Hi guys,

Silly mistake I know but just mounted the gearbox I had from an old dinner car in to the Haynes I am building and tried out the gear seating position etc when I found out the box I have is a 4 speed. Would there be a benefit dumping the box and trying to locate a five speed?

yes, the benefit will be an extra gear

I think it depends what you want from your car. I am building a Haynes and plan to fit the 1.6 Pinto (59bhp when new!) and the 4 speed box which came in the Sierra I'm breaking...

there is plenty of info online about the gear ratios to expect from the 4 speed (which I think is not a Type 9 as they are all 5 speed?) Ford boxes and the 5 speed so you can see the practical difference between them.

I think you have to have a good look at a gear speed calculator to really understand how the ratios will work with your diff ratio and your tyre size.
If your replacing a 4 speed type 9 with a 5 speed there is little difference apart from not having a cruising 5th.
If the 4 speed is an earlier type E then there are versions that have much superior ratios, or worth more than a type 9 5 speed

I have an unmodified 5-speed type 9 in my Locost. The first gear is so low that it is near-enough useless apart from very low-speed manoeuvring, and I generally start in 2nd unless I'm on a steep up-gradient. So, effectively, I have a 4-speed box. So, as snapper says, look at the ratios as the 4-speed box that came before the Type 9* may well be better suited. The 4-speed's ratios are more spread out and therefore more useful.

The T5-speed is really good when modified with a new set of gears that spread the ratios across the range. Unfortunately, this costs a fair bit of money!

* Note: IIRC, the Type 9 box is the 4-speed Type E with an extra gear housing attached to the back.

What he said ^
The T5 is good out of the box
Type 9 needs a longer 1st gear
I run V6 Type 9's which whilst not ideal has a better 1st (3.36) and a much better 2nd (1.81) with a 1.21 3rd
Ideal gearbox for the road would be a V6 Type 9 with the 2.7 1st

It's all about getting the gears closer together and fitting a diff and tyres to tweek.

Having an engine that has a higher rev limit effectively shortens the gap between the gears and as in the Cosworth turbo torque further masks big gaps