[Steveti]

Link to UK gear and diff ratios :

http://www.roadster-nc.com/2013/07/roadster-transmission-drivetrain.html

I will quite happily stick my neck out here and suggest that in a real life road driving situation you will not find any real difference in the driving experience.

Any theoretical gains in acceleration with the lower ratio diff are simply offset by a higher ratio in the gearbox gearing, any and all road tests that I have seen of the five and six speed cars show no performance differences, I'm sure that there may well be a tenth here or there to be gained or lost at some given point in the rev range between the two cars but sufficient to start thinking about swapping gearboxes around to make theoretical performance gains? No chance far to much work involved for no real world gain of deficit.

As I suggest if you are looking for something that will provide a real life performance gain then a simple BBR Super 175 kit would be the way to go. Prior to installing the charger to the GM I did run it for a while with what was in effect the Super 175 conversion and that reall made a huge difference to the car, it really freed it up and made an already willing to rev engine even more willing to rev.

Also worth bearing in mind that there are differences between the NC1 & NC2 onward 2.0 engine that give it another 500rpm before the redline and limiter, that of course equates to more spees available in each gear for the 2.0 litre NC2 onward.

My view on the gearboxes is straight forward enough, if you start mismatching gearboxes and diffs i.e. a diff from a five speed with a six speed box or a diff from a six speed with a five speed box then you will have an inaccurate speedo and will start playing around with the fuel consumption. If you are tracking the car, then the six speed box is the sturdier box so that would be my choice.

For road use, leave the gearbox and diffs matched up as they left the factory, as stated in real life driving you will not notice the difference and you won't feel that you are missing out on anything for the lack of that one intermediate gear, the engine has sufficient power and torque in the right places to ensure that this is the case. What is often not understood is that there is a lot of time and effort put into the calculations to determine the optimum ratios for engine power/torque curves the weight of the car and resulting performance figures. In fact you will more than likely find that the manufacturer will have a desired performance criteria right at the start of the design process, the Engineers then do the required calculations to determine the gear ratios required to give the desired performance. There is no guesswork involved when it comes to the gearbox and final drive ratios.

 

[Beaky]

Great insight and advise Steveti, decision made will fit and enjoy and put some money toward the BBR175.
I can get all the adrenalin rush, etc from my air cooled Porsche, but since these are now trading at silly prices wanted to reduce its usage by supplementing it with the MX5

 

[Rubik]

My view on the gearboxes is straight forward enough, if you start mismatching gearboxes and diffs i.e. a diff from a five speed with a six speed box or a diff from a six speed with a five speed box then you will have an inaccurate speedo

Speed is calculated from an average of all four ABS wheel sensors so changing gearboxes and diffs wont affect the speedo reading.

 

[Steveti]

My view on the gearboxes is straight forward enough, if you start mismatching gearboxes and diffs i.e. a diff from a five speed with a six speed box or a diff from a six speed with a five speed box then you will have an inaccurate speedo

Speed is calculated from an average of all four ABS wheel sensors so changing gearboxes and diffs wont affect the speedo reading.

That's handy to know, I thought the boxes still had a speed sensor albeit electric/ non cable. So only tyre sizes will bigger abouut with Speedo accuracy, nice one and just shows that every day's a school day. Cheers.

n.b. I've just had a look in the workshop manual for the six speed box at least and it does have what appears to be a speed sensor on the output shaft of the gearbox. This could of course be for a variety of purposes with modern cars being littered with so many sensors but interesting that it shows a vehicle speed sensor as part, or an option (?) on the box.

The Five speed box manual shows a similar set up.

 

[Roddy]

My 2.5 had a sticky speedo. Sometimes it worked and sometimes it didn't. I changed the cog doo dah on the end of the speedo cable that connects it to the gearbox and all has been good since.

I think I'm telling lies again. The cog doo dah in the gearbox doesn't fit on a speedo cable. It attached to a sensor with wires to the speedo. It's a 5 speed box. I also have ABS on all four wheels.

 

[steveh]

I wonder if there were any mk3 /NC without ABS.

 

[Kev5]

My 2.5 had a sticky speedo. Sometimes it worked and sometimes it didn't...

That's interesting, Rodders, all the indicators on mine are like that - you know, keep going on and off...

 

[Roddy]

It's a nylon cog Kev and the teeth were worn. As soon as I stuck a new (secondhand) one in, the problem was cured. Just one or two small fixing bolts and the electrical connector. Best off done on a ramp though, otherwise it's not easy to reach.

 

[Rubik]

It's a nylon cog Kev and the teeth were worn. As soon as I stuck a new (secondhand) one in, the problem was cured. Just one or two small fixing bolts and the electrical connector. Best off done on a ramp though, otherwise it's not easy to reach.

Im an old git and my first car was a Ford Pop 100E 3 speed which sounded like a jet plane when you gave it some in second. Eventually 2nd gear gave up juddering with a broken tooth so I drove round the busy London south circular using 1st and 3rd to buy a recon box advertised in the Exchange and Mart. When I got there they didnt have one in stock which taught me to ring ahead in future. So I drove home revving the nuts of it in 1st before grabbing 3rd. Then i noticed the speedo wasnt working. Later when I changed the box I found the nylon speedo gear had melted. It was a hot summers day too which probably didnt help. Those were the days.

 

[Rubik]

Another important factor we must consider is shift time when comparing an all out drag between 6 and 5 speed 2.0 NC cars. To the point of minimum transmission loss (5th in a 6sp car and 4th in a 5sp car ) the driver of a six speed car needs to make one more horizontal shift losing momentum for 1s. Most 6sp cars have been at the pies too and are loaded up with unsportscar like goodies which doesnt help. In 4th onwards the 5sp car has the advantage with its 4.1 diff. The 6sp car will of course have a slight edge in the lower gears around the houses. Early 6sp boxes could be notchy so check gear change is acceptable before laying down the wonga.

Shift times

• A long shift time is considered anything over 625 ms
• The average manual car driver: 500 ms - 1 s (vertical gear changes e.g. 1st-2nd, 3rd-4th, 5th-6th): 1s - 2s horizontal gear changes (2nd - 3rd, 4th - 5th, 6th - 7th). As well shift times can change depending on gear throws (distance between gears), its easement of movement, ergonomics of lever and age of gearbox.