Can't get above 55mph

Got a series truck cab 88"on a shortened 90 chassis with all 90 running gear recently put a Auto box in out of a Disco 2 it changes gear nice and smooth up the box but once it locks in at 52mph it doesn't go any faster so as soon as you hit a bit of uphill road it immediately changes down until you build up enough speed for it to lock in again it's a 300tdi running 265/70/16 summer and 285s in winter is it the transfer box or the axles need changing to get more speed.atb Ed

It should do more than 55 whatever transfer box is fitted I would have thought....
For top end speed though, a disco transfer box has a 1.2:1 ratio which is the highest geared, standard transfer box.
There should be a label on the box, visible from below, which will tell you what ratio it is....


1.6:1 as fitted to mil 110s and others (I’m generalising now) is the lowest ratio, and will give the lowest top speed.
1.4:1 as fitted to most 90s are somewhere in between and should easily achieve higher top speed than 55

Theres no label on transfer box so how could you tell,do the axle ratio not make a difference. Ed

Serial number...
https://www.shop4autoparts.net/technical-info/LT230-Transfer-Box-R-Type-T-Type/

Dunno what diff combinations are available to change axle ratios.

I’m assuming you have verified the speedo reading with gps?