diff ratios, tyre sizes, lack of power........help!

[Saxon78]

Hi All,

 

 My mk2 escort is a fantastic car in every way, but its not the fastest thing in the world. 

 I used it as my daily driver for a while which was great fun but i did find the 4000rpm at 70mph a little tedious, the tyres i were on were 165/60/13 so i think this means i have a 4.1:1 diff ratio? (4 speed box)

 Im now thinking about changing my tyres to 175/50/13, which i think im correct in saying would worsen my current screaming crossflow issue, would changing my diff to a 3.9:1 ratio help this? has anyone got this setup?

 

 Any help or comments would be truly appreciated.

Ta muchly

James

[Vista]

The 175/50 is a lower profile tyre so yes it will make your situation worse. To know whether a 3.9:1 CWP would alleviate the problem sufficiently I'd need to run all the numbers through a gear calc program on my desktop at home.

 

What makes you think you have a 4.1:1 CWP though? It's not dictated by your tyre size

[Saxon78]

Thanks Vista,

 

 Well i explained my situation a while ago to a bloke who claimed to be wise about these things and he thought it was likely to be a 4.1:1 diff, I've also done a bit of research and it seems that the smaller engined escorts  had the 4.1 diff. 

 

 I understand the tyre size doesnt dictate the diff ratio but it has to be taken into consideration when calculating it doesnt it? 

what gear calc programme do you use? 

 

many thanks for your help

[nickdanny]

If you run the RS 2000 diff at 3.5 to 1 you should be doing 70 mph at 3500 rpm

185/60 13 is probably a better size tyre for the car

[Veemax]

My cortina is around 5000 at 70mph so think yrself lucky lol, as said 185 x60 would be better, jack the car up at back and mark the prop flange at the diff, then turn one of the wheels and count the turns on the  wheel for the prop revolution and you will have yr diff ratio

 

Pete.....

[warrenk]

My Sport has a 3.89 ratio diff in it and I'm running 175/50R13's.

At 4000rpm I'm doing just under 100km/h , approx 60mph

[warrenk]

Forgot to ad.

My is running a 3.54 ratio, also on 175/50R13's...

At 100km/h or 60mph its sitting on just over 3500rpm

[quickrack]

I was exactly in this boat until earlier this year. Got a ierra 5 speed. Drops revs loads. 1000+ at 60/70mph