Diff Gears, How they affect acceleration

[delcowizzid]

2640.................

 

[Calaber]

To get a better idea of the actual difference between the Commodore and Navara wheels, I just measured my Berlina. The diameter is actually close to 25", not 22. After calculating the circumference of both wheels, the Navara's is actually 57.39% greater in circumference. This means that the Navara wheels would be turning much fewer times per kilometre than the Commodore's would (Approx 252 per k for Commo, 145 for Navara)

When you then multiply the number of wheel revolutions per kilometre, by the diff ratio, you get the following:

Commodore - 252 x 3.08 = 776.16 tailshaft revolutions per kilometer.
Navara - 144.6 x 4.11 = 594.31 talshaft revolutions per kilometer.

The difference in the number of tailshaft revolutions, which is what the speedometer is calibrated from, is 23.43%. If my calculations are correct, then the speedo reading difference between the two diff ratios will be substantial. Your speedo is going to need adjustment to cater for the vastly different ratios in the diff or it will read way too fast. (ie 123.43 kph indicated at an actual road speed of 100kph.

(Gees, I hope my calc's are right - I failed maths every year from Kindergarten up)

 

[greenacc]

i'm not sure how you worked out that 33inch tyres are 57.39% bigger than 25inch ? I got 32%, using pie x D.

 

[graemevb]

It's a navara kingcab, so a smallish to med 4wd...and yeh using commodore auto. So run me through this, if it's a 12% difference, say I was cruising along the h/way in a commodore with std diffs at 100klm an hour revs at say 3000 (use this figure as eg)...would the 12% difference mean if I was in 4wd with 4.11 diffs and 33" tyres, revs would be at 2640 or 3360? Did I follow that correctly.

According to specs navara kingcab runs a 255-265/70/16 which has a diameter of ~30.5in
comm 205/65/15 has diameter 25.5in

overall gearing fraction are 4.08/3.08=1.32 and 30.5/25.5 = 1.20, the diff increases the revs by 1.32 and the wheels reduce it by factor 1.20.
The diff factor is larger so the 4wd will rev faster at the same road speed by factor 1.32/1.20= 1.11 or 11% more.

 

[HoldenOnn]

To Calculate RPM;

168 x Diff Ratio x MPH / Tyre Radius x Gear Ratio

eg. 235/35/17 tyre with 3.08 diff and 0.7 overdrive gear @ 110km/h

168 x 3.08 x 68.31 / 11.74 x 0.7 = 2108 rpm

*** Notes;
* Tyre Radius is in Inches
* Speed Needs to be converted to MPH (formula: 1km = 0.621 miles)

 

[holdenvtman1123]

i found this in the book for the VT.

1st gear ratio is - 3.059:1
2nd gear - 1.625:1
3rd gear - 1.000:1
D Drive - 0.696:1 (overdrive)
R reverse - 2.294:1

my diff ratio is 3.08:1

i have a vt 5.0lt 4sp auto

 

[Calaber]

i'm not sure how you worked out that 33inch tyres are 57.39% bigger than 25inch ? I got 32%, using pie x D.

I used pi x radius squared, but that's area, not circumference, isn't it?

I DID say that I failed everything in maths after kindergarten. At least I have an excuse - fourty years since I left high school.

Just ignore everything I said while I look for a nice dark hole to crawl into.

 

[-scyth-]

lol aww poor calaber...:cry::drink:

I used pi x radius squared, but that's area, not circumference, isn't it?

I DID say that I failed everything in maths after kindergarten. At least I have an excuse - fourty years since I left high school.

Just ignore everything I said while I look for a nice dark hole to crawl into.

 

[Tsunamix]

After speaking with quite a few diff specialist around the country (not just Bris) this is what the consensus is.

Going from a 308 to a 346 diff ratio will not add much in terms of acceleration. It will be noticeable, but only just noticeable.

Apparently a 371 ratio is the preferred option. by coincindence, this is the ratio that appears in manual 6 speed commodores as a final drive.

Apparently the first gear ratio on auto v6's is a bit high and responds very well to a lower diff ratio.

I woudl have spoken to a dozen diff builders about this, in the process of trying to find a cheaper diff build - didnt want to fork out $1700 for it. The bad news is that 3.7 gear sets are not easy to find any more. One importer of them has gone belly up.

 

[graemevb]

i'm not sure how you worked out that 33inch tyres are 57.39% bigger than 25inch ? I got 32%, using pie x D.

why not just go 33/25.

and for others use Tire size calculator rather than do rough measurements to find the size of the tyre.