inter d1310 4x4 diff ratio

Hi all.
I am trying to find out the ratio and type of differentials in my D1310 4x4? As I understand it they could be anything from 4.88 and up. It is currently geared that low that 70 kmh is all that is achievable out of the 4 speed gearbox. It has 8 stud axles and I would like to keep it that way as the wheels I have, I like. Can a retrofit of another diff remedy the issue?
All information is great fully received.

Standard factory ratio is 4.88.....as you mention the centre was available in slower ratios to suit the 1500 and 1600 4x4 truck which used the 1300 centre as front drive.....Dont forget you need to match the front diff ratio ....The diff ratio is marked on the end of the pinion as a fraction...such as 7/43......the easiest way to increase speed is bigger tyres and or wheels.

Thanks for the info John.
I have increased the tyre profile already and that has given me 10kmh greater speed. I am now thinking gearbox upgrade to an overdrive or changing the diffs. But these are expensive solutions. I have a friend looking into the diffs to see if the centres can be changed however there are no plates identifying the type or ratio on the diff housings I have. I will have to open them up I think to see if there is a diff centre that can be retro fitted. Either that or replace the housing to something from a f250 or land cruiser to drop the ratio to a 4.11?

4.8 to 4.11 is a bigger change than most realise. I went from 4.11 to 4.8 on a custom car I have and even with a huge overdrive (around .6:1) the increase in revs was significant. Going the opposite way you risk dropping too low in the power band so you are constantly shifting for every hill or gust of wind. I have another vehicle I would like to go from 4.11 to 4.8 as it is a bitch on the freeway as it isn't revving hard enough to drag itself up a hill.

I'd look into the overdrive gearbox over diffs as it would cheaper and easier.

Standard ratio in the 1300 4x2 is 4.3...IMHO this is a good ratio for 7.50x16 tyres as it has plenty of load starting on hills and also gives 100km/h at 2800revs......You will then need to match the front diff,which is a Dana 60 you can probably find a diff from a Inter or Dodge for this..incidentally,the front and rear ratios in the 4x4 1300 IHs werent a perfect match.