Diff Ratio

[In the Shed]

Hi Folks,

Question for the techo’s………. My Ute is pretty much stock in terms of motor, trans & diff from what I can gather. It runs 185 x 70 profile x 13” tyres. With the tacho fitted it seems to spin at near 2000 rpm (60km/hr) and 3000 rpm (approx 85km/hr). I don’t know if the diff ratio is standard 3.89, but wondering if it is worth changing to a 3.55 diff centre, being the next nearest so as to drop a few revs? In the near future I will be looking at tyres and wonder if I should be going for the 175 x 80 profile 13” tyres being taller and not waste time fiddling with the diff?

Whilst a job for a rainy day, there is a 3.55 centre on bookface for $320 and one on gumtree for $300.

https://www.gumtree.com.au/s-ad/edwards ... 1315948437

If I do look at these diff centres is there anything I should be specifically looking for other than a smooth turning centre, no chipped teeth and minimal wear marks?

Interested in your input please. Thanks

Stephen

[Errol62]

I would definitely consider the taller tyres Stephen, for looks as well as revs. Gary Deslands runs 14” rims on his wagon with stock diff and happily sat on the speed limit all the way to Echuca and back, though it did surprise me.

I reckon you could pick up a 3.55 centre for $120 if you are patient, but driveability in traffic may suffer. If you were considering the drive west you could stick some big tall 14” tyres on the back just for the drive over.

I’ve always considered second hand diffs to be a worth while gamble. They can look ok and still whine.


FB ute fixer upper, EK van on rotisserie

[ardiesse]

Stephen,

My two cents' worth -

I have a Humpy with a stock 132, and I fitted a 3.55 to it. It's now much quieter at cruise.

My FC has a 3-1/8" bore motor, and a 3.36. I used to run 185/70-13 tyres, and it was fine with them. Now I have taller profile Nankang whitewalls. It's not super-fast from a standing start, but on the highway, it's very relaxed.

I'd suggest a 3.36. Give Simon at the HKTG Garage a call.

The important thing to look for is condition of the crownwheel and pinion teeth: no scuffing, and no water damage. If there's no resistance at all to turning the pinion flange, the bearings have lost their pre-load. Leaky pinion flange oil seals are common. It's worthwhile selecting a good centre, and then getting it rebuilt.

Rob

[Blacky]

I have tried lots of different ratios and tyre sizes to try and find the happy medium between being able to drive up big hills and across treeless plains - have come to the conclusion that with the 13” rims I use a 3.36:1 and with the taller 14’s I run a 3.55:1 - tried a 3.08:1 on one nationals trip and found you had to hold it flat to maintain 110, it used more fuel too.

[Errol62]

I've got a couple of centres you can have z look at Stephen. One is a 3.55 and the other might be 3.36.

Sent from my SM-G781B using Tapatalk

[In the Shed]

Thanks all for your comments, it is appreciated.

Clay, thanks for your offer and gladly accepted to have a look at them.

Regards
Stephen

[Stygian]

Hi Stephen,

My two cents based on my personal experience over the years:

• For a stock grey grey motor, the 3.55 ratio is probably the limit. I ran a 3.36 with a stock grey years ago and, whilst great on the highway (except hills), it was not that crash hot around town.

• Running a 3.55 in my car with a warm grey and 185/80R13 tyres gave me an almost spot on speedo reading. I concluded the 185/80 tyres have just slightly less than the rolling diameter of the original Olympic Air ride cross ply tyres.

• Blacky is right in that 14 inch rims work well with the 3.55. The variety of profiles for 14" rims means you can more easily get the rolling diameter you want compared to 13" rims.

• I found the 3.55 was too short for a warm grey with 185/80R13 tyres, but really great around town. So I put in a 3.36. The speedo is now out uniformly by 5 mph across the whole range.

• The 3.36 is great on the highway with a warm grey. No dying too much on hills.

• Anything higher than a 3.36, like a 3.08, makes for unpleasant driving with a grey motor as a general rule, even a warm grey. The low down torque just isn't there.

Hope this helps.

[Blacky]

Also , speedo correction boxes are available through the Rod shop in Castlemaine to correct your speedo once you settle on a ratio and tyre size, you just let them know what your speedo reads when you are travelling at 100kph - use a phone with a GPS speedo app for this - and what speed you are doing when the speedo shows 100kph-or 60 mph in our case.
They are $250 plus you need a short speedo cable to get the thing to fit , about another $80.

I have 2 of them, one for the 14's and one for the Tasmans.

[parisian62]

If you do change the diff make sure you get a coarse spline.

[In the Shed]

Thanks for the valuable advice.

Regards
Stephen