90ps135 luton van low top speed

[rampking]

Mk7 just died and had to get another luton on road quick to save contract. I bought a 54plate mk6 90/135 luton. It used to be a flatbed and has had a luton body with slimjim tail lift put on it at some pint recently. 83k genuine miles. Starts and drives fine. BUT, 3000rpm in 5th gear is 50mph and screaming nuts off it to get to 60 if i'm lucky on motorway. Question: Are tippers/flatbeds lower geared or has mine had another box fitted at some stage. I don't think rear axle/diff has been changed as speedo almost same as tomtom indicated speed. So, if it's gearbox, what box should i be looking for to change it over and what top end should i expect. I know it's underpowered for a luton which runs at 3500kg continually. Thanks in advance.

[knobby1]

It'll probably have a 5.13 or possibly higher ratio diff in it....I guess originally it was used for pulling heavy loads being a flatbed, not built for speed. Gearboxes are all the same ratio.

I suspect the only thing you can really do is put a lower ratio/taller diff in it. I'm guessing a 4.27 is a good compromise between speed and load pulling.

Lord Knobrot

[rampking]

Thanks Knobby1. Will start looking for a diff replacement. cheers.