Tyre Sizes - Rough Guide Thread

I believe the fuel consumption issues associated with bigger tyres are as much due to speedo/odometer inaccuracies as they are the extra effort required to turn said tyres.
E.g. if you fit 10% bigger tyres and drive at the same indicated speed (e.g. 10% quicker) and you will be working the engine around 20% harder. Yet the odometer will read 10% fewer miles than you have actually covered. You can soon see how this adds up to a massive perceived drop in fuel consumption, even if it hasn't actually changed.

Having the same sized spare is just common sense, for all the reasons you mention. Most old Jimny's (the only ones I can afford!) have had the spare swapped for a bald or flat tyre anyway. I rotate my tyres (including the spare) which means I can get 25% more miles out of the set of 5 tyres. After 24,000 miles my relatively expensive tyres still have around 50% of life left in them.

So I'd say it's a false economy to fit 4 cheap tyres!

I spend approx 50% of time off road (that includes rough/damaged gravel roads).
I consider that Jimnys have good ground clearance even in stock form as they are very short and nimble, so having an extra 10 or 17 mm of ground clearance is not so much needed in practice, except for psychological advantage when crossing over some demanding terrain.

The driver's skill is much more significant than a few millimeters of gain here or there.

Let me quote my neighbor, a retired mechanized army corps colonel:
"One horsepower at the steering wheel is more important than all the others under the bonnet."

I did label the size which I advocate (205 / 75 / R15) as the optimal sensible upgrade.
By "sensible" I meant with no required amount of body trimming (except in a few very mild cases which I described), with no perceived gearing change and no significant speedo/odo inaccuracy, and still with a relatively good gain in ground clearance.

I never said that 205 / 75 / R15 is the absolute optimum, as such term can never be achieved - optimality depends on the criteria. In my case, the criteria was "sensibility", and I pointed that at the beginning.

Size 215 / 75 / R15 does get you 75% more ground clearance gain than 205 / 75 / R15 when compared to stock size, but it comes with additional issues, mainly with body trimming.

My point is that everyone needs to know all the parameters and issues, in order to be able to make informed decisions. At the end, everyone will have their own "optimal" size.


I agree that comparing MPG with different tyre sizes is not a trivial math, and most people don't take into account odometer discrepancies, leading to misleaded calculations.


EDIT:
About the 5th spare tyre - I would agree that having an "active" 5th spare tyre in the same size as the others is the best practice, and it does not present a waste of money, as the 5th tyre will be rotated with the others during their lifetime.
In that case, having a proper 5th spare tyre is entirely justified.
However, there are people (like me) who have the spare tyre cover (either plastic or vinyl), and those are rather impractical to take on and off, so the spare tyre is in "passive" mode (does not get used until a puncture occurs).
So, my remarks about the spare tyre issues were for those people.

Hi friends,
does anybody have experience with the tire size 175/80R16 or 185/85R16?
This sizes are well spreaded on japanese Jimnys....
Thanks and cheers,
Chris

A quick look on the Internet suggests that it's only the 175 that is even slightly available in the UK and then only in Dunlop grand trec at2 which aren't a bad tyre. also a quick bit of maths reveals that the 175 is about 3/4 of an inch taller than stock so could offer some rubbing on certain unlifted models. In theory.

i run 225/75/15 bfg a/t , no mods at all , fit fine , no rubbing etc

mag wrote: i run 225/75/15 bfg a/t , no mods at all , fit fine , no rubbing etc

Mag what size rims do you have ?