help! need more top end speed

[tranny van man]

can anyone help i have a mk3 which was a 2 liter petrol now a 2.5 di, what could be the problem? is it the rear axel, or gearbox? i have bigger tyres on which gives it about 6 mph more at 50 mph it screams its bolloks off what can i do?

[FredTransit]

Whis gearbox have you got? The petol axle is rated differently to the diesel, if it bothers you a lot think about changing the diff for a different ration, ask madmark!

[tranny van man]

cheers fred i think its a mt75 the thing is quick on set off even with a 5 birth caravan + all the kids n luggage and 55 mph is pritty much it at top end. the gearbox is sound it seems to be the 2.0 axel. i need it to about 80 ish as it will be used on long runs with 7 people in it soon. what ratio would i need ? o yea its a 1990 "H" swb mk3 semi hi minibus

[rover]

Cant say what ratio you need but yeah its def the back axle/ diff that needs changing, you need one from a diesel, be carefull though, the ones that were made in Germany have alarger diameter drive shafts, when you go to get one you would be best to take one with you so you know it fits into the diff, changing the diff is easier in one respect cos you dont have to disconnect all the brake pipes and leaf springs, if you decide to change the complete axle though you dont need to worry over the size of the shafts.

[tranny van man]

nice 1 so will that work then? what sort of speed will i be looking at,how long do you think it will take?

[rover]

Well a standard DI will normally do between 80 and 90, some slower some faster, they are not the quickest accelerating vehicles, but will do their top speed all day long.

[tranny van man]

cheers mate i will get straight on with it!

[madmark]

firstly mate you need to get under the van and find the tax on one of the bolts round the diff, remove this and cleen it up and let me know what ratio is on the tag! it sounds like a 5.14 , what you need is a diff with 4.1 ratio these are hard to fine or middle of the road 4.56 this will give you around 70 reasonable ,if you nock it down to much and you are towing you will be in 3rd all the time on the hills 5.14 is the best for towing but not much top end ,4.56 is best allrounder!

[rover]

I always wondered what those Tags were for.......some bloody mechanic I am

[tranny van man]

thanks mad mark, great advice as well as rover. I wont be doing much towing as i have sold the caravan, i just need it to have more top end and hopefully better fuel economy. I will have a look at the rear diff tomorrow and i will post you the ratio. The sooner i get this done the better