what was I thinking..??

Silly (stupid) me; I have miscalculated the size when I bought a second set of wheels with A/T's, in order to preserve my 265/75/16 MT tires.

I just realised that by 215/70/16, the 70 stands for the % of the tread with (see picture below).

--Now; does this improve my fuel consumption? (it feels like there is a lot less power needed to move along)

--If I need to use my MT as a spare wheel would that cause any damage to the diff???
I know, a lot of questions, but I kinda regret now.
JS.



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Reply By: garry r - Saturday, Feb 23, 2013 at 17:12

Saturday, Feb 23, 2013 at 17:12
Hi there silly, one big problem that stands out is that your speedo will be well out of wack. Some where on this site is a tyre diameter to speed conversion of one size to the other. As for placing two odd size diameters on the vechile is one will rotate faster than the other and I would expect some very angry spider gears in the diff yelling to you. Good luck. I replaced all my tyres including the spare when I go away, and swap the normal road tyres back on again to save wearing out my good ones for off-road. When needs be, I shall change half used off road tyres onto my alloys, to put new ones onto the steel rims, then it won't matter if I need to use and odd steel or alloy.
AnswerID: 505394

Reply By: Member - John and Val - Saturday, Feb 23, 2013 at 18:18

Saturday, Feb 23, 2013 at 18:18
Hi,

Suggest Google tyre markings to find an explanation of just what all those numbers mean. There's a lot of information there if you know how to decode it!

265/75/16 means it fits a 16" rim, and the height of the cross section "balloon" is 265mm which is 75% of its width.

215/70/16 also fits a 16" rim the height though is only 215mm from the rim, and that is 70% of the width of the "balloon".

That 50mm difference makes a change of about 13% to tyre diameter, so the speedo will read 13% high and of course you'll need 13% more RPM to achieve the same road speed with the smaller tyres. You will not be able to mix sizes without damaging your diff, so you'll need the right sized spares too!

Sorry....... think I'd try to sell them and start again.

Cheers

John

It will work ok, but





reserve my 265/75/16 MT tires.

I just realised that by 215/70/16, the 70 stands for the % of the tread with
J and V
"Not everything that can be counted counts, and not everything that counts can be counted."
- Albert Einstein

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AnswerID: 505398

Follow Up By: Keir & Marg - Sunday, Feb 24, 2013 at 07:52

Sunday, Feb 24, 2013 at 07:52
It's actually the circumference of the tyre that drives the speedo, and the circumference changes as the square of the diameter, so a 13% change in diameter means a 17% change in the speedo reading. Hope that helps!
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FollowupID: 782351

Follow Up By: Member - John and Val - Sunday, Feb 24, 2013 at 08:29

Sunday, Feb 24, 2013 at 08:29
No!

The circumference is proportional to the diameter. It is the AREA of a circle that is proportional to the square!

Cheers

John
J and V
"Not everything that can be counted counts, and not everything that counts can be counted."
- Albert Einstein

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Follow Up By: Keir & Marg - Sunday, Feb 24, 2013 at 12:57

Sunday, Feb 24, 2013 at 12:57
Oops, sorry John - you are entirely correct - my bad
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Follow Up By: phil300 - Sunday, Feb 24, 2013 at 21:37

Sunday, Feb 24, 2013 at 21:37
"215/70/16 also fits a 16" rim the height though is only 215mm from the rim, and that is 70% of the width of the "balloon"."


J and V 0n a tyre 215/70/16 fits a 16" Rim & Width is 215 and height is 70% of width.

section width = 215 mm
height = 150.5 mm
diameter =707.4 mm


tyre size calculator


http://www.miata.net/garage/tirecalc.html



regards Phil.

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FollowupID: 782407

Follow Up By: Member - John and Val - Monday, Feb 25, 2013 at 08:38

Monday, Feb 25, 2013 at 08:38
Phil,

Thank you for pointing that out !! I must have been having (another) serious seniors moment! You are of course absolutely correct - the "215" etc refers to the width, sidewall to sidewall, and the "70" etc gives the ratio of height to width. Don't know how I got distracted into that nonsense as I've known better for many ( too many) years!

Cheers

John



J and V
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FollowupID: 782424

Reply By: Ross M - Saturday, Feb 23, 2013 at 22:53

Saturday, Feb 23, 2013 at 22:53
PradoMad
Well, you do have a problem.
The bigger tyres can't be used with the smaller ones, sorry, or even fitted to and used on the vehicle.

The front or rear diff will not like the constant difference in rotational speed.You have gone to 97mm smaller dia and that is around 12+% smaller.
The smaller wheel is 305mm less circumference than the bigger wheel.
The rotational speed may be enough to trigger traction control if fitted.
Definitely the ABS will not know what to do and it would appear to be very dangerous to me.
If used on one wheel or even two wheels on the same axle then the centre differential will also be running it's spider/differential gears to try and make up for the difference in drive rates being supplied to the front and rear axles.
It will be working overtime, as it is designed to only cater for slight differences of rotational drive speeds when cornering, never constantly being in that situation.

If you have DSC or TC, Directional Stability Control or Traction Control or both, it/they too will be unable to allow for the difference and strange things will happen if and when those systems are activated by the electronics. Probably not good things.

Did you buy those additional rims or did a tyre fitter fit them to those rims? if a tyre fitter did, they would be held responsible for the ensuing accident.

Despite the obvious instability and sudden additional wear rates which will be happening the vehicle will be dangerous and it will sit down on that end or side wherever one of those wheels is placed.

There will be a myriad of other reasons in addition to what I have mentioned to convince you to sell them and NOT to use them at all.

Normal recommended tyre difference when changing from one set to another size is 2.5% You are nearly 13%.
The jury will decide. I have.

Ross M
AnswerID: 505415

Reply By: Tim HJ61 (WA) - Sunday, Feb 24, 2013 at 00:40

Sunday, Feb 24, 2013 at 00:40
I bet the Prado accelerates really well now! Might have a much lower top speed, but your gearing will make for quick take off, burnouts maybe? Whoo hoo.

And I'd think it looks pretty silly with those tiny wheels inside the arches.

I can't believe a tyre place fitted those tyres knowing they were going onto a Prado.

Sell them, quickly, to someone with a Commodore.

Tim
AnswerID: 505417

Follow Up By: Outnabout.. - Sunday, Feb 24, 2013 at 07:19

Sunday, Feb 24, 2013 at 07:19
those little wheels look like they are the standard Prado off the RV model which didn't have flares. They are also only 6" from memory versus 7.5 for the GXL. I agree the vehicle would look a bit odd. Most RV owners ditch those wheels and fit the GXL rim with some cheap flares
1
FollowupID: 782350

Follow Up By: AlbyNSW - Sunday, Feb 24, 2013 at 07:53

Sunday, Feb 24, 2013 at 07:53
Agree Outnabout, or they may be off an SR Hilux. Either way they are of little / no use for the intended purpose.

1
FollowupID: 782352

Reply By: member-PradoMad - Sunday, Feb 24, 2013 at 07:26

Sunday, Feb 24, 2013 at 07:26
Thank you all.
I bought them with the rims. They came off a Prado.

I will put my MT muddies back on, sell those skinny ones and try to find a set of original wheels with road tires.
Just thought that I want to preserve those MT's as they are awesome in the high country.
...a well, lesson learned and money wasted; some..!
JS.
AnswerID: 505420

Reply By: Kimba10 - Sunday, Feb 24, 2013 at 09:21

Sunday, Feb 24, 2013 at 09:21
Hi there JS, they are off an RV prado which makes them only 6 inch wide rims and 16 in diameter (meant for 90 prado without flares). Your is obviously a GXL with steel wheels and a 90 series going by the picture. Yours would be 7 inch wide but you also went up a size from the original factory specs on the GXL when you have put the mickeys on, the original only been a 70 profile. Been a series 2 90 series you may or may not have traction control ?? (was optional) I would imagine it would certainly not feel to stable running those skinny wheels. You could run them if you really needed to, to get yourself out of trouble but you would need to run all 4 at the same time and you would not be able to run the mickeys even as a spare on one corner more so due to it been an AWD set up. As mentioned above Id get rid of them and buy another set. Problem is its hard to shift those 6 inch skinnys as everyone wants the 7 inch wide rims. You would certainly notice a big difference in power. I went up to a 75 on my grande V6 90 when I had it and I noticed the power loss and the fuel bill climb. I kept the 70 profile when I next got my 90 grande turbo diesel as to not lose the power.........I have 120 now V6 and am in the process of trying to find some muddies in a 65 series profile, BF'g in muddies only make a 70 for mine which are illegal. Im only allowed 15mm extra rolling diameter over standard, the BF's put me 28mm over...
AnswerID: 505422

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