Fuel consumption on 285/70r17's

[spoondog]

Hi, this has probably been covered before but can't find much on it. I'll be up for new tyres within the next few months on my 3.6 jk and am wanting to go to 285/70 r17's from the stock 245 75's, my only concern is fuel consumption. I get offroad when I can but most of my km's are on the highway at 100kph and I clock up about 3500k's per month so can't really justify 285's if I'm going to get smashed too much in the fuel department. Can anyone share their experiences changing to bigger tyres so I get a bit of an idea? I realise it's dependant on a lot of other factors too but just a ballpark figure would be great. My back up option is just play it safe and go to 265/70's.
Thanks

[Big Dave]

I didn't notice any significant change in fuel consumption going from stock 245's to 285x70/17 muddies on my 12 3.6 manual and I have corrected the speedo with a procal so it's accurate.

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[Banshee]

I think I noticed maybe 1L/100km more. That's going to muddies as well, from A/T. May have been other things added around the same time, but calculated after speedo correction. (GPS to fine tune it to be accurate for distance/speed.)

That is calculated from Km traveled & litres used to fill tank. (I got into the habit, 30 years ago, of always recording it.) That's in my '08 3.8 manual.

[Nanook]

That's good to know as I'm getting close to ditching the 265 muddies and going up to a 285 as well.

I just have to modify my garage door first to give me a bit more height so I'll fit.

[StuieG]

I have Cooper Discoverer STT's in 285/70 R17 on my JKU CRD and I don't complain about it. Around town my fuel economy is horrible, but i live in the Eastern Suburbs of Sydney and every road is like a car-park. Out on the highway and even off-roading, no complaints at all.

I haven't recalibrated mine, but I've used 2 different GPS speed measurements at the same time and clocked it against what my speedo reads and it's 99% accurate. The speed i'm showing is what i'm doing to within 1%, usually all vehicle from factory can be as much as 10% out

[nevillebartos]

305/65/17 and I average around 14L/100km in the shorty, long highway inclines are the worst and keep having to knock back as far as 4th which is not helping. Putting around town is actually better and seems to drink the 12 - 13L I was seeing on stock. Based on phone GPS measurement speedo is now precise, need to be mindful for cameras.

[Banshee]

Originally Posted by nevillebartos

305/65/17 and I average around 14L/100km in the shorty, long highway inclines are the worst and keep having to knock back as far as 4th which is not helping.

I have to do that with the 285's! But I also have a 3.8 manual, so...

Invest in a Procal or similar, so you can recalibrate the speedo easily. Particularly if you have an auto, otherwise it can play merry hell with the shift points, so I hear. I just want to have an accurate speedo!

As StuieG said, the speedo is not necessarily accurate from the factory. As per ADR 18/03:

TEST OF SPEEDOMETER ACCURACY
FOR CONFORMITY OF PRODUCTION
1. Test conditions
The test conditions shall be as set out in paragraphs 5.2.1. to 5.2.6. of this Regulation.
2. Requirements
The production shall be deemed to conform to this Regulation if the following relationship between the speed indicated on the display of the speedometer (V1) and the actual speed (V2) is observed:

In the case of vehicles of categories M and N:
0 ≤ (V1 – V2) ≤ 0.1 V2 + 6 km/h;

In the case of vehicles of categories L3, L4 and L5:
0 ≤ (V1 – V2) ≤ 0.1 V2 + 8 km/h;

In the case of vehicles of categories L1 and L2:
0 ≤ (V1 – V2) ≤ 0.1 V2 + 4 km/h.

Off road 4wd is category M. What this means is that at 100kph (actual), the speedo can legally read anywhere between 100kph and 116kph. Although my TJ & JK were both very close. Motorbikes though are L3 & so can read up to 118 at 100kph. In practice their odometer is usually spot on & the speedo typically reads +10%. On all the bikes I have owned if I do, say 80kph actual, the speedo is reading 88. I have never had a cop look at me twice when doing 122kph indicated on the motorway in a 110 zone, or when I pass then doing 88 in an 80 zone.

It also means that you can be doing 100 indicated and be actually travelling at only 85kph! No wonder some of those slow drivers give you dirty looks. They might think they are travelling at the speed limit and have no idea.