Any gear/diff ratio experts?

[s2sol]

I've recently bought a van which is very low geared. Fortunately, there is a confusing number of alternative diff ratios. It currently has 12 teeth on the pinion, 47 on the crown wheel, giving me a diff ratio of 1:3.92.

Top gear is overdrive @ 0.791:1. The van has 350nm of torque, between 1400 - 2800 rpm. Currently, it is at just under 2500 rpm at 60, and about 2800 at 70. I'd like to change the diff ratio to drop as much as I can down the rev range.

Here are the alternatives: 13/41(1/3.15) - 13/43(1/3.31) - 13/47(1/3.62) - 12/47(1/3.92)11/46(1/4 18) 9/40(1/4 40) 9/44(1/4 89) 9/46(1/5 11).

I guess I want something like 1:3.31 or even 1:3.15. Is it as simple as changing the pinion and crown wheel in the diff, and will it absolutely knacker the driveability of the thing?

[swifthobo]

Quaife?

[s2sol]

Iveco's own...

[Classic Grad 98]

If you gave the tyre size on the driven axle, or better still the tyre diameter or circumference, someone with some time on their hands could do the maths for you quite easily to calculate where each ratio would have the engine revving at 70MPH, which would help.
Regarding decreasing the axle ratio, you need to bear in mind the following;
-Your speedo will no longer be accurate (it will underread)
-Your low gears will be longer, which will mean more clutch slipping to get the heavily laden van off the line (IME most vans are geared too short, whenever I rent one I set off in 2nd unless it's heavily loaded so this may not be an issue at all)
-It's quite a big job. If it's a RWD van like a Sprinter or a truck chassis, you can just swap the differential for a donor. You can just fit a Crownwheel and Pinion to your existing diff but they need setting up to adjust the mesh of the gears. In a FWD van, the gearbox would need to be removed and dismantled.

[swifthobo]

Find out what diff they used on there minibuses i had a transit minibus which used to sit at speed lovely and they a swb the same that didn't

[s2sol]

Tyre size is 195/65R16, and it's RWD. The speedo's a good point, but I'd imagine it would just need plugging in at the dealer. As you say, it's pretty low geared at the moment. We tend to use 1st out of habit, but it's a bit of a waste of time.

It's an RWD Iveco Daily, and there's a guy local to me who rebuilt the diff in the last one, so I'd trust him with this.

It's returning 20.3mpg at the moment, and if I can't get it up to around 25, I'll just get shot of it and get a 7.5t and an operator's licence.

ETA - I'm slow posting. Doogz, if that's right, then great. I could save a fortune. Swifthobo, the minibuses are generally over 3.5t GVW, so tend to be geared even lower. That's an Iveco thing, though. Transits may well be different.

Edited by s2sol on Thursday 26th January 16:23

[Maximum Bobs]

If it's an older Iveco it may be worth you swapping the whole axle with one that has the required ratio, they're readily available & I can imagine cheaper than getting the ring & pinion changed.

Also I have a low geared Iveco daily & the word on the street is that you should always use first gear because apparently it reduces the stress & strains of pulling away in second with not very strong gear boxes.

[s2sol]

It's a 2008, and I'd rather pay a bit more and know what I've got, iyswim.

It does about 50k a year, so a used diff would be a bit of a risk. That's also why I want a few more miles out of a gallon!

[LooRoll1000]

s2sol said:

Tyre size is 195/65R16, and it's RWD. The speedo's a good point, but I'd imagine it would just need plugging in at the dealer. As you say, it's pretty low geared at the moment. We tend to use 1st out of habit, but it's a bit of a waste of time.

It's an RWD Iveco Daily, and there's a guy local to me who rebuilt the diff in the last one, so I'd trust him with this.

It's returning 20.3mpg at the moment, and if I can't get it up to around 25, I'll just get shot of it and get a 7.5t and an operator's licence.

ETA - I'm slow posting. Doogz, if that's right, then great. I could save a fortune. Swifthobo, the minibuses are generally over 3.5t GVW, so tend to be geared even lower. That's an Iveco thing, though. Transits may well be different.

Edited by s2sol on Thursday 26th January 16:23

This maybe a shot in the dark, did you ever put an diff in? If so, what did you use and how did it go?

[s2sol]

Crikey, I'd no idea I'd been on here that long. I did nothing about i but put my rates up. It was a lovely van, but it was thirsty. And it ate rear calipers for fun.