D40 fuel consumption with bigger wheels

[GunMentalGrey]

Hi all, just got back from a quick highway trip and expected the fuel consumption to be a bit lower than usual, but it was higher.

‘07 STX manual with around 190k on it. Prior to fitting bigger wheels (286/65/17) on the highway id expect around 10% less fuel use. This time it actually used more than usual, at 110km/h over a tank full. With virtually all of it at freeway speeds. Calcs showed 11l/100km when I’m getting around 10.4 around town. I’ve done 2 similar trips with bigger wheels with same results. Both times with one passenger and a couple of overnight bags so almost unladen.

I know the bigger wheels have skewed it a bit because of travelling further per wheel rev, but I’m comparing consumption with the current wheels. With stock wheels round town was calc at around 10 with dipping to low 9 on highway.

Any thoughts or experience with fuel consumption on the highway after a 50mm lift and 33” tyres on? Or should I be looking for engine problems?

 

[Old.Tony]

The lift makes little difference. I've found that the biggest effect adding to fuel consumption comes from speed.

The faster you go, the more aerodynamic drag the car experiences. It is really obvious above 80km/h when my fuel consumption rises from about 6-7LPHK (60km/h) to 9-10LPHK (80km/h) to 15-16LPHK (100km/h). Admittedly I have a roof basket up top and a full canopy creating a large flat area at the rear, which at speed creates a low pressure area trying to pull my car backwards.

Changing to larger tyres sometimes means a longer lever, but in your case you've chosen a lower profile than stock, so your road circumference has actually reduced from 2522mm to 2520.5mm. The change is so minor that fuel consumption shouldn't be altered by anything noticeable.

 

[GunMentalGrey]

The lift makes little difference. I've found that the biggest effect adding to fuel consumption comes from speed.

The faster you go, the more aerodynamic drag the car experiences. It is really obvious above 80km/h when my fuel consumption rises from about 6-7LPHK (60km/h) to 9-10LPHK (80km/h) to 15-16LPHK (100km/h). Admittedly I have a roof basket up top and a full canopy creating a large flat area at the rear, which at speed creates a low pressure area trying to pull my car backwards.

Changing to larger tyres sometimes means a longer lever, but in your case you've chosen a lower profile than stock, so your road circumference has actually reduced from 2522mm to 2520.5mm. The change is so minor that fuel consumption shouldn't be altered by anything noticeable.

Thanks Tony.

The tyres are significantly bigger. 285 wide, so equivalent to 33”. Combined with the lift the whole truck sits about 75mm higher. The speedo has gone from reading 10% over to almost spot on due to the bigger rolling diameter.

Previous to the tyre swap I’d always got better fuel economy after freeway trips. Usually use the GPS to stick to the limit, 110km/h both with stock tyres and the larger so aero shouldn’t be making a difference, unless the lift hurts it. Also haven’t changed the configuration of the ute. Soft tonneau, no roof racks etc, just he wheels and lift are different.

Our r51 pathfinder runs stock tyres and normally drops from 10l/100 around town down to low 9 or high 8 after highway driving. It’s an auto but that previously doesn’t seem to have made a lot of difference to revs at speed.

It’s just a bit odd to me all other cars I’ve measured consumption on have used less on the freeway compared to round town, but I’ve never made a change like bigger wheels before.

 

[Old.Tony]

Yeah I did the math on the wheels, the width contributes to the axle height because the profile (in your case, 65) is a percentage of the width as the wall height, so your tyre walls should be 185.25mm high vs 185.5mm on a standard 265/70R17 tyre. This means your axle based on this tyre should sit 0.25mm lower than with a standard tyre.

Unless I've got the standard tyre size wrong?

 

[MANNING]

@GunMentalGrey - what was the original tyre size.?

the tyre size you quoted -

...bigger wheels (286/65/17)

- dont seem to be as tall as your saying..
my tyre size calculator says 31.6"

Lots of variables when working out fuel consumption.. even if the tyres were excatly the same rolling circumference, the wider tyres would be giving different fuel usage and then you've got all the other long list of things too..
how are you working the fuel average?

 

[GunMentalGrey]

My bad, new tyres are 285/70/17. Stock is 255/65/17. The axle is about 25mm higher.

I record all fuel usage in an app. Fill to full each tank, record odometer. It could be wider tyres causing more drag.

I was interested to see if anyone else who had upsized tyres had similar experiences.

 

[Cowboy550]

I seen the same result from the old XB ford ... a mate put in a really tall geared diff for the highway ... but ended up using heaps more fuel and shortly after removed it again, to maintain the speed you had to give it a boot full all the time, like driving up a hill in top gear with your foot to the floor instead of changing down a gear and using half the throttle, even thow the rev’s go up your still only using half the pedal,
also the top speed was reduced not increased because the 6 cylinder engine wasn’t strong enough.

worst fuel economy ever a mate filled his tyres with water for better traction like on his tractor, he said it worked but you can guess the result... hehehe less than 100km per tank

 

[Cowboy550]

Online says 07 STX ran 255/70/16 ?

 

[GunMentalGrey]

Oops forgot original wheels were 16” but I upgraded to later model 17” stock tyres before going to now aftermarket wheels and bigger tyres.

 

[220]

My fuel consumption went to shit after fitting bigger tyres. That was until I realised the odometer was reading under by 10%. When I factor in the real world km not much difference.

 

[GunMentalGrey]

My measured consumption is about the same for fuel used per wheel rotation, calculated off litres in and odometer. Just happened that each rev goes a bit further now due to bigger tyre diameter I haven’t corrected for that.

The main thing that has changed for me is using more at highway speeds rather than less like it used to.

 

[MANNING]

Just happened that each rev goes a bit further now due to bigger tyre diameter I haven’t corrected for that.

Ive never in depth, chased my fuel consumption so don't really know excatly what the car measures and how.. i would assume it's set for standard size wheels..?
If this is the case and the graph you posted is what it says, then every wheel rotation your seeing an unrecorded 8.6% wheel travel.. Thats no really to far off the 10% in your original post

..(286/65/17) on the highway id expect around 10% less fuel use. This time it actually used more ..