How does one determine the perfect diff ratio?

[IndianaJones]

Hi guys

How does one determine the perfect diff ratio?

I’m doing a 3UZ with 5-speed auto conversion into an everyday 4x2 workhorse ute.

I won’t be entering burnout competitions or drag racing it, thus I’m not after ultimate acceleration, and I won’t be doing top-end runs, thus not after a particularly long ratio either.
The most prominent consideration is probably highway fuel consumption, and ability to tow a car trailer/transporter, like once a month.

Overall the empty ute would weigh less than a Lexus LS430 in which the engine comes standard, around 150 to 200kg less. The ute is not wider, but stands taller, thus probably around 10%-15% more frontal area and more wind resistance.

Do I try to achieve the same RPMs as standard in the LS430 at highway speed, which would be around 2360RPM?
A similar factory V6 powered ute would be doing 3400RPM at highway speeds, but the V6 has much less grunt than the 3UZ.

Regards

[Daddio]

Match the diff to the gearbox...3UZ has about 375Nm toque at 2000rpm or 3/4 of the 420Nm max, so aim for a diff ratio to achieve cruising speed of say 120km/h at 2200-2500rpm in highest gear ratio (use intended wheel size for calculation since your 4x2 will probably differ a bit from the stock wheel size)..this should place you in a range to pull the car trailer when required (you might have to drop a gear up steeper hills), and have relaxed cruising when not towing....depending on wheel size, this is likely to place you close to the stock diff (assuming you are using the stock auto) so around 3.5xx: 1 to maybe 3.9:1 (Stock being 3.769:1)
(I'd be thinking about a decent trans cooler too for when you tow)

[IndianaJones]

Thanks Daddio

I'll definitely add a trans cooler, haven't yet decided whether to go with a separate cooler, or use a radiator with a built-in cooler, as I have to change to a bigger radiator anyway.

Yes, I'm using the A650E gearbox, the stock 5-speed auto the 3UZ V8 comes with. As far as the diff ratio is concerned, my stock ute diff ratio is 4.636:1. My thinking is that if I change to a 255/70R16 tyre size, I'll be running at 2850-2900RPM at 120km/h in top. If I had to drop a gear to 4th for a steep hill when towing, I'll be hovering over the torque peak of the engine at 3500RPM, when running between 100 and 120km/h.

Would that be too high for decent fuel consumption? Should I gear it longer for lower RPM's?

[Daddio]

Personally I suspect even with 255/70 16's that the 4.636:1 diff will still be too short, and would suggest around a 4.1 would be better...