# Plus-sizing, Tire pressure, & Load capacity (MKIV Jetta wagon & General)



## flashfasbo (Oct 29, 2005)

So many questions. Tire pressure seems like it should be so simple but I'm finding it anything but clear. The forums here and elsewhere, as well as tech pages from tirerack and others, have provided some insights, but some of the forum advice is completely incorrect (like to pump your tires to whatever pressure is marked on the sidewall!! :nono: )
Anyway, I will add more detail and other links to threads and sites I've been referring to later on, but for now I'll just state the basic facts and questions to get the ball rolling.
I am a new owner of a 2006 Jetta TDI Wagon (mkiv) and the sticker on the driver's door says simply to use the following pressures:
*FRONT: 36 / REAR: 45*








I've read that on previous model years they used to list two sets of pressures, one for full load, and one for light load. And I believe the numbers listed on my car (any new car?) are the "full load" values.

So the first question would be... what should I use when I don't have a full load? I've read some posting a formula to use as a baseline:
_Vehicle weight (lbs) / 100 +2 psi, + another 2 psi for fronts assuming FWD._
For my vehicle, according to spec sheets from vw.ca, the weight is approx 3000 lbs, so the above formula gives 34F, 32R.
Sounds reasonable for the fronts, but that number is waaay less than the vw-recommended pressure for the rears. But I've also read for wagons this may be due to the fact that a fully-loaded wagon may indeed have most of the weight in the back... so.... I guess all of this may still be reasonable and not contradictory.
[just as a side comment, doesn't the 45 psi recommended by VW come surprisingly close to the max pressure of many tires? My new tires have a max of 51 psi.]

*Complicating things further* though is that I've plus-sized my wheels and tires from the stock 15" 195/65R15 to *18" 225/40R18*. This is where things start to get unclear to me. Some opinions I've read have simply stated that you use the door-marked pressure no matter what the tire size. But some manufacturers list higher pressures for plus-sized rubber (say, 1 psi per plus size), which would appear to contradict the one-pressure-fits-all recommendation.
Also interesting is that the VW dealer that installed the tires for me said that they just left the tires at the pressure they were inflated to when they were mounted/balanced by VW before shipping them up here, and that those should be the correct pressures for them. Guess what? All 4 were set to approx 45 psi.

*Even more unclear* is how load-capacity figures into this. My new tires are rated at 88, and haven't looked up yet if that's the same as the stock tires were rated. But my rough understanding is that if the plus-sized tires have a lower load-cap than orig rubber then you may need to compensate with more pressure... or something like that.

Anyone care to wade-in to this one?


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## Sheep (Dec 21, 2000)

*Re: Plus-sizing, Tire pressure, & Load capacity (flashfasbo)*

Actually, inflation becomes a bit more significant the wider/lower aspect ratio you go since there is less sidewall to absorb shocks from bad roads/dividers, RR tracks, etc etc. With a 225/40x18, you'll experience far more feedback into steering and cabin than on 15 or 16"ers, even 17's. But you for that sacrifice over 15/16's you gain better handling and steering response. I went from 17's to that size on my previous Jetta and you just need to experiment and monitor pressure. You can start safely at around 38lb (I go a bit softer in rear because there's no weight back there and that's where you get the road-noise back into cabin). I hope you upgraded your suspension, even over a factory spt suspension. It won't handle the increased grip on turns if you have hi-perf 18's, unless you're going to drive conservatively anyway.
Rotation is extremely important, no more than 5K between swaps. Watch your wear patterns across tread and adjust as necessary depending also upon how much harshness you can take.
I had an H&R/Bilstein upgrade from OEM spt susp on my Jetta4. Had good wear using 38f/36r (never went above that) with the 225/40's on 18x8's, but I don't mind a bit of harshness. The wider the rim means you usually can add a bit more pressure. We have excellent roads down here though.
I believe those pressures are for fully loaded vehicle. There is no way you need/want those pressures for only 2 people and unloaded. 45lb in an unloaded vehicle on 225/40x18's will guarantee you a very harsh ride.


_Modified by Sheep at 7:43 PM 11-11-2005_


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## Alex @ Tire Rack (May 3, 2005)

*Re: Plus-sizing, Tire pressure, & Load capacity (flashfasbo)*

Your OE pressures are exactly as stated 36 F 45 R. 
Euro metric tires ( with out P before the tire width)
like these may have higher inflation pressure maximum ratings BUT maximum load is achieved at 36 PSI. The additional inflation pressue is used in the rear to stabilize the rear's capability for additional load bearing capacity.
Your OE tires have a maximum load of 1356 lbs ( achieved @ 36 psi) 
All 225/40/18's that are euro metric have a maximum load capacity of 1389 lbs. 
This being said you have exceeded your OE's internal volume therefore OE pressures are more than adequate. 
If you care to tailor them personally a probe style pyrometer would allow you to analyze the tire's acutual working tendancies, by compairing tread temperatures across the tread face.
My guess on the high side inflation pressures is 2 fold - 
A - its a wagon, meant to be loaded
B - Most deisel buyers are concious of mileage 
(a motovating factor for most when selecting deisel) 
Tires with higher inflation pressure create a narrower and longer contact patch (the crown or dome when Floored on inflation pressure) this position while it created uneven wear it yeilds the least rolling resistance - therefore the best mileage.
Alex


_Modified by Alex @ Tire Rack at 4:52 PM 11-11-2005_


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## IndigoBlueWagon (Aug 9, 2004)

*Re: Plus-sizing, Tire pressure, & Load capacity (flashfasbo)*

I have an '02 TDI Wagon and it does offer two sets of tire pressues: for up to half load it says 28 front/26 rear. That's with 15" wheels and 195/15 tires. BTW, I think the OE Michelins are load rated at 91, a little higher than your 18" tires.
FWIW, my '97 shows pressures for two different 14" tires (185 and 195 section) as well as 195/50-15s (on the Jetta GLX of that vintage). All the pressures are within a pound or two of each other but they do suggest higher pressures for the lower profile tires.
I think 28/26 is too soft, but the 2 lb. differential (with the rear lower when lightly loaded) is something I follow. I'm not a mileage freak and I like a good ride so I typically run 32/30. I've arrived at this pressure after much experimenting, and I now have 205/55-16s on the car.
With 18" wheels I'd start with 34/32 or 36/34 and fine tune from there. You do want to be careful to not run low profile tires too low as you can be subject to impact damage to the tires and wheels if you drive on bad roads.


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## flashfasbo (Oct 29, 2005)

*Re: Plus-sizing, Tire pressure, & Load capacity (Sheep)*


_Quote, originally posted by *Sheep* »_Actually, inflation becomes a bit more significant the wider/lower aspect ratio you go since there is less sidewall to absorb shocks from bad roads/dividers, RR tracks, etc etc. With a 225/40x18, you'll experience far more feedback into steering and cabin than on 15 or 16"ers, even 17's. But you for that sacrifice over 15/16's you gain better handling and steering response. 

Yes, the thinner profile and increased exposure to pot hole damage etc, is definitely something I'm keeping in mind. I am not unhappy with the ride at all compared to the stock 15" wheels and didn't really find the difference in road noise significant... at least on these tires (Michelin Pilot Sport a/s). 
I just went out and took some profile measurements of my tires, and here is what I found (as measured from the bottom edge of the wheel to the concrete floor:
*F: 2-5/16" (36.5 psi)
R: 2-1/2" (34.5 psi)*

















_Quote »_I hope you upgraded your suspension, even over a factory spt suspension. It won't handle the increased grip on turns if you have hi-perf 18's, unless you're going to drive conservatively anyway.

Nope, not yet. Still debating my options. To lower or not to lower. But definitely would benefit from tightening/firming things up a bit. But even on the stock (non-sport) suspension, the grip is unbelievable. The roads have been consistently wet around here since I bought this thing, and these tires on these wheels are still easily the grippiest setup I've ever had. I have to push amazingly hard to get the ESP to kick-in at all! So sweet.









_Quote »_I had an H&R/Bilstein upgrade from OEM spt susp on my Jetta4. Had good wear using 38f/36r (never went above that) with the 225/40's on 18x8's, but I don't mind a bit of harshness. The wider the rim means you usually can add a bit more pressure. We have excellent roads down here though.

If I go with a lowered setup, I'm pretty decided now on going with the Eibach Pro-System package(approx 1.3" lower front and rear, and approx 20% stiffer springs with matched dampening), but... I'm not 100% decided I want to lower at all, (fearful of losing practicality for driving to trail-heads, getting to ski areas, putting mountainbikes on a hitch rack, etc). The roads are great here too, btw.

_Quote »_I believe those pressures are for fully loaded vehicle. There is no way you need/want those pressures for only 2 people and unloaded. 45lb in an unloaded vehicle on 225/40x18's will guarantee you a very harsh ride.

I can vouch for that! For the first day after the dealer mounted them, I was driving around with 45 psi all around! I could have used my car to detect cracks in the pavement.



_Modified by flashfasbo at 12:32 PM 11-13-2005_


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## flashfasbo (Oct 29, 2005)

*Re: Plus-sizing, Tire pressure, & Load capacity (Alex @ Tire Rack)*


_Quote, originally posted by *Alex @ Tire Rack* »_
Euro metric tires ( with out P before the tire width)
like these may have higher inflation pressure maximum ratings BUT maximum load is achieved at 36 PSI. 

How did you determine this? Is it something you calculate? Or do I have to refer to the manufacturer's specs or something?

_Quote »_The additional inflation pressue is used in the rear to stabilize the rear's capability for additional load bearing capacity.

So increasing pressure above 36 psi does not increase my load bearing capacity? But it seems that _decreasing_ the pressure does actually decrease my load cap, right? 
Here's a related question. Is there a way to accurately calculate a _minimum_ safe pressure? I want to know what is the safe *range* of pressures I could use, so I can experiment without impacting safety. If (as you say above) 1356 is the max load cap, achieved at 36psi, I suppose the minimum would be just enough air pressure to carry the weight of the that corner of the vehicle? 
If the entire vehicle (empty) weighs approx 3,000lbs, then very crudely speaking, each tire carries 1/4 of that weight, or 750lbs. Obviously with an empty wagon, the front would be heavier due to the engine, so let's assume a 60/40 weight split (guessing). With a 60/40 split, the weight on each front tire would be 900lbs, and each rear would carry 600lbs. So how low could one go with the pressure while still providing enough to safely carry these weights? 


_Quote »_This being said you have exceeded your OE's internal volume therefore OE pressures are more than adequate. 

You're saying my low-profile 18s actually have more internal air volume than the stock 15s? Am I correct in saying that increased air volume gives more load bearing capacity?
Thanks for the info.


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## flashfasbo (Oct 29, 2005)

*Re: Plus-sizing, Tire pressure, & Load capacity (IndigoBlueWagon)*


_Quote, originally posted by *IndigoBlueWagon* »_I have an '02 TDI Wagon and it does offer two sets of tire pressues: for up to half load it says 28 front/26 rear. That's with 15" wheels and 195/15 tires. 

Wow that is low, but instructive. Does your "full load" show the same recommended pressures as mine (36F/45R)?


_Quote »_With 18" wheels I'd start with 34/32 or 36/34 and fine tune from there. 

Yeah, that basically the range I'm playing with now: 36F/34R.


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## dennisgli (May 17, 2003)

*Re: Plus-sizing, Tire pressure, & Load capacity (flashfasbo)*

Mine says 30F/28R for half load and 32F/41R for full load. But it isn't a wagon and the maximum load is 925lbs and not 1109lbs. Plus I assume that a wagon is heavier in the rear? This is for 15, 16, or 17inch wheels - 18 shouldn't be any different.
I've always assumed that if 1109lbs was full load then 555lbs would be half load - ie. one person in the front seats; one and a half in the back seats; and the rest in the boot.


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