different tyre sizes

Submitted: Saturday, May 03, 2014 at 12:10
ThreadID: 107597 Views:1769 Replies:4 FollowUps:0
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I want to lower the tray on my ute and so am looking for a narrower spare tyre to slot in under the tray. I currently run 31*10.5R15 (265/75/R15). I have run a range of different tyre sizes on a 15 or 16 inch rim through a tyre size calculator looking for a tyre size with the closest radius to the 31*10.5R15.

This spare tyre will also be used only running, at most, short distances at lower speeds, most likely without load, around town. When I go bush I ditch the spare under the tray and carry 2 spare 31*10.5R15 on the tray.

I understand the calculated radius is for a new tyre and a tyre that is 50% worn will be approx 5mm less in radius.

My question is: for minimising risk to handling, diffs or transmission what is the maximum difference in tyre radius I should accept?

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Reply By: disco driver - Saturday, May 03, 2014 at 14:48

Saturday, May 03, 2014 at 14:48
If you can carry two tyre/rim assemblies on the tray when going bush, why not mount one tyre/rim on to tray up against the cab rail as a permanent fixture.
then you could lower the tray to suit your requirements.

AnswerID: 531741

Reply By: Member - Darwin Dave - Saturday, May 03, 2014 at 17:04

Saturday, May 03, 2014 at 17:04
Hi Dermie66,

If you just want a get you home tyre that you can have out of sight and out of the weather when not travelling and is legal.

Circumference of 31 x 10.5 x 15 on half worn tyre is 2.430.

700 x 16 on split rim half worn is 2.440 and half the width.

I take my 2 good travelling wheels out of my canopy and fit 2 split rims in their place when around town. Half the population don't know what they are and the other half are frightened of them.

Just watchout there are a couple of offsets.

Darwin Dave

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AnswerID: 531756

Reply By: The Bantam - Sunday, May 04, 2014 at 10:08

Sunday, May 04, 2014 at 10:08
For a start don't obsess over small differences in tyre diameter, if its only a spare and it will only be run for short distances and in two wheel drive.

Arround town vehicles travel in actual real straight lines a very short amount of the time so you diff is always working to accomodate the difference in wheel paths.

While there are a number off reasons why it is not good to have wheels of serioulsy differing tyre diameter, small differences are of very little concern.

There is no particular measurement that makes a given difference a go or no go matter.

If however there is a considerable difference....say 2 inches diameter.....you probably wont break anything, but it will look realy odd, it will cause the suspension to load up in strange ways and the vehicle may not drive well at all.

BUT...you should not be expecting any sort of performance or handling with a significantly different width tyre either.

Personally I will not carry a non matching spare on any vehicle & I would not buy or own a vehicle with a space saver spare tyre.

In this day and age we soo commonly travel at 100KMH on freeways and even in slow moving traffic, I dont want to be driving a vehicle that is effectivly braking and steering on 3 wheels.
In my over 30 years of driving I have never had a highway accident and all the accidents and close calls I have had have been round town or in daily driving situstions.

I want my vehicle to be 100% all of the time.

NOW here is another solution.

If you have a ute that stores the tyre above the chassis or accesses the tyre over the rear chasis rail, like a 70 series.
have you thaought about doing away with the standard spare tyre accomodation all togeter and fitting something like a hilux spare tyre winch under the chassis for arround town.

It would be a pain in the ass off road because the rear clearance and departure angle would be compromised...but arround town...not an issue and you could carry a matching spare.

AnswerID: 531793

Reply By: Member - Phil G (SA) - Sunday, May 04, 2014 at 20:20

Sunday, May 04, 2014 at 20:20
Just go for a 205R16. Very common size that's just a little smaller than the sizes you mention.
I assume your ute is 2wd with selectable 4wd, so it doesn't matter, unless for some crazy reason you decide to select 4wd on a firm surface.
So if you needed to use that spare, its better off on the front.
AnswerID: 531857

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