Tyre Sizes

Submitted: Friday, Feb 27, 2004 at 02:53
ThreadID: 10830 Views:1441 Replies:6 FollowUps:2
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Hello to all,
Just looking at new tyres and wondering what would happen if i went bigger than the originals. I have a manual 100s with 275/70/16, if i went slightly bigger would there be a difference in terms of revs as opposed to what i have now, say for the gear changes, or what i get at say 100km per hour. I hope this makes sence.
Any help would be appreciated.
Thanks Brad
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Reply By: Wayne (NSW) - Friday, Feb 27, 2004 at 07:43

Friday, Feb 27, 2004 at 07:43
Depending how much bigger will effect how much the following will change.

1. The gearing will appear to be higher.
ie If you pull 2500rpm to go 100 kph taller tyres may pull 2250rpm at 100kph. This is good for hiway driving but can also mean when going down hills in low range the vehicle will want to go faster.

2. The ground clearance of the vehicle will be raised.
Suspension will raise the body of the vehicle, but taller tyres will increase the ground clearance of the diffs.

3. Acceleration will be slower.
0 to 100 kph might take 20 seconds instead of 15 seconds.

Wayne
Always Out'N About
AnswerID: 48319

Reply By: Phil G - Friday, Feb 27, 2004 at 08:15

Friday, Feb 27, 2004 at 08:15
Brad,

Your 275/70 are 792mm diameter.

Options are 265/75R16 - cheaper tyre, stronger with LT construction. 810mm diameter (+2%)
or 285/75R16 - if you want a taller tyre that fills the guards, looks good and gives you a little more clearance - about 835mm diameter (+6%) and you'd have to look into the local regulations - may require engineers certificate.

If you have a standard 100 series with the 1HZzzzzzz motor, then go the 265's as it struggles pushing the bigger tyres. If yours is a petrol or TD, then the difference won't be as noticeable. And your speedo may be a bit out, so don't get caught speeding.
AnswerID: 48322

Reply By: RainMaker - Friday, Feb 27, 2004 at 10:39

Friday, Feb 27, 2004 at 10:39
Brad,

Try this site.

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

Regards,
Tony
AnswerID: 48344

Reply By: ceredig - Friday, Feb 27, 2004 at 11:31

Friday, Feb 27, 2004 at 11:31
When you put taller tyres on, the following happens:
(I'll talk in terms of what happened to me when I went from 29" to 31". Thats 7%change.)

Less braking power (7%)
Less torque (7%)
Less acceleration
Less off-road performance at low speed (ie steep uphills)
Slower speedo- more speeding tickets
Apparent better fuel consuption (not really, it probably goes down offroad and improves on the freeway)
Gear changes are required earlier (in my case the two hills I go up in 3rd every day, I now have to use 2nd. (wot-a-pain!))

The only improvement is ground clearance for me it is 1"
Now there is a big discussion waiting to happen about the true value of that lift.
(Apparently the bigger tyres look better too, whatever that's supposed to mean.)

I have two sets of wheels and tyres. Big for offroad and normal for onroad. (I don't mind the small loss of grunt with the biggies 'cos Iv'e allways had enough anyway, and, it feels very nippy around town with the littlies. As well, the M/Ts last longer cos I don't use em on the tarmac.)

have fun.

AnswerID: 48355

Follow Up By: Allfour4x4 - Friday, Feb 27, 2004 at 21:23

Friday, Feb 27, 2004 at 21:23
ceredig,
Looking at similar increase (7%) for our T/D Prado........So if you had to keep your biggies on all the time, would you bebleepoff or have you just got used to it?
Glenn B.
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FollowupID: 310291

Follow Up By: ceredig - Monday, Mar 01, 2004 at 09:40

Monday, Mar 01, 2004 at 09:40
Hi allfour4X4
Most people seem to get used to the bigger tyres, others put up with them cos they like the look ...?...
Me? I didn't like loss of performance, thats the 'all round' performance I mean. I keep the big tyres now for when I think that I will really need them.
I prefer the standards and have found that the are perfectly OK, so far. The muddies have been in the shed for about six months now and I am surprised just how well the normal A/Ts perform.
I think the big tyres will stay in the shed except for when its wet and slippery, or if I'm going to a new location and I don't know what I'm going to find.
have fun.
0
FollowupID: 310521

Reply By: basecamp15 - Friday, Feb 27, 2004 at 14:49

Friday, Feb 27, 2004 at 14:49
Another thing to note is that you will get more surface contact (called the footprint) of the tyre to the terrain you're driving on. This is good news as it increases grip.
You'll also get a longer life out of them too because of the larger rolling diameter.
Cheers, Mark.
AnswerID: 48396

Reply By: Member - Stephen (WA) - Saturday, Feb 28, 2004 at 19:45

Saturday, Feb 28, 2004 at 19:45
Brad,

All of the above comments are true.

A chap offered me some Cooper some 285/75's for the same price as the 265/75's. I took it.

I reckon when I'm up for new tyres next I'll go put the 265/75's on as the 285/75's are a bit tall for my liking.

They certainly do drop the revs when cruising on the highway - but after correcting for the tyre diameter, I found that the ecomony was no better (or worse) than before.

The increased clearance is a bonus, but I'm not sure how many times I've really used it (knowingly or unknowingly) - wouldn't be many times at all.

I miss the crisper starts a bit and there is a noticeable difference when speeding up to overtake at highway speeds (poorer acceleration with taller tyres).

The constant need to drive at 5 km/h below the speed limit (according to the speedo) is a pain.

Don't get me wrong - I'm very happy with my tyres - It's just that I'll try the smaller set next time I'm due for a change.

Cheers
Stephen J.
AnswerID: 48554

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