Thread: Thoughts on gearing and cruising rpm

I'm assuming that you've got the 4speed?

I have a Isuzu 5speed with 1.2 ratio lt230. First is crazy low and I start in second mostly. Therefore I'm proberly close to your gear ratios as you are now. I find highway revs about right with 33s. I'm suspecting once Turbo'd I might need 35s.
I didn't find much difference from 31 to 33 apart from a bit nicer at 100, so I'd recommend trying that first. Definatly easier n cheaper than fitting new gears. Especially if you may be looking at turbo later.
My 5c.

Originally Posted by Boof Ed

I'm assuming that you've got the 4speed?

I have a Isuzu 5speed with 1.2 ratio lt230. First is crazy low and I start in second mostly. Therefore I'm proberly close to your gear ratios as you are now. I find highway revs about right with 33s. I'm suspecting once Turbo'd I might need 35s.
I didn't find much difference from 31 to 33 apart from a bit nicer at 100, so I'd recommend trying that first. Definatly easier n cheaper than fitting new gears. Especially if you may be looking at turbo later.
My 5c.

Yes, its a 4 speed. I take it you dont have a tacho? If you know the 5th ratio in your box, I'll plug it into my spreadsheet and tell you your rpm at 100 km/h.
I think the overall driveability of mine would be worse if I went to 33's with the 0.996 gearing and the 4 speed. Maybe it's down on power...

Originally Posted by mark2

Yes, its a 4 speed. I take it you dont have a tacho? If you know the 5th ratio in your box, I'll plug it into my spreadsheet and tell you your rpm at 100 km/h.
I think the overall driveability of mine would be worse if I went to 33's with the 0.996 gearing and the 4 speed. Maybe it's down on power...

That's what I thought also, but you notice less than you think. Maybe my gears just suit nicely. Don't get me wrong, it's still s.l.o.w., but not hugely noticeable between the two tyre sizes.
On power, have you had the injectors serviced. This made a difference to mine. And clean out the strainer in the fuel line banjo.

My 5th is 0.776.

Oh, also, I do have a tacho:
If I can hear my kid in the back seat ~1000 rpm
If I can hear my wife in the front ~ 2000rpm
If all I can hear is my fillings rattling
~ 3000rpm. ;-)

Up to 3000rpm is fine for power. But your fuel consumption will keep climbing. It takes a lot of fuel to just turn these engines over at that rpm all day.

Even geared in the low 2000rpm's with no turbo you should be able to pull 100km/h fine.

Originally Posted by Boof Ed

I'm assuming that you've got the 4speed?

I have a Isuzu 5speed with 1.2 ratio lt230. First is crazy low and I start in second mostly. Therefore I'm proberly close to your gear ratios as you are now. I find highway revs about right with 33s. I'm suspecting once Turbo'd I might need 35s.
I didn't find much difference from 31 to 33 apart from a bit nicer at 100, so I'd recommend trying that first. Definatly easier n cheaper than fitting new gears. Especially if you may be looking at turbo later.
My 5c.

According to my spreadsheet that equates to 2133 rpm @ 100 km/h with a true 33" diamater tyre. In reality a 33" is probably a bit smaller based on loaded diameter (measure distance from the hub center to ground level and double it).

Originally Posted by mark2

According to my spreadsheet that equates to 2133 rpm @ 100 km/h with a true 33" diamater tyre. In reality a 33" is probably a bit smaller based on loaded diameter (measure distance from the hub center to ground level and double it).

Not quite that simple - the effective rolling diameter of a tyre is approximately the diameter of the steel belt plus a bit - if you think about it, as the tyre rolls, if the distance travelled is less than length of this virtually incompressible belt, the tread must be slipping on the road, which is clearly not the case.

In fact, the effective diameter of a tyre is pretty close to the nominal diameter (twice the wall height plus the rim diameter) over a very wide range of pressures and is almost independent of tread wear. But it does vary between brands and models of tyre.

John

Or put another way, while the height of the tyre can change with tyre pressure or load, the length of the tread (tyre circumference) doesn't change and will still travel a set distance each revolution.


In your case, the 7.50-16 Steeltrek fitted to Perenties (or maybe you have the Michelin 4x4) measures 800mm dia (31.5in), but then when fitted to the vehicle, the axle hub centre is 375mm (ie 750mm "dia" for calculation purposes) off the ground. Clearly, installing the tyre doesn't shorten the circumference of the tread by 6 inches.