What Sized Tyres can I fit?

Peter Aawen
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Re: What Sized Tyres can I fit?

Unread post by Peter Aawen » January 30th, 2014, 9:02 pm

Yamba, your 'very old thread post' up there isn't quite correct. There very definitely IS a point of diminishing returns on fitting bigger tyres.

In your instance/example, for your vehicle & what you are prepared to put up with in the way of compromise, going from 29's to 31's may have been manageable & could even have improved your fuel economy, and you are right that the Rocky engines & gearing can certainly handle tyres that are a little larger that 29's; buuut, our vehicle must have had different gearing to yours, cos our figures were quite different to those you quoted, and of course, the point of diminishing returns - if you tried fitting 33's, you'd find it is probably a whole different ball game!! Sure, it's gonna be even taller & have longer legs/fewer revs per km, but on just about all of them it will be beyond the ready ability of the engine to turn them with ease, regardless of what stock gearing yours came with; it's also likely to have lifted the truck so that the air resistance will be increased exponentially, adding to the engine load even more just to push the bigger brick thru the air; and the bigger tyres will really whack around the low range gearing/crawl speed too, & some find that important!!

We ran 33's on a 95/96 RV TDi for a while, then dropped back to 32's cos of all that above, and ended up settling on the 31's as being the best compromise all around for us, altho many wouldn't/didn't like it and it did whack around our fuel economy and low range gearing a bit, as well as pick up from stop & a few other things (welll, until we changed the gearing, anyway...) & we did do a little bit of other modding too, in order to help get it thru some of the harder stuff that we regularly tackled that needed low down grunt and a low crawl speed.

So it's probably not quite correct to say that "Anyone saying putting on bigger tyres is a bad thing does not have a clue, or are just stupid" - it's just that going up to 31's might not be an issue for you/your vehicle/the way you drive, while it could well be for others (who might really need/appreciate the lower gearing to tackle the harder stuff more often) and going even bigger, say up to 33's, IS almost certainly going to be an issue with stock gearing, in more ways than are obvious at first glance/thought, so maybe people who say that sort of thing are a just little smarter than you might think... ;)
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Yamba24
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Re: What Sized Tyres can I fit?

Unread post by Yamba24 » January 30th, 2014, 9:39 pm

Peter,
Your earlier post advised the owner of the thread to not even go to 31", and stay at 29".
Peter Aawen wrote:While 31's might make it look good and even give you a little bit more ground clearance, the gearing changes will be pretty hard to live with, especially if you like working it in the tough stuff off road...
Sorry mate, but I think that is deadset bad advice, and by the look of the thread, the guy went with the smaller dia tyre on your recommendation. Maybe not "does not have a clue, or are just stupid", maybe instead just "wrong". Scaring people by talking about changing gear ratios or living with a sub par vehicle is not helpful to anyone. If that bloke ever went up to 31" tyres, he would be regretting forking out for smaller ones on your advice, I am very sure.

I agree that for our conditions, 31" is probably the working limit, but this has nothing to do with
air resistance exponentially adding to the load even more...
(all nonsense) but is down to the simple fact that going up 2 tyre sizes is already flirting with being over sized in NSW according to the continuously changing technical specifications. To stay legal, you would need the change to be engineered. Also, anything bigger than 31" will involve either spacers (illegal in NSW) or body modification - with the extensive work required to make the tyres fit requiring an engineers certification. Usually tyres bigger than 31" would also require spacers on the drive shafts, and I have seen a few that require the engineering of completely new shafts incorporating an extra UJ to make the angles work. Was your vehicle properly certified? Blue slip? Plates?

I have a 94 F78 with brand new 31" highway tyres care of the previous owner. I am getting around 9.12l/100km, cruising at 101km/h care of GPS (showing 94km/h on the dash), and am quite happy with that!

I suppose I should have started my post with "I know this is a very old thread, but for those who read this and got bad advice, here is the truth".
At least nobody annoys me when I am alone

Peter Aawen
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Re: What Sized Tyres can I fit?

Unread post by Peter Aawen » January 30th, 2014, 9:54 pm

Funny how most of the other Rocky owners who also posted about what they found when they fitted 31's to their Rocky's back then pretty much agreed with my comments, isn't it?! Given what they said as well, even tho some admitted they'd do it again despite the issues & one said he found it OK, effectively what you are saying now is that most of the people who posted in this thread back then are lying thru their teeth, but that you are delivering the undeniable truth!! :eek:

I think I'll just settle for being happy that when asked, I told the OP what my experiences revealed & made a recommendation based upon them, a recommendation that also happened to be pretty much supported at least to some extent by the other posters and their experiences too - and I managed to do it without telling anyone else they either didn't have a clue or were just stupid! What he did with that is up to him, just as what people do with your advice now is up to them. ;)

Ps, our vehicle was fully certified & engineered for the mods it had done to it back then, which may be a whole lot different to what needs to be done these days!
An Ex-Service person is someone who thought enough about their country & how great it is, how lucky we are to live here, to write a blank cheque made out to 'The People and Commonwealth of Australia' for the value of 'Up to & including my Life!'

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