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afternoon gentlemen, I have a v6 with 18" alloys with grabber tyres on , I put 39 in the front 45 rear. is this correct ? someone mentioned to me that those pressures
are for 17" with normal tread tyres. any help would be appreciated.


regards dave.
 

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If you are running fully laden, yes. If not those pressures will make the ride very harsh. 38 all round is a good compromise.
 

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If you are running fully laden, yes. If not those pressures will make the ride very harsh. 38 all round is a good compromise.
I have a Pathfinder with 18in rims and recently swapped road tyres for Grabber AT3s. Nissan recommend 35psi front and 35-40psi rear (increasing to 44psi when fully laden). On the road tyres I ran 35 front and 40 rear upping to 44 rear when towing caravan with fully laden truck. On the AT3s I started with 35 all round, then quickly increased that to 40 rear when I checked the fuel consumption. But am still losing 2 to 3 mpg on the Grabber AT3s. I can't drive any more sedately than I am at present so am faced with 3 options: put up with it, play about even more with the pressures, or get it remapped. Is 2 to 3 mpg loss normal when swapping to AT tyres? Any comments welcome !!
 

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I'm guessing losing only 2-3 mpg is probably very good, compared with standard road tyres, even with optimal pressures.

A remap, depending who you believe, is unlikely to improve mpg and runs the risk of clogging the DPF - just an observation and not based on proper science or big statistics... I heard Billcar no longer offer remaps for towing vehicles, owing to DPF issues, but I'm not certain.
 

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I have a Pathfinder with 18in rims and recently swapped road tyres for Grabber AT3s. Nissan recommend 35psi front and 35-40psi rear (increasing to 44psi when fully laden). On the road tyres I ran 35 front and 40 rear upping to 44 rear when towing caravan with fully laden truck. On the AT3s I started with 35 all round, then quickly increased that to 40 rear when I checked the fuel consumption. But am still losing 2 to 3 mpg on the Grabber AT3s. I can't drive any more sedately than I am at present so am faced with 3 options: put up with it, play about even more with the pressures, or get it remapped. Is 2 to 3 mpg loss normal when swapping to AT tyres? Any comments welcome !!
Are the grabbers the same size? Reason being that if they are larger you will get an mpg drop, not a real drop, just that the car doesn't know it is going further for each tyre revolution.
 

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I run Grabbers on my D22 and I find the mpg isn't that good, I am thinking of changing next time to the new Mitchellin Cross Latitude tyre to see if that makes a difference
 

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Yes - never trust the 'computer' mileage. It is calibrated(-ish) to the standard tyre circumference at standard pressure. Always measure mpg based on what the fuel pump reads on a fill up, against actual mileage covered.

Nevertheless some tyres types do have a higher rolling resistance and will affect MPG even if identical in circumference...
 

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Are the grabbers the same size? Reason being that if they are larger you will get an mpg drop, not a real drop, just that the car doesn't know it is going further for each tyre revolution.
Good point, thanks, but yes the Grabbers are identical size - different speed rating but that's the only difference.
 

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Yes - never trust the 'computer' mileage. It is calibrated(-ish) to the standard tyre circumference at standard pressure. Always measure mpg based on what the fuel pump reads on a fill up, against actual mileage covered.

Nevertheless some tyres types do have a higher rolling resistance and will affect MPG even if identical in circumference...

Yes, I suspect rolling resistance too. The Grabbers seem to sit more square, so a bit more tyre in contact with the road. And I doubt that increasing the pressures is going to make much difference to the way they sit but, on the other hand, I'll be grateful for that extra grip when off-road :smile2:. Trying to keep a positive spin on things (apologies for the pun).
 

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Well, you let that pun slip, so get a grip, man.
Don't slide off, when the pressure is on, just stay in the groove...
 

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Good point, thanks, but yes the Grabbers are identical size - different speed rating but that's the only difference.


A brand new AT tyre can have a rolling diameter a full inch or more greater than a HT tyre with the same size stamped on the sidewall. Or even compared to a fully-worn version of itself.
 

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A brand new AT tyre can have a rolling diameter a full inch or more greater than a HT tyre with the same size stamped on the sidewall. Or even compared to a fully-worn version of itself.
Thanks JDO, that makes sense. Great website - such a wealth of knowledge out there among you guys
 

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A quick bit of maths says that the standard tyre at 31" in diameter is 97" in circumference.

A 32" tyre is 100" in circumference.

So if your diameter has gone up by 3% (ie 3") then the fuel consumption reading will appear to be 3% worse. It isn't, but that is what the gauge will tell you.
 

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I have been reading this and now have a question .
I have Cooper discoverer AT3 tyres fitted they are inflated to 35psi as per the info on the door label .
My question is should I maybe increase this pressure and if so how much the car is only used on road
 

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I have been reading this and now have a question .

I have Cooper discoverer AT3 tyres fitted they are inflated to 35psi as per the info on the door label .

My question is should I maybe increase this pressure and if so how much the car is only used on road


The door placard pressures are for on road driving, so follow them.

For off road driving you’d use anywhere between a little and a lot less than that.
 

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Thank you

I must have read the previous posts and miss understood it then , I thought it was saying the AT tyres had different pressures to road tyres

If you have changed the tyre size from standard, then you may also have to change the pressure. Sorry, I didn’t connect the two questions in my mind.
 

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Thank you
I must have read the previous posts and miss understood it then , I thought it was saying the AT tyres had different pressures to road tyres 😁
Generally no. Tyre pressures are about maintaining the tyre shape on the bit that contacts the road.

The best way of testing is to put a broad stripe of chalk across the tread, then drive in straight line for a few yards. The chalk should wear off evenly, if it is too much in the middle, reduce pressure, too much at the edge increase pressure.
 
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