Tyre Sizes

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

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.

Brad,

Try this site.

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

Regards,
Tony

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.

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.

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.