Need a bit of help from thoses that under stand these things! Going to change the xflow to mx5 power wth the gear box but leave in the live axle from the mk2 escort,
Now i have a 3.54 diff which i know has been handy with the 4 speed i have but will it be ok with the mx5 box? I sorta think it will as its not far from the diff ratios of some of the mx5 as far as i can see but only of the ones with 6speeds and i'm going with 5 speed,
Gear ratios 
1 3.136 
2 1.888 
3 1.330 
4 1.0
5 0.814 
Any one have any ideas what they will behave like? Will it be over geared or will the weight advantage over the mx5 sort that? Or have i got this all wrong(lol wont be the 1st time)
Cheers

Which MX5/engine/year? What was the original diff ratio?

From those gear ratio's I'd guess NA or NB 1.6/1.8 which had 4.1:1 diffs. It's a big jump to a 3.54 so you might be pushing it

Not got anything yet but will be a 1.8 non vvt these are the only ratios i can find, seem to be "international" so presume uk had the same? The locost at the mo is just over 550kgs so pos 600 with new lump and bigger tank i will fitting, if that makes any difference?...if its to to high i will change the diff.

Given the huge difference in weight between the 7 and Mx I would suggest you stay with the diff you have, considering that gears 1 to 4 will be similar to your current setup and 5th gear will be a great economy cruising gear.

You can always change the diff later at your leisure if you find that you want more performance.

If you are running smaller tyres (diameter wise) than the MX5 then that will help redress the balance.

Use a gear/speed calculator to find the road speed @ revs in gear
I run 3.36 1st 3.9 diff 13 & 14" wheels
Your 3.1 seems ideal

What are the ratios of the Ford box you have at present?

Usually, top gear (4th) is 1:1, so will be identical to 4th on the Mazda box, whilst 5th on the Mazda will be a slightly more relaxed 'overdrive'.

If you've been happy with your current diff. with the Crossflow, chances are you'll be equally happy with it coupled to the Mazda - particularly if you like the idea of a slightly more relaxed and economical cruise.

3.54:1 is a little on the tall side, by the standards of this type of car, but then not all of us enjoy driving everywhere with the engine screaming its nuts off to no effect.

Try playing with this

When I had a mk2 escort a 3.54 diff gave me 20 mph in top gear 4 spd with was 1.00 to 1.00 at 1000 rpm on 18570 /13tyres so in 5th it would be approx 3500rpm at 80 mph if your running 185 60. /13 5th gear would be about 75mph at 4000 rpm which would be ok as the the car wieghs about 40 percent less than the mx5 it should be ok remember higher diff makes all gears longer which is better hope this helps Ian

I run 195 50 15s but could go to 13s to lower it(have been thinking of going 13s any way) a over drive on the motorway would be handy i dont mind revving stuff i'm use to bike engines so its normal but if theres no point? But i dont want it to be over geared on track days especially on the shorter tracks like brands etc but with the extra power i'm was hoping it would be better? Maybe not!

Gearing now is
1, 3.656
2, 2.185
3, 1.425
4, 1.000

[Edited on 23/12/14 by threadbare wallet]

My striker had a x flow in it swopped for a Toyota 4age totally transformed the car I'm running a 3.9 diff I wouldnt want to run a lower diff as you will be changing gears all the time of I could get a slightly high ratio I would as less gear changes is better in my book 5th is only used for cruising not on track .i would try your diff first to see what it's like .the Toyota engine just loves to rev as the mx5 engine will .

Cheers will give it ago with the diff and change wheels and/or diff if need be as dont seem miles out.