22 replies to this topic

[S pack]

3.08:1 should drop it down to around 2800 rpm or less at 100kph.

[BIG KEV]

Go 4:11 and have some fun

[Bigfella237]

Using your tyre size @ 100km/h in top gear, I get these revs for the following diff ratios...

 

2.60:1 = 2,200 rpm

2.78:1 = 2,350 rpm

3.08:1 = 2,600 rpm
3.36:1 = 2,850 rpm
3.55:1 = 3,000 rpm
3.90:1 = 3,300 rpm
4.11:1 = 3,500 rpm

 

...based on your tyre size doing 507 revolutions per kilometre.

 

(Please note that I rounded off the rpm figures)

[rexy]

You should get an overdrive 5 speed gearbox.
Best of both worlds, zippy off the mark and nice highway cruise rpm.

[Dave6179]

Just remember, the taller the diff the worse in town/hills/towing/acceleration. As I did (and above) a 5 speed gives alround 'driveability'. A too tall diff can actually make fuel use worse. 

[Bigfella237]

I was looking at a manual trans from a VE Commodore and sixth gear has a ratio of 0.50:1, that means with a 285/30R18 tyre and even with a 4.11:1 diff, you'd still only be doing 1750 RPM at 100 km/h, damn that's tall!

 

Apparently manual VE Commodores typically have a 3.45:1 diff and HSV's have a 3.70:1

[S pack]

That is retarded, when you have OD gearbox you usually shorten diff gears not the other way around.

Chasing better fuel economy no doubt.

[Dave6179]

Yep. My old stove hot 179 prefered a 3.9 diff over the 3.55, 3.36 and the 3.08. Economy and driveability got worse with the bigger diffs.

But, the 202 hated the 3.9, 3.55 in there now... and feels like it wants more, but 3.45 BW to go in soon.

[S pack]

Yep. My old stove hot 179 prefered a 3.9 diff over the 3.55, 3.36 and the 3.08.

Not enough guts to pull the crust off a rice pudding with a 3.08 eh!

[axistr]

With no overdrive gearbox 3.08 ratio would be my choice for all round driving. Depending on diff choice 3.55-3.45 if you rarely go on the highway & 3.7 or lower if you never go on the highway or don't want to drive at a constant speed more than 90kph.

[S pack]

With no overdrive gearbox 3.08 ratio would be my choice for all round driving. Depending on diff choice 3.55-3.45 if you rarely go on the highway & 3.7 or lower if you never go on the highway or don't want to drive at a constant speed more than 90kph.

+1, but if you don't mind revving the ring off your engine at 100kmh then go for the lowest ratio you can find.

Edited by S pack, 15 March 2015 - 08:44 AM.

[axistr]

When choosing a final drive ratio it's always going to be a decision between specific use or compromise. If you have a specific use like only going in a straight line down the quarter mile the choice is much easier, go for a ratio that will see the engine rev to its maximum as it passes the finish line.

 

If your only going to use it for the highway go for gearing that will give you a low R.P.M at cruising speed 2.78 ratio or simular. But the compromise choice is a much harder decision.

 

The Torana is still a light car so I reckon for the average 308 engine with a four speed you can easily get away with a 3.08 ratio for every day cruising around in a variety of conditions. Many high performance engines don't like to drop to low in the rev range and get a bit harder to get of the line so I would go a bit lower ratio. Most high performance engines generally have a bad ignition curve when it comes to economy driving so as mentioned above the economy may not be very good with a high speed ratio.

 

That's where E.F.I comes into it, the best of both worlds as there are so many variables that the engine computer can adjust for.

Personally I like 6 speed overdrive boxes with final drive gearing of around 3.9-3.7 for street use. You don't have to use the overdrive gears around town if you don't want to, and than on the Hwy you have got an engine that can take it easy and all your money doesn't go out the exhaust.

 

What surprises me is my hatch 513hp & 502ft torque. Its running a 3.0-1 diff gears with a TKO Tremec with a .65 overdrive. The car had this diff ratio in it when I bought it and when I restored the car I was always going to change the gears for a set of 3.7 but never got around to it. When I take it to Toranafest each year its a round trip of 450km and uses 55-60 litres of fuel as the engines is only ticking over at 1,650 RPM. and because of the high torque output it still sinks you into the seat at 120kph.  Around town however I can easily use 50 litres just going to Parramatta and back a round trip of only 85km.     

[Bigfella237]

I thought I read somewhere recently that diff ratios in the high 3's were typically noisy?

 

Is that true only for 9" diffs, or only for a particular brand of gears? Is it limited to high 3's or would any low gearing, like say 4.11's, be noisy too?

 

Is it a case of the faster things spin the noisier they are or is it just a harmonics kind of thing with those particular ratios?

[Dave6179]

A regular trip I used to make was 540km one way. Roughly half way I would top up. The std 3.55 and 5 speed would use $28 (yes, it was a long time ago... same car). With the 3.08 it then used $49, no other changes!. It would also do 165km/r in third@6000rpm. Nothing left in fourth. 3.55 it was still pulling in fourth at 5500 pushing 170, with more to go when into fifth. The 3.9 really woke up the 179 (186 +30) and would run out of gearing in fifth. But the economy was at its best.

[axistr]

Bigfeller237 I think it has more to do with the brand and setting up of the gear set than the ratio. My 9" 3,0-1 makes no noise. Over the years I have has to redo many diffs and resets do to other people setting them up totally wrong.

 

Dave6179 Sound like your engine has a better ignition advance at mid range and maybe the vacuum advance isn't working properly. Or the carby in the lower range is running way to rich. Correctly set up ignition & fuel delivery will always produce better economy when the engine isn't working hard and generally at lower R.P.M

 

And so people wouldn't have a clue how to drive to economy. 

[IMORAL]

I thought I read somewhere recently that diff ratios in the high 3's were typically noisy?
 
Is that true only for 9" diffs, or only for a particular brand of gears? Is it limited to high 3's or would any low gearing, like say 4.11's, be noisy too?
 
Is it a case of the faster things spin the noisier they are or is it just a harmonics kind of thing with those particular ratios?

I ran 4:11's and had a period noise. Had it rebuilt with decent gears and it was good. Only a very very slight period noise but in the 4 region it is expected due to the tooth contact. Depending on how well it's built will depend how little the noise is as previously said.