[SuziBlu]

I'm a bit lost folks. My new toy, a maruti, has 215/15s, approx 28.5" tyres, and this is an old tyre that is no longer available, im told the closest is a 225/75/15, so by rights, higher geared than normal.

Im doing 4,000 rpm at 100k, or there abouts, and im thinking, this must be out, as surely it would or should be lower with the larger dia tyres.

So I lift the front wheel, and count the prop shaft revs to tyre turns, should tell me the diff gearing, right?

I am doing 2 prop shaft revolutions to one tyre revolution, meaning my diff ratio is 2-1, er, ok, so what am I doing wrong. Shouldn't I get 4.1 prop shaft revs per 1 tyre rev.

[tanshi]

you need to lift both front wheels, if you only lift one it will move the wheel twice as fast. i think

[tanshi]

also your 225s are smaller than, 28.5, they are only very slightly taller than 28s

[SuziBlu]

Ok, just tried that, same deal, 2-1 ?

And both front wheels turn the same direction ?

[tanshi]

odd, thought thats how it should work.

what are you trying to figure out? the diffs should be 4.1 so you can probably assume they are

[SuziBlu]

I thought open diffs meant that the wheels turned opposite directions when free turning, and only limited slip turned them the same direction.

I want to work out my gear ratios so I know what is needed gear wise for 33s.

as it stands, in theory.

28.5 to 33" is 14% higher, from 4000 revs to 3500 revs is 12.5% lower revs.

So, as many people say, 3500 is a good rev for 100k, IF I do nothing at all, and simply put on 33s, I will go from 4000 revs at 100kph to 3440 or so rpm with 33s.

[JrZook]

Do you know your diff ratio's and gearbox ratios?

[SuziBlu]

JrZook wrote:

Do you know your diff ratio's and gearbox ratios?

lol if I knew, I would not be trying to work them out.

[tanshi]

3500 is fine, except that a 1 litre motor will never push those wheels at that revs

plus low range will be killed

[whatulookinat]

Interesting you say about 4000rpm at 100klm because I used to get the same running 205 70 tyres but since I had the front wheels bearings done now she only revs at 3500rpm at 100klm

[shep]

SuziBlu wrote:

And both front wheels turn the same direction ?

there might be a lockright in that diff. does it make a ratchet sound when one is on the ground and ya spin the other?

[SuziBlu]

shep wrote:

SuziBlu wrote:

And both front wheels turn the same direction ?

there might be a lockright in that diff. does it make a ratchet sound when one is on the ground and ya spin the other?

Shep, just rang a mate, he said that is normal for front wheels to do that for some reason, did as he said, its somewhere around the 4-1 mark, thats without being super precise.

Now I have the back off the ground, and will check the transfer gears.

[SuziBlu]

I need a beer.

ok, so I have approx a 1.5-1 transfer case. 4.1-1 diff ratios.

28.5" tyres, meaning approx 89.5" dia, or, 2.274m

So, engine revs/1.5/4.11*2.274 should give me my road speed at that revs, yes/no.

Something is amiss, and I cannot see it, this tells me at 4,000 revs, im doing 147.9kph, but im doin 92.

Thats a miscalculation somewhere of 37.79%.

Maybe I need a few beers.

[JrZook]

Your missing the gearbox ratio or are you using 1:1 no overdrive?

If so calculation is right and will give you 1475 meters/min. /1000 and *60 to get km/h which will give you 88.52KMH

Dan

[SuziBlu]

engine rpm/1.5/4.1*2.274/1000*60

DOH, *60, 1479 is the meters per minute. Bloody hell, now I really need a beer for being such a dill.

That was at 1-1 or top thru the box, as I cannot get the engine and box to turn by hand to work out the overdrive, or 5th.

Just went and double checked the transfer ratios, dun it 5 times, and its more like 1.7-1

With a 10% overdrive, 4k revs means I should be doing, on paper, 90.9kph, via GPS its 92kph, the slight difference can be my eyesight on the speedo.

Ok so Im over that hurdle.

Now the next.

Maruti came out with, 205/70/15s, this equates to a 26,3" tyre, the ones I have on are 28.5", so by rights, I should be doing faster than the speedo says, but im going slower by 8%, where as I should be doing 7.8% faster.

26.3/28.5 = 92.2 or 7.8% change.

In my mind, this means a variance of 15.8%

IF I go from 28.5" to 33" a 13.6% increase, it means I should be running 2.2% underdriven.

Yes or no. ?

No need to change gear ratios.

[tanshi]

it will be around .8 over drive

unless you already have transfer gears your transfer will be
High 1.580
Low 2.511

[SuziBlu]

ok, thanks.

Still, im sort of on the money regarding gear ratios (?) and upgrading my tyres to 33s, and if I go from the 1.3 carb donk to the 1,6 baleeno, revs will be around spot on, and the far greater drag caused by the much larger tyres will be offset by the much more powerful engine, in it correct(ish) rev range.

lol Please tell me im correct and have not just spent a couple of hours barking up the wrong tree.

[SuziBlu]

All this came about because I have a set of 5.12 RnP, and wanted to work out if I need to fit them. If I don't, Suziblu will get them.

[tanshi]

the way i figure it you will be doing around 3300 at 100 ks


your low range will be dread full though

[SuziBlu]

tanshi, I very seldom, if ever use 4wd, and if I get stuck, I only pop the hubs in pull out, then back to 2wd, sand is my play and I love doing it in 2wd, lots of skill, in 4wd its no challenge at all.

IF I use 4 low, its to pull someone out of a hole they have dug.

By my calcs, low will be 38% lower than high, so still lots of grunt.

If I were doing rocks and stuff, yes, I understand fully, but as my medium is soft sand, it hopefully will be fine.

Suziblu is stock gears and runs 31s, an she does really well, summer sand she is perfect, winter sand, a tad high geared, so I would like another 500 or so revs for the same speed, or, same revs for a bit less speed. Where im at is, if suziblu does it with stock and only a 1.3l donk, my other with 33s and a 1.6 "should be fine, I hope.