Altering ratios on 6 speed box

[craigyb]

Has anyone any knowledge of changing the ratios on a 6 speed to make it lower revving on the motorway?

[Jonathan Kay]

What final drive ratio are you starting from?

Jonathan

[craigyb]

I don't know, but it's standard

[oilyhands]

The box design means that 6 is direct through the box, 1:1 ratio, I think the only option is to go for a higher diff. It seems counter intuitive that the 5 speed box which has an overdriven 0.82:1 fifth speed also has a higher diff (3.62) and the 6 speed which has a 1:1 top gear has a lower diff (3.92:1) . Not that sensible on a road car, but maybe justifiable on a track focussed car.

Oily

[Paul Richards]

Not often I disagree with Oily, but the higher 3.62 diff is normal for the 6 speed and 3.92 for the 5 speed. The lower diff is an option on the 6 speed.

[Jonathan Kay]

How to check what ratio you've got... 

... and which rear wheels?

... and which type of diff?

Jonathan

(Amended)

[oilyhands]

Paul, that is news to me, every single 6 speed Caterham I have had here has the 3.9 diff fitted, we learn a little every day. Still if the OP has a 3.9 them the 3.6 would be an option.

Oily

[Mucus72]

Damn. This is all really confusing. I thought that the BMW sourced diffs came with a final drive ratio of 3.62. I didn't know that the gearbox you specify at purchase makes Caterham provide you with a diff with a different ratio. I've just ordered a new Caterham with 5-speeder and plan to try for 6 months before swapping (maybe) to a 6-speed one. This now means that the diff ratio needs changing too according to Oily I think?

[Mark w]

My R400K had a 3.62 and six speed . ( and it went to Oily for verniers ! ) 

My 620R also has a 3.62 but the top gear in the Sadev is much longer legged .

[OliverSedlacek]

As others have said, a diff change is your obvious option. I drove mine for years thinking it had a 3.6 final drive when it actually had a 3.9. The only other option is bigger wheels/tyres.

[Mark Durrant]

If you have a Ford De Dion rear another option is to fit a 3.38 diff which will drop the revs by 500rpm when cruising at 70mph but I would only consider this option if you have 200bhp+.

[MADMALC]

My SLR had a 3.62 dif and 6 speed gear box and would give 144 mph at the 8,000 rpm in 6th on ACB10s. Entirely theoretical as I could not get to 8,000 in 6th.

I have a good spread sheet if you want to see what diff you have and what difference changing the diff or the tyre size would make. If anybody would like a copy just blatmail me your email.

[Slipper man]

You should bear in mind that you don't have to change the diff. By changing the crownwheel and pinion, you can change the ratio quite cheaply. Road and Race Transmissions have done mine twice.