craigyb Posted November 29, 2015 Share Posted November 29, 2015 Has anyone any knowledge of changing the ratios on a 6 speed to make it lower revving on the motorway? Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted November 29, 2015 Member Share Posted November 29, 2015 What final drive ratio are you starting from?Jonathan Link to comment Share on other sites More sharing options...
craigyb Posted November 29, 2015 Author Share Posted November 29, 2015 I don't know, but it's standard Link to comment Share on other sites More sharing options...
oilyhands Posted November 29, 2015 Share Posted November 29, 2015 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 Link to comment Share on other sites More sharing options...
Area Representative paul richards Posted November 29, 2015 Area Representative Share Posted November 29, 2015 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. Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted November 29, 2015 Member Share Posted November 29, 2015 How to check what ratio you've got... ... and which rear wheels?... and which type of diff?Jonathan(Amended) Link to comment Share on other sites More sharing options...
oilyhands Posted November 29, 2015 Share Posted November 29, 2015 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 Link to comment Share on other sites More sharing options...
Mucus72 Posted November 29, 2015 Share Posted November 29, 2015 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? Link to comment Share on other sites More sharing options...
Mark w Posted November 29, 2015 Share Posted November 29, 2015 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 . Link to comment Share on other sites More sharing options...
OliverSedlacek Posted November 29, 2015 Share Posted November 29, 2015 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. Link to comment Share on other sites More sharing options...
Mark Durrant Posted November 30, 2015 Share Posted November 30, 2015 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+. Link to comment Share on other sites More sharing options...
MADMALC Posted November 30, 2015 Share Posted November 30, 2015 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. Link to comment Share on other sites More sharing options...
Slipper man Posted December 6, 2015 Share Posted December 6, 2015 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. Link to comment Share on other sites More sharing options...
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