Do bigger tyres really affect economy?

[Dave]

265/70/16 is roughly a 31 inch tyre.

Heavier and bigger tyres take more effort to turn them.

and more brakes to stop them

^ What he said to.

 

[BravoBrad]

Your right Dave. For some reason I thought I read 265/75. even so, still a 2" jump

 

[D21 dualcab Z24 4X4]

On my old D21 ute I went from 29AT to 33MT. On the monash freeway inbound going down the hill to warrigal road, I put it in 5th at 90kh and lost speed.... Went back to 29 and excellerated up that same hill in 5th outbound...

 

[Guido]

To the OP......YES.

Went from 265 70 AT to 265 75 AT and lost about 40-50km's to a tank. Was getting nearlly 600 now getting 560ish. Nearlly all town driving so the effect off the lights was more than imagined. Needs more pedal and more clutch. If the tyres weren't cheap I wouldn't have got them. Needless to say I'll be dropping size next set of rubber.

Guido

 

[joe2006]

Guido,

Were you relying on the odometer to calculate the distance travelled and if so did you adjust the calculations to account for change in accuracy of the odometer post fitting the larger tyres?

All this stuff is quite interesting.

 

[LLadnar]

Well guys, I can't do the maths, but since I put 285/75/16's on my ute, doing long runs out too mudgee and back, my fuel usage has dropped. I have suffered from a loss in torque, noticed it most around town stopping and starting, but out on the golden highway my engine speed in down and it just cruises along alot easier.

 

[nakedape]

Well guys, I can't do the maths, but since I put 285/75/16's on my ute, doing long runs out too mudgee and back, my fuel usage has dropped. I have suffered from a loss in torque, noticed it most around town stopping and starting, but out on the golden highway my engine speed in down and it just cruises along alot easier.

This type of experience is kinda what I thought might happen - more effort (fuel) getting it going but less effort (RPM & thus fuel) to keep it at speed (e.g. 100kph).

Arghh - now my head hurts. Where's Dr Karl when you need him?

 

[Al]

Hes headed down to the Antarctica over Christmas, I believe he goes every year on a cruise boat thingo as a guide. Guy must be raking it in.

 

[nakedape]

Hes headed down to the Antarctica over Christmas, I believe he goes every year on a cruise boat thingo as a guide. Guy must be raking it in.

Yep - and I'm as jealous as all hell!

 

[ben85]

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

So 33s are legal on a d22 now?

 

[Guido]

Guido,

Were you relying on the odometer to calculate the distance travelled and if so did you adjust the calculations to account for change in accuracy of the odometer post fitting the larger tyres?

All this stuff is quite interesting.

Joe, can't say I thought about that as having a effect on my odo.

Is it true then with bigger tyres you are effectively putting less km's on your vehicle than the distance actually travelled?

Guido

 

[joshman]

Actually, more if my math is correct. I know the difference between 265/70/16 and 265/75/16 is about 3.4%

So my fuel calculations go like this:
[Liters / (Odo * 1.034)] * 100

Should give you an accurate reading of L/100km (for my tyre size)

Edit:
Done the math, the 285/75's are about 7.3% BIGGER in rolling diameter to the stock 265/70/ 16 tyres.

So if your speedo reads true on 265/70/16's, then on 285/75/16's you need to drive with your speedo at ~92km/hr and you'll actually have a road speed of 100km/hr. what this means is that when you do your fuel calcs, multiply your speedo reading by1.073 before you do the calc.

Eg. You got 600km out of 60L on 265/70/16's, that's a nice easy 10L/100km.
You up the tyre size and suddenly your odo is only reading 540km out of the same 60L. Ok. Our calculation for this would look like:

[60L / (540*1.073)]*100

Which would give us a true fuel consumption reading of:

10.35L/100km.

Please note that I totally made these values up as an example. Use your own figures to do the proper calculations.

 

[fzrr]

Joe, can't say I thought about that as having a effect on my odo.

Is it true then with bigger tyres you are effectively putting less km's on your vehicle than the distance actually travelled?

Guido

I've found this to be the case. The trip from the servo to my house used to be 4kms on the stock tyres. Now it shows 3.8kms after going up to 265/75/16 MTZ's.

So my odo shows less kms then whats actually been travelled. If my maths is correct then for every 95kms I see on my odo i've actually done 100.

 

[mojo]

Wouldn't a simple analogy be - "getting rid of first gear and adding a sixth gear" on my STR ? Let's say I increase my tyre size enough so that taking off from standstill would be the equivalent of taking off in second gear with my current tyre size. Once I can overcome the hurdle of starting the car off from standstill, then power/acceleration would be the same no matter what tyre size, correct? And if I'm doing a lot of freeway driving, this is by far my best option.
(I'm a bit late to add to the discussion, but I'm about to purchase new tyres soon)

 

[tommy222220_0]

Wouldn't a simple analogy be - "getting rid of first gear and adding a sixth gear" on my STR ? Let's say I increase my tyre size enough so that taking off from standstill would be the equivalent of taking off in second gear with my current tyre size. Once I can overcome the hurdle of starting the car off from standstill, then power/acceleration would be the same no matter what tyre size, correct? And if I'm doing a lot of freeway driving, this is by far my best option.
(I'm a bit late to add to the discussion, but I'm about to purchase new tyres soon)

Weight, engine power/torque also comes into it.

Also how efficient the engine is at certain RPMs. Thats why most people fit exhausts and intercoolers, increases the efficiency of the motor.

If the gear ratios/tyres are to tall, the engine wont be able to push the car and you'll be constantly changing down on hills.