Do bigger tyres really affect economy?

[joe2006]

Will going from 265/70/16 to 265/75/16 have a detrimental affect or a possibly a positive affect to economy?

To drive economically in the D40, top speed needs to be limited to just over 90kph which is at times a little embarrassing & often frustrating. Over 2000rpm it is said that fuel economy suffers.

While I understand the lever affect & the fact that it may take slightly more fuel to accelerate, bigger tyres is effectively changing the gearing & accordingly the top speed for any given RPM.

Obviously correct gearing would depend to some extent on the speed at which you generally travel & the desired rate of acceleration. Other factors such as towing may also need to be considered.

When going to bigger tyres, I understand that the accuracy of the speedometer may be closer to reality, unlike to odometer which will then be incorrect and read less distance than that actually travelled.

I wonder people are still using the odometer figures when calculating fuel usage which may go some way to explaining why consumption figures are often quoted as being greater after the tyre size change? Perhaps tread pattern may also go some way to explaining reported increases in fuel usage?

 

[Old.Tony]

It's an often confusing subject - so it's a great discussion topic.

First and foremost let's talk aerodynamics. The Navara doesn't have any. As such, the faster you go, the more force you need to make it go that fast - period. Gearing doesn't come into the equation.

Think of a bicycle. With the normal gearing you can happily cruise at about 30km/h. Why not put gearing on so that you can cruise at 80km/h? Because wind resistance is so significant beyond about 45km/h (I believe that's the figure) you simply need to exert too much energy to push the largely non-aerodynamic human body faster. Racing cyclists wear special outfits and have stupid-shaped hats to give themselves an extra tiny advantage - also, they aren't cruising, and they're athletes too.

This translates directly into any vehicle and the Navara, with its housebrick-like aero styling is going to suffer a lot worse than something sleeker like a Porsche or a Nissan 350Z.

So the cost of going faster is ALWAYS going to be higher with the same shape vehicle.

Now add a taller gearing that, as you acknowledge, is going to make the engine work harder - and so you are definitely looking at requiring more fuel to go faster.

If you were to choose a shorter gearing, your engine would rev harder to do the same speed, so you'd back it off to the sweet spot again. You could drop your diff ratios to 4:1 (I think we're running 4.3:1 now) and your sweet spot speed would fall to 85km/h.

For my vehicle, I've gone from the standard 255/70R16s to 265/70R16s - that's a 2% increase in wheel circumference, so my wheels turn over 2% less than standard tyres. My "sweet spot" has moved from 95km/h to about 97km/h (which is right mathematically as well).

My fuel consumption calculations take this into account. Here's the formula from my spreadsheet:

=IF(E187>0;(E187-F187+G187)/(C187*1.02/100);" ")

where E contains the litres bought, F contains how much of the fuel I just bought went into jerry cans, G contains how much fuel I added FROM jerry cans, and C contains the km done from the trip meter. Notice that I modify that by 1.02 to accurately reflect that I've done 2% more travelling than indicated.

There's no shame in being part of the Jerry Hatrick club.

 

[joe2006]

Tony,

I will have to take your word about the formula

So were the tyres of similar tread pattern and do you know how the pre & post tyre size change economy figures compare at 97kph?

In the good old days, 4.11:1 gears for the drag strip & 2.75:1 for the highway, unless the car didn't have enough power & would therefore use too much fuel with the 2.75:1's.

I find this stuff like time travel, the more I think about it the more confusing it becomes.

There is an optimal. Have Nissan found it?

 

[Old.Tony]

I don't know if they've found it, but they've probably done computer-modelling and graphed the various ratios to come up with something close. Odds are the entire power-plant-drivetrain is designed around providing a cruise speed of near 100km/h at optimal power output. You might be able to get better economy at slower speeds with shorter gearing OR a more restrictive entry into the turbine area causing higher turbine forces at lower RPM (more boost earlier) - but these are the fiddlings we generally don't do.

Pre-tyre-change I was averaging 12-13s around town, 10-11s on the highway (not towing). Post tyre change and the figures are hovering from from mid 12s to mid 14s (at a guess, that's from harder takeoffs). All of these figures are from tankfuls sourced from Shell or Caltex.

I've since gotten a 11.39 on a highway run (same tyres) using BP that I grabbed from Port Macquarie service centre. We did have an awesome run home on that fuel, very little headwind and pretty much just cruised - so it's not a perfect indicator of improvement and may have just been a fluke. I've been using BP diesel for a few tankfuls now and this is the first outstanding result, so I'm more inclined to think it was a case of favourable traffic/wind and a lighter foot than some bonus dispensed from the diesel bowser.

 

[ONLOCK]

Honestly, theres sweet f all difference, wouldnt bother

 

[BravoBrad]

If you compare true figures you will find very little difference. The reason people notice the difference so often is

a) heavier wheels (when not using the orignal alloys)
b) Before their speedo is out, so they were actually travelling slower (less wind resistance)
c) After the Odometer is out, so you actually do more Km's than the odometer shows.

but, if you do the calcs, taking all the variables into account, the difference will be minimal.

 

[pointblank5]

if you want bigger tyres then this is not a big enough problem to stop you, you wouldnt notice the difference unless you logged your fuel usage very very accurately

 

[gadgets]

Ive done a few tests now on my d22 since i put bigger tyres on, gps versus speedo, I did this over a few tanks and basically for every 500k I get from a tank, the gps says ive actually travelled 530k.

 

[joe2006]

I'm not changing tyres on the Nav yet but rather on wifies car. It came standard with 265/70/16 & were last replaced by Cooper ATR's 265/75/16's. I noticed no difference apart from noise when they got quite a few ks on them. I really did not pay a great deal of attention to fuel consumption although don't think it changed much. They have now done nearly 90,000ks (probably a bit more with odometer error after fitting them).

A few weeks ago the tyre shop said the AT3's were not yet available in 265/75/16 but I noticed this evening that they are now listed as available on the Cooper site.

Depending on how they go on her car, I will probably get then on the Nav when they are due. That means I will be going up from 255/70/16 (I quoted the incorrect size in the first post) to 265/75/16 or roughly 39mm in size.

Vicroads state a maximum of 50mm increase in size so I think they must have recently relaxed the regulations?

There is an article by Berrima Diesel in a current magazine discussing the amount of extra torque required by changing tyre sizes. I must read it properly one day

 

[gadgets]

I find freeway driving is still the same on bigger tyres but in the city it seems to take way more right foot to get going, went from all terrain to mud terrain too so that probably does not help.

 

[Leprechaun]

Vicroads state a maximum of 50mm increase in size so I think they must have recently relaxed the regulations?

Yep your right, new regs as of last month. http://www.vicroads.vic.gov.au/NR/r...mber8Guidetomodificationsformotorvehicles.pdf

They have dropped the 15mm increase and gone back to 50mm. So I'm legal again

 

[Dion]

Yep your right, new regs as of last month. http://www.vicroads.vic.gov.au/NR/r...mber8Guidetomodificationsformotorvehicles.pdf

Holy **** I feel like it's Christmas!!

 

[FatBoy89]

When will NSW follow this!!

 

[mike1303]

There is an article by Berrima Diesel in a current magazine discussing the amount of extra torque required by changing tyre sizes. I must read it properly one day

Which magazine was this article in?

 

[joe2006]

Which magazine was this article in?

Caravan & Motorhome on tour

I have scanned it but I don't know whether I can post it as an attachment or whatever??

It is not all that earth shattering but if somebody knows whether I am allowed to attach it I will do so later FYI.

 

[joe2006]

Well here is the document. Please remove the post if it is not appropriate.

 

[Old.Tony]

I think they're saying what we've been saying. Bigger tyres = more force required to turn 'em over = more fuel to use for the same speed.

 

[davess]

I think they're saying what we've been saying. Bigger tyres = more force required to turn 'em over = more fuel to use for the same speed.

and more brakes to stop them

 

[Sean]

I recently changed from 235/70's to 265/70 on my auto Nav. I do a bit of towing and instantly noticed that it really made the car feel gutless on the highway, as soon as I touch the accelerator it wants to change back gears, even on flat ground doing 100kph. I am really dissapointed with the result!

Cheers

Sean

 

[BravoBrad]

that is a fair change in tyre size. It would be equivilent of going from around 29" to nearly 32" (would be 32 if it was a muddy) so yes, that would knock the punch out of the engine.