Thread: 18" Tyres V 19" Tyres

Originally Posted by discojools

Just a couple of points re your discussion: With regard to tyre sizes in Vic the rules have changed for offroad vehicles from an increase of 15mm to 50mm for offroad vehicles.

Good news...but please cite the regulations where this is stated. NCOP doesn't count, it's not been adopted by Victoria.

Originally Posted by rmp

SNIP
The harder and further you go offroad, with a heavier load and the more often, the better the low-profiles become. SNIP

Hi rmp,
I think that this is the comment to which Gordon was referring. I agree that it is probably a typo and inconsistent with your more recent posts.

Best Wishes,
Peter

All eloquently put Robert, and all true. But you can't put 18" tyres on a D4 3.0 ltr, so it's pointless arguing that the car is not as good as it could be. I'm not disagreeing that a mythical D4, with 18" tyres would marginally outperform one on 19's - that's simple physics (but only if it had the 19" car's brake package). I'm arguing that a current D4 3.0 ltr on 19" tyres will out-perform a current D4 2.7 ltr on 18" tyres. Re-reading my last post, maybe that wasn't quite clear:

1. In Sand: As you previously pointed out (and we've done countless times), for sand one can air down a 19" AT to 16psi comfortably. It will climb dunes and wander through soft sand with as much ease as those on 17's and 18's, simply because there is enough longitudinal footprint to apply it's torque advantage easily. And it's not just numbers, but the speed of engine reaction to throttle placement - much better with the 3.0 ltr motor - remember the D3 lag?

2. In rock/shale: About even - the 3.0 ltr driver will need to be more careful with wheel placement, and should limit tyre pressure to 25psi. But he'll have better traction control, better HDC performance, better engine response. I've cross-axled - many times - my 3.0 ltr at these pressures. So far no dramas to report!

3. Dirt road touring: I haven't done long-distance touring with 19" tyres yet. Some of my clients have, one with MTR's, and a couple with the Pirelli ATR's. The tyres were never flagged as a weak point (and they did some serious milage on these trips, as you do in WA), but I don't recall what tyre pressures they ran at. The 3.0 ltr however, is again more economical, more responsive and a better tower.

4. Long distance bitumen: 3.0 ltr

I don't think an argument can be made that the 2.7 ltr on 18" tyres is a better package.

On a side note, before people race off to retro-fit 2.7 ltr brake packages. It's not just an argument as to whether the effect on on-road peformance is noticeable. The difference in the two brake systems also influence the traction control performance - it's sensitivity and heat dissipation. One specific issue - I've had the HDC overheat on my D3, whereas the same tracks with the RRS didn't raise any concern. Admittedly the RRS is a little lighter than the D3, but I think the point is still valid.

Cheers,

Gordon

Hi Gordon,
How did you go with the 18"rims that may or may not have fit?

Robert,
Here is the quote from Vicroads Vehicle Standards Bulletin 14 from page 16:

OVERALL DIAMETER
Section LS Tyres, Rims, Suspension & Steering
The overall diameter of any tyre fitted to a passenger car or passenger car derivative must not be more than 15mm larger or 26mm smaller than that of any tyre designated by the vehicle manufacturer for that model.
The overall diameter of any tyre fitted to an off-road passenger vehicle or a commercial vehicle must not be more than 50mm larger or 26mm smaller than that of any tyre designated by the vehicle manufacturer for that model.
Speedometer accuracy must be maintained for the selected tyre and rim combination to within the degree of accuracy specified in ADR 18 where applicable.
The web address is:
http://www.infrastructure.gov.au/roa...g_3Feb2006.pdf

Hope you can find it.

By the way, have just fitted 245/70/17 BFG MT KM2 to my D3 as an alternative to my 18" Grabber AT2s. These KM2s are hardly noisier than the Grabbers and so far on road I am pretty impressed. Am looking forward to giving them a good test on a Sunset Country trip in a few weeks.. Will report after that.
Strangely the KM2s are not available in 265/70/17, a much more common size for Toyotas and Nissans.

Oh rats,I have just realised that this info is from the NCOP. However my local Tyrepower man told me that Vicroads have change to that recently. I am sure I came across that NCOP Bulletin through the Vicroads website...Here's hoping!!

Originally Posted by discojools

Oh rats,I have just realised that this info is from the NCOP. However my local Tyrepower man told me that Vicroads have change to that recently. I am sure I came across that NCOP Bulletin through the Vicroads website...Here's hoping!!

Yes, that's NCOP...which is NOT law in Victoria at this time anyway.

With all due respect to your local TP man I've found the big chains to be notoriously poor at knowing what they are talking about. For example, I've just been involved in un-doing fitment of incorrectly load rated tyres to a 4WD which had been recommended by a "professional".

Originally Posted by gghaggis

All eloquently put Robert, and all true. But you can't put 18" tyres on a D4 3.0 ltr, so it's pointless arguing that the car is not as good as it could be.

I wasn't arguing that point at all, I was discussing hi vs low profile in general.

The point is relevant for those considering a D4. For really tuff trips I'd go a 2.7 D4 with 17s over a 3.0 with 19s unless it's all sand.

Originally Posted by gghaggis

I'm arguing that a current D4 3.0 ltr on 19" tyres will out-perform a current D4 2.7 ltr on 18" tyres. Re-reading my last post, maybe that wasn't quite clear:

Hmmm...I think it would be situation-specific. On sand, yes. On really rocky terrain where the extra 3.0 grunt can't be applied...no. Much of a difference overall...not really. We're not talking about big differences here, it's not like the difference between say a mud terrain and road tyre. Also I would compare 17s to 19s given the 2.7s can work with 17s.

Originally Posted by gghaggis

1. In Sand: As you previously pointed out (and we've done countless times), for sand one can air down a 19" AT to 16psi comfortably. It will climb dunes and wander through soft sand with as much ease as those on 17's and 18's, simply because there is enough longitudinal footprint to apply it's torque advantage easily. And it's not just numbers, but the speed of engine reaction to throttle placement - much better with the 3.0 ltr motor - remember the D3 lag?

In sand I agree the extra power of the 3.0 will give it an advantage. The low profiles are less of a problem there. I also would take the 3.0 with 19s over a 2.7 with 18s for a sand drive.

Originally Posted by gghaggis

2. In rock/shale: About even - the 3.0 ltr driver will need to be more careful with wheel placement, and should limit tyre pressure to 25psi. But he'll have better traction control, better HDC performance, better engine response. I've cross-axled - many times - my 3.0 ltr at these pressures. So far no dramas to report!

Here's where I think a 2.7 with 17s would have an advantage through lower pressures, and the 3.0's extra power isn't the benefit it is in sand. I'm not saying the 3.0 isn't capable, not at all, both vehicles will get there.

Originally Posted by gghaggis

3. Dirt road touring: I haven't done long-distance touring with 19" tyres yet. Some of my clients have, one with MTR's, and a couple with the Pirelli ATR's. The tyres were never flagged as a weak point (and they did some serious milage on these trips, as you do in WA), but I don't recall what tyre pressures they ran at. The 3.0 ltr however, is again more economical, more responsive and a better tower.

The 3.0 isn't very much more economical than the 2.7 in cruise mode, but that's nothing to do with the tyres. My concern about 19" tyres with dirt-roading is resistance to rock damage. Mileage for me depends on where you go not how far.

Originally Posted by gghaggis

4. Long distance bitumen: 3.0 ltr

I don't think an argument can be made that the 2.7 ltr on 18" tyres is a better package.

Agreed, nobody has made that argument from what I can see. However the 2.7/18 is a VERY good package, even if the 3.0 is better.

Originally Posted by gghaggis

On a side note, before people race off to retro-fit 2.7 ltr brake packages. It's not just an argument as to whether the effect on on-road peformance is noticeable. The difference in the two brake systems also influence the traction control performance - it's sensitivity and heat dissipation. One specific issue - I've had the HDC overheat on my D3, whereas the same tracks with the RRS didn't raise any concern. Admittedly the RRS is a little lighter than the D3, but I think the point is still valid.

It is valid, I also made that point elsewhere. The days of easy retrofitting mods are gone with the inter-linked computer age. I never use HDC as I consider it largely a waste of time, probably another argument there I suspect ;-)