1977 Toyota coaster diff Ratio

3.55 was a pretty common ratio in the HT-HG models too. In fact, Holden offered optional diff ratios from 2.78, up to about 5.33, from memory.

Those were the days, more options than a lolly shop.

Bob.

Diff ratios in the 6 cyl Holdens were;

4.10:1
3.90:1
3.55:1
3.36:1

These diffs were all 178mm diameter ring gear (crown wheel).

HT to HZ Holdens used a Banjo diff for 6 cyl engines, and a Salisbury diff for the V8 engines.
WB 6 cyl vans and utes used the 178mm diameter ring gear in a "small" Salisbury housing.

Diff ratios for the V8 Holdens were;

3.36:1
3.08:1
2.78:1
2.60:1

These diff ratios were all 190mm diameter ring gear (crown wheel).
All the 190mm ring gear diffs were Salisbury diffs.

All Holden 1-tonner diffs were 190mm ring gears in V8 Salisbury housings, regardless of whether they were 6 cyl or V8.

I'd suggest the 202 would benefit from a Yella Terra head and a 5 speed gearbox from a Celica.
Talk to Dellow about what they would suggest as the best improvement for more speed with less revs.

The focus seem to have gone to a Holden diff and the diff he has will most likely be the Toyota diff as Thinkin has said.

Probably best to comment on what it does have rather than what it doesn't have.
Thinkin is close in mentioning the ratio should be much greater than 3.1.

Nigel, to get the ratio, place a piece of paper around the tail shaft to get it's circumference.

Divide the distance into 10 and tape it to the shaft.
Jack up one wheel and reference mark the wheel and the shaft.
Now slowly revolve the wheel exactly ONE turn.
Take up backlash slack first though!

The number of times and parts thereof the shaft does turn will be exactly twice the final drive ratio so divide by two. ie, 12.4 turns of tailshaft = 6.2:1 pinion to crownwheel ratio.

Have a go and see what it is. Then you know where you are starting from with the ratio. Then you can decide on what to do about it.
Cheers
Ross M

yea now you mention it all those old comercial vehicles not confined to toyota had pretty low diffs. a bit of googling indicates 4-5:1 is probablly more like it

yea pretty high

Just as long as we all talk about high and low ratio the same way.

A high ratio is a slower speed diff usually associated with bigger wheels.

So a Toyota diff is a HIGH or LOW?

get outmore, you have just has referred to it as both high and low so a tad confusing to some who aren't real familiar with this stuff and have to get heads around an issue.

Cheers
Ross M

WHOOPS

should have said high ratio

basically the bigger the number the higher ratio it is an the less speed you will get for your revs

Hi all,

Thank you for the info.
I did my ratio calculation by jacking one wheel clear of the ground,
marking a reference point on both the wheel and tail shaft and rotating the wheel 10 times, counted the number of tail shaft revolutions, which was 31, divided by 10 giving a ratio of 3.1:1, I did this several times to confirm the numbers.
The bus is quite old and is starting to show it's age (rust) there for I don't want to spend too much money on it.
I was hoping for a cheep fix but it doesn't appear there is one.

Thanks again to all
Cheers
Nigel C5

With one wheel on the ground and 10 turns of the free wheel resulting in 31 turns of the tailshaft indicates a ratio of 1.55 : 1

You'd better go back to school, Ross M. Nigel has the ratio figured correctly.
At 3.1:1, the diff has been changed from the original, which would have probably been around 5.0:1 or 6.0:1 ratio.

Of course, we're working on a major lack of information here - such as whether Nigel has a single wheel or dual wheel Coaster, and what the tyre size is, and the all-up weight of the vehicle.

Regardless, as soon as you start playing with gear ratios, diffs, and overall gearing, you're looking at serious dollars - and you're also opening a can of worms.

A Holden 202 usually pulls best at 3000-3500RPM at highway speeds, and if it's pulling a lot of all-up weight, then gearing the engine to run slower will only reduce its pulling ability, and possibly do nothing to reduce fuel consumption.

Bottom line is, if you have a rusty old bus, that's done a lot of k's, been repowered, and has seen better days, you're better off to put the money you're going to spend on altering gearing, towards another bus in better condition.

More than one quite good-looking vehicle has been seen riding on a tilt-tray down to Sims, because the repairs or modifications required are more than the vehicle is worth.

Age is what catches up with all of us - and it catches up with vehicles much faster than humans.
If you plan on spending serious money on anything over 15 yrs old, it had better be a classic, and in good condition, otherwise you're throwing money down the drain.
Older vehicles become a nightmare to try and find parts for, and no-one wants anything to do with them.
If you have a vehicle from the 1970's and 1980's, the only buyers you'll find when you want to quit it, are those with a nostalgic bent.

Ron
Are you allowing for the differential action doubling the turns of the tailshaft?
Why is 31 divided by 10? there is no 2 X factor of differential gears in the equation.

10 turns of the rear wheels with a locked/welded diff will give 31 turns of the tailshaft if a 3:1 ratio.
With and open diff of 3:1 ratio and 10 turns of the rear wheel ( one stopped , one revolving will result in 62 turns of the tailshaft. Differential gears are now in play.

Now work out what is the ratio is if 10 turns with a diff ratio of 3:1 and a "test diff gears doubling the tailshaft numbers) results in 31 turns of the tailshaft.

Woops - I forgot to take into account the other side rear wheel being locked by sitting on the ground.
This, however, halves the number of tailshaft turns when the one wheel is turned and the other is locked - as compared to both rear wheels turning.

I checked this, today, on a spare reconditioned Holden rear axle I have, to make sure.
It's a 3.55:1 ratio diff (I know because I overhauled it) and it took two turns of the wheel to get the pinion shaft to turn just over 3 1/2 times.

Thus, 10 turns of the Coaster rear wheel to get 31 turns of the tailshaft makes for a 6:2:1 ratio, which sounds about right for the original Toyota diff with dual wheels (and 7:00x16 tyres from memory).

The Difflap file shows a 6 tooth pinion and 37 tooth ring gear for the early ('73-on) Coaster, making it a 6.16:1 ratio.

Difflap - rear axle ratios

Just did some quick calculations on 202 engine RPM with a 6.16: 1 diff, and it would be pulling over 4000 RPM at 90kmh on 7.00x16 tyres, without allowing for any torque converter slippage.

Thanks Ron N , finally after ten posts you have the prize for for the correct answer. As you can see I had it spelled out in the first sentence.

Diff ratios seems to give brain haemorrhage to a lot of folk.
I wonder how many people went straight to Difflap as I suggested.
Someone suggested a diff ratio of 1.55. No such thing in the automotive world.

Ron

Who is going to school???????? Ron after ten posts does not have it correct at all.

In a 3.1 : 1 ratio diff,
Having one wheel stopped on the ground and turning the free wheel one turn will result in 6.2 turns of the tailshaft. It is not halved AT ALL it is DOUBLED. The simple gearing of the diff gears drives the crown wheel carrier twice, THAT IS TWO TIMES, ahead of its start position

I didn't suggest a ratio of 1.55 : 1 at any time. I said, from the figures given be the original poster the ratio works out to be 1.55 : 1.

Ron just checked his Holden diff and has made the same mistake as the poster with the coaster. You best overhaul the diff again cos it isn't working correctly.

With a Coaster diff of 6.2 ratio with one wheel on ground and 10 turns of the free wheel will turn the tailshaft 124 times. YES 124 turns.
One turn of the free wheel yields 12.4 tailshaft revolutions.

Sometimes GOOGLE is not your friend. Every apprentice mechanic SHOULD know the above.

I used to do this quite often with a motor wrecker friend, he was good at getting diff ratios in vehicles assessed for replacement. MANY MANY local mechanics had to ring him to know how you do it because they could not. AND, it seems they still can't. Must be an Australia wide phenomenon.

The principles involved is Cave Man Basic mechanical gearing.

The desks in the school house are waiting.

Sorry, Ross - Over here on the Left Coast we don't count on our fingers and toes, and drink a lot of beer while we watch wheels go around, and argue about which way the wheels are turning. [;-)

I rest my case, M'Lud ......

Finding your diff ratio - what gear ratio do I have?

I still reckon one of those school desks is reserved for you. [:-)

Cheers, Ron

Thinkin - Hey, did you count on your fingers and toes, too? TEN posts??
I made it only THREE posts I put up, before yours - sounds like you might need a school desk, too? - alongside our old confused mate, Ross ... [;-)

Hi Ron, this is the 15 response or post to this third reply to this subject.
If you read back to my first sentence ( reply 3 of 6) quote, " Ratio of 3.1:1 don't sound right! might be around 6.1." then bla bla.

FACT: Toyota Coaster 1977 came out with 6 tooth pinion x 37 tooth crown wheel = 6.16 ratio.

As you can see the first mention of 6.1 ratio was made by me,
( reply 3 of 6 ) I had it figured out from the start. ( Nigel made no mention of change of original diff, only Holden/ 202 Trimatic gearbox conversion).

Can't see how you think I might need a school desk and I still have all my fingers and toes.

As I said, ( diff ratios seems to give brain haemorrhage to a lot of folk). cheers Thinkin.

RON

According to Nigel it revs A LOT and it aint' going to do that with 3.1 diff and bigger wheels is it? OBVIOUSLY NOT!!!!!.

When you question someone on here they automatically begin to claim the opposing view is a result of grog. Sorry Ron but you are way off the mark there. Posibly by your own admission it could be the other way around.

Most don't read or understand the question and comment on other unrelated things too.

A Coaster with with a 37/6 gearing as suspected by "Thinkin" and myself, will cause 37 teeth per WHOLE axle, BOTH AXLES turning to pass a certain point.
When one wheel on ground and one free and turned 10 times means, because of differential action, 740 teeth to pass that point.
Divide that by the 6 teeth on the pinion and you get 123.333333 turns of the tail shaft. NOT 31. (as mentioned by Nigel the OP). and pretty close to 124 turns too, don't you think.

Ron possibly the the school collapsed on someone.

Not the TOO HARD basket as Thinkin meant, but it must in the "3 hard basket", as it is 1 more than 2 too hard for some/most.

I found a Meccanno Set very good when I was young. Perhaps we should have a Seniors Meccanno for skill upgrades for those who didn't get it way back then but still became a mechanic despite it all.

As mentioned, this is a basic gearing. Some are very lucky not to have to work out planetary gearing in a six speed auto, me thinks.

More Cheers
Ross M

I've got to agree with get outmore, the Trimatic is an awful transmission, and the 202 is losing a lot of power through it, as well as making the engine rev a lot more than it would with a manual, because the Trimatic has no lock-up feature, like the better, later-model auto transmissions.