Thread: Tyres: Slow on uptake and continuing from what Vlad said...

That all depends on what size tyres you have on now and what size your looking at. From memory 32" is standard on the defenders so that should give you a neutral speedo reading.

Potentially yes to all of the above, if you are fitting larger diameter tyres, you will be increasing your gearing, your speedo should be more accurate, but check it with a GPS to find out its real % error.

As for fuel economy, higher gearing can be a double edged sword, if you have power to spare yes maybe, but if you in a heavily loaded defender struggling for power you might actually use more as the engine has to work harder, in real world terms I`d be very suprised if it made any real difference at all.

Originally posted by JamesH
Sorry to be dim guys but am I right on the following in theory?

Vlad commented earlier that rims aside if a tyre has a larger diameter then it would tend to make the gearing higher.

Could this therefore have a positive effect on fuel economy on a long run as it would cruise at lower revs? It would also mean that it would take more engine power around town to get it moving.

Also I am thinking that it would mean that my speedo (currently over reading by 8%) may become more accurate.

At the moment I am on Michelins (not the originals) and wondering if they have a smaller diamter than the originals.

J

An engine's fuel consumption is a function of its revs and throttle opening. Low revs, high throttle opening means high consumption.

Larger tyres will reduce the gearing required to cruise and that will help with fuel economy, provided the engine is not labouring in too high a gear.

On the downside taller tyres are heavier and are also rotating weight, so this requires more energy to move. Drag (air resistance) is also increased.

So as usual, the answer is "it depends".

A Defender's standard tyre is 235/85/16, or 31.7 inches (same as 265/75/16). This is the tallest tyre of any 4WD with possibly the exception of the F250.

G'day All
I have 245/70X16 simex road trekker fitted to std RR alloys, according to the conversion doohicky i looked at they are very slightly larger in overall diameter, according to the GPS the speedo still reads 8km/h over true speed! I can't go any bigger on rubber until I do coils & dampers.

Cheers Scott