at 19:02
Hi Michael,

I didn't do the conversion myself but instad have somebody do it for me. We did have a donner car that we migrated the parts from so I was sure to get everything I needed.

In the end we needed quite a bit but we could have spent more time fiddling around with things and done it with less.

Regards
David
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