Gearbox update

5-Speed vs. 6-Speed box

The bottom line is that I am unsure as to which gearbox I will end up with. I have placed the order based on the standard 5-speed gearbox which is now a Mazda sourced unit. The 6-speed unit is a Caterham box, but also comes with a pretty steep £2,500 optional extra charge.

The consensus is that the 6-speed box is better suited to track days, and the 5-speed more appropriate for touring, with more widely spaced ratios and more of an ‘overdrive’ 5th gear.

My problem is that I enjoy track days and road driving, and with a fairly torque rich Duratec 2 litre engine (certainly compared to my old K-series engined Caterham), does it matter that much, seeing as I’m on track for fun, not to chase an additional tenth of a second?

Gearbox analysis

I downloaded an Excel tool from available here  to try and see if I could work out the best option just based on simple maths and gear ratios / tyre sizes and final drive (the differential gear ratio).

I also think, but am not 100% sure, that the current differentials shipping from Caterham are from BMW (used on 1-series cars) and have a final drive ratio of 3.64:1, irrespective of gearbox option purchased.

So I plugged all of the ratios and wheel sizes into this spreadsheet and got the following results:

Mazda 5 Speed data

5 speed

Caterham 6 Speed data

6 speed

Analysis of results

A few things strike me with these results, and to be honest, I’m not an expert here, and probably need guidance from someone who is. Feel free to comment on this post, all advice and thoughts welcomed!

I like the fact that with the 5-speed box, the engine revs are around 3,000 at 70 mph, as this makes for more relaxed driving on more tedious A roads and motorways (compare this to 4,000 @ 70mph for the 6-speed box).

With this ‘overdrive’ though, comes the problem that on track, I will only be using 2nd,3rd and 4th gear, and there are some fairly big gaps between those gears and larger rev drop-off’s, so it will be harder to keep the car on the boil. But having said that, my understanding is that the engine torque characteristics will cover much of this problem.

The 6-speed box is obviously more suited to track work, with 2nd, 3rd, 4th and 5th coming into play, with all of these ratios closer to one-another, giving better options of being in the perfect gear for all corners. Its a bit of a shame that the 6th gear isn’t considerably longer.

I’m still not sure what to do here. Perhaps the most obvious thing would be to test drive cars with both configurations and simply get a ‘feel’ for the right decision that way.

Currently the order has been placed for a 5-speed box, and I could always swap this out at a later stage.

The specification


Deciding on the specification was not difficult for me. The first time I bought a Caterham, I had little or no idea, used some guidance from BlatChat, and rather flukily managed to purchase an awesome car, just the right spec for me.

This time around, having been in the Caterham world for a while, I know exactly what my perfect specification will be, and ordered exactly that.


  • Seven 420 – Complete Kit
  • Wide-bodied Chassis (S5)
  • Self Build
  • R Pack
    • Limited-slip differential
    • Lightweight flywheel
    • Sport suspension pack
    • Uprated brake master cylinder
    • Carbon-fibre dashboard
    • Composite race seats
    • Momo steering wheel
    • 4-point race harnesses
    • Black pack
    • Unique key, gear knob & instruments
    • Shift light
  • 5 Speed Gearbox
  • 13″ Apollo Black Alloy (6″, 8″)
  • Ventilated discs + quad piston calipers
  • Full Windscreen, hood, side screens
  • Tonneau cover
  • Boot Cover – carbon vinyl
  • Fully Carpeted Interior
  • Quick Release for Momo steering wheel
  • Lowered Floors
  • Heater
  • Track Day Roll Bar
  • ZZS Tyre upgrade
  • Filler Cap – Aero option – Black
  • Paint Colour – Ford Frozen White (7VTAWWA)
  • Full Decal Pack – Caterham Blue


This was the biggest challenge. I spent ages looking at other Caterham paint jobs and just car colours in general. In the end I decided on a base that means that I can swap decals and stripes if I choose, and a colour that’s easy to touch up.

So I picked white. But then there are many shades of white, some flat, some pearlescent, some creamy, some at the blue end of the spectrum.

Ford Frozen white has been used for a few years on many of their cars, looks particularly good on that little Fiesta ST. And a very easy paint to get hold of.

My future Caterham will probably look a little something like this:


The purchase decision

I have owned a Caterham 7 for the last four and a half years, and it’s been a wonderful experience. There was nothing wrong with my car, it was in great mechanical condition, I looked after it well, I just got to the point on track days where I wanted the next level of performance.

Now those in the know say, and I completely agree, that there is much more performance to be gained from self-improvement and driver training.

To them I say “yes, but first I have to stop getting frustrated running out of puff on medium length straights on any race track”. Its disheartening to have cars that are so much slower into the braking and cornering zones then sneak back past a few moments later.

After trying a few cars out last year, most notably the Supersport R (180 Bhp) Duratec engined Caterham, I knew that I needed to be in the over 200 Bhp territory. The Supersport R did not feel that much quicker than my very well sorted circa 150 Bhp K series.

I then started thinking about buying second hand, but then the price difference between a very good condition 2-3 year old well-specified car was not that different to new.

And I am 6’5″ (198 cm) tall, and feel extremely uncomfortable in a Series 3 chassis car, so it had to be SV, with lowered floors. When I was looking on the used market, at the time nothing was jumping out at me.

So then I started playing around with the Caterham new spec and pricing sheet and at that point realisation dawned that not only could I guarantee the perfect specification, I could also have a crack at building it.

Then I started reading the Assembly guide and some other self-builder blog sites. Then I chickened out and went back to drawing board. I mean how many people achieve a smooth build, with no problems or no missing parts? A big fat zero.

Then I had a reality check. Its not going to be perfect, I know what Caterham is like. I know that its a bunch of awesome people trying their best in a low margin, low volume challenging environment. I don’t really care about that kind of thing. I spent four and a half years just pleased that my car started and got from A to B most times, and the unknown is all part of the fun. Damn it, I’m no perfectionist and have no delusions of grandeur. I don’t get upset with creaky, rattles and noisy diffs (ear plugs are great!),  and when things drop off, there’s always a tie-wrap close to hand.

So I went for it. Placed my deposit, ticked the boxes, picked a colour.

The whole of the above process took about a year. My target is to complete the build in a quarter of that!