Longer 6th Gear

[JasonNT]

Hi Folks,

I have a six speed box and was wondering if anyone had replaced 6th with a longer gear ratio?

5th and 6th are pretty close together and the Cat is screaming its head-off on a motorway. It would be nice to drop in a longer 6th with a view to having a more reasonable rev count.

Any thoughts or advice?

Thanks

Jason

[Jonathan Kay]

Would you settle for a longer final drive? What do you have at the moment?

Jonathan

[JasonNT]

I have to admit I don't know, what I do know is its at about 4700 at 80. 

Do you mean tweaking the Diff?

 

[Jonathan Kay]

I have to admit I don't know, what I do know is its at about 4700 at 80. 

What wheels and tyres?

Do you mean tweaking the Diff?

Depends on how pedantic anyone is feeling, but I'll go with "Yes". :-)

Jonathan

[Mark Durrant]

The 6th gear ratio is 1:1 and you cannot change this so if you want to reduce the revs at 70 mph you will need to change the diff. Do you know what ratio you have, this is likely to be could be 3.6 although some have a 3.9. If you change the diff ratio it will impact on acceleration. What engine do you have?

 

[JasonNT]

Tyres 195/45/R15 Avon Cr500

 

Yeah not sure on the final drive :)

[JasonNT]

2.0ltr Duratec

[Jonathan Kay]

Tyre diameter is 556mm (21.91").

Now we need to work through the stupid inconsistent units... gives a final drive ratio of 3.25. Are these known to exist?

3.9 final drive gives 66 mph at 4,000 rpm.
3.6 final drive gives 72 mph at 4,000 rpm.

Jonathan

[Mark Durrant]

The Duratec has sufficient power for a different diff ratio without blunting your acceleration. I would check to see what ratio you have and then consider the options. I have a spare Ford 3.38 Crown Wheel & Pinion which could work and let me know if this is of interest.

[JasonNT]

The car goes back to Caterham next week so will see what they have to say on ratios (have to say have no idea). Will reach out with feedback.

Thanks for the feedback already :)

J

[JasonNT]

How did you work that out? Interested to know.

So suggestion would be to head the 3.6 route?

[Jonathan Kay]

Pick a calculator, change the final drive value and see the effect on engine speed at 70 mph etc. Those calculators also allow you to predict the speed in any gear once you've entered the ratios. 

:-)

Jonathan

PS: I'd measure the final drive ratio directly: jack up the back end, turn a road wheel by hand and count the ratio of turns of the prop shaft to turns of the wheel.

[Bricol]

That'll only work if the diff locks or is very stiff - either the other wheel will spin the other way and the prop stays still, or the prop will rotate twice as fast as ou expect.

I've always want a longer 6th - or an overdrive.   But I've been prepared to accept the tradeoff for the better acceleration which I use more often.

Given a suitable amount of cash, some drawings or a box to take apart and reverse engineer, I would have thought it wasn't impossible to make 6th into a overdrive ratio like the 5spd?

[Wrightpayne]

Caterham parts have a recon 3.38 diff assembly on offer at just under a grand.

[revilla]

I think 6th "gear" doesn't actually have any gears, it simply locks the input and output shafts together directly, so not really possible to make it anything other than 1:1 without a major redesign of the box.

[simon_h]

If you want a good cruising gear I'd get a 5 speed built with close ratios that suit your engine and match it to a good diff ratio. This would be good if you have a duratec that has plenty of torque anyway. 

[Mark Durrant]

I have a spare 3.38 crown wheel and pinion that covered just 6k miles for £250.

[Ian B]

Any thoughts or advice?

Yes, stay off the motorways 

[Rex]

The low flying archivist Martin Phipps checked my ratio during one of his visits to Caterham. My six speed has a 3.92 final drive. It works out to about 4100 rpm at 70 mph with a Zetec engine and 16" wheels.