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Easiest way to compare ratio's
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Author:  oneinkedgeek [ Tue Nov 05, 2013 6:10 am ]
Post subject:  Easiest way to compare ratio's
Ive been told the ratio's in the diffs for my 1999 SWB, 2 Litre J20A GV would be 4.x

I pulled the rear end out of my 1600 GV with 5.12 ratio diffs, and moved the entire housing and diff over (was in much better condition and l like the fact i dont have to drain oil and re seal if i didnt diff center)

The idea is that before the weekend, ill pull the front alloy diff out and put my steel diff in the front which also has 5.12 ratio's

When i jumped in the truck this morning, i didnt notice any reduction in gearing, I hit the motorway and noticed that i got exactly the same 100kph@ 3200rpm

im thinking the ration in my truck is actually the same... i dont want to pull the diff center out of any of the diffs at the moment so my question is....

if i put the truck up on stands, mark the top of each tyre and have 4WD engaged and all wheels turning, am i going to be able to see the difference between 4.x and 5.x easily ?

If the ratios are the same, it means i dont have to rush getting the steel one in and i can look at doing a air locker... ideal time to look at it

Image
Author:  Jas [ Tue Nov 05, 2013 7:07 am ]
Post subject:  Re: Easiest way to compare ratio's
If its exactly the same speed and rpm then the ratio should be the same.

If in doubt, either:

jack it up, Disconnect the prop shaft, put a reference mark the input shaft and casing. do the same for out put shaft (or tyre), hand turn it and see what the difference is.(make sure you mark it where you can see both when turning)

ie rotate the tyre/axle ~1/5 revolution = if the input shaft is back/near at the mark then its around 5.

or if you have an external non contact tachometer measure input shaft rpm and axle rpm. divide input rpm by axle rpm = reduction ratio
Author:  Gwagensteve [ Tue Nov 05, 2013 8:32 am ]
Post subject:  Re: Easiest way to compare ratio's
How did you measure your road speed @ rpm? If you did it with the vehicles speedo, it will read the same regardless of axle ratio.

I thought the J20 cars had 4.8 diffs (?) If you've swapped 4.8 to 5.12 it won't feel dramatically different.

Steve.
Author:  henno [ Tue Nov 05, 2013 8:38 am ]
Post subject:  Re: Easiest way to compare ratio's
Gwagensteve wrote:
How did you measure your road speed @ rpm? If you did it with the vehicles speedo, it will read the same regardless of axle ratio.


^^ this x 2. It's a common mistake made when people change diff ratios; don't feel bad if that's all it is. :)
Author:  oneinkedgeek [ Tue Nov 05, 2013 9:51 am ]
Post subject:  Re: Easiest way to compare ratio's
Gwagensteve wrote:
How did you measure your road speed @ rpm? If you did it with the vehicles speedo, it will read the same regardless of axle ratio.

I thought the J20 cars had 4.8 diffs (?) If you've swapped 4.8 to 5.12 it won't feel dramatically different.

Steve.


Yeah thats the one, i just figured that if i was in the same gear doing the same speed, the revs would increase say from 3200 to 3500 with a lower ratio.... but obviously thats not the case.
Author:  henno [ Tue Nov 05, 2013 9:59 am ]
Post subject:  Re: Easiest way to compare ratio's
They will... for the same "actual" speed, not speedometer-measured speed. Obviously the number of revolutions the driveline is making will not change (as calculated and reported by your speedo) as that is measured before the gearing of the diffs.

Use a GPS to measure and I bet your speed is lower than 100kph.
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