I raced the car with the 4 speed and got a 16.22 sec 1/4, then put in the 5 speed 16.5sec. 3.77 diff gears. Several runs with each box so I know these results are conclusive

I am assuming you have actually checked (ie stuck your head in there) the T9 gear ratios in YOUR gearbox, not just reading technical documentation which is often of a general nature.
 
With the change to the 4.125 diff ratio the super short standard Escort first gear will run out of puff much faster than with the 3.77 and especially the 3.54 ratios. Same with the second gear. If your engine is producing 130HP, torque will also be increased. So the engine wants to work because of the extra torque, but can't apply the torque correctly with the shorter diff ratio. So the solution is to change the gear ratios, either in the gearbox, or the diff, or even maybe both. A change in wheel diameter can also help a lot.
 
I turbocharged my RS2000 in 1985 and retained the 4 speed gearbox and the 13 inch wheels. Acceleration and cruising effort and the entire characteristics of the car changed when I fitted a 2.72:1 diff. The car went from being just "fast" to "a weapon" that none of my friends with modified V8's could touch. The secret was in the gearing.
 
Three years ago I changed the diff ratio of my stock standard 1979 1600 Escort panel van from 3.77 to 3.54. The car is my daily driver. Even with good loads and a full tank of fuel going up hills there is more than enough power. The benefit is longer legs, reduced in car noise levels and much improved fuel economy. I am a fan of longer legs on street cars.

Fantastic info there Peter. I really like the taller gearing as well. And a 3.54 should be fine on freeways with 4 speed box. Around 2800rpm, not that the car spends a lot of time on freeways. After reading the above I'm looking forwards to running the 3.54. Thanks very much

If you want to use the car cruising at higher speeds often you will probably like the 3.54 however i much rathered the 3.77. I run a 2L with around the same power (probably a touch less) and 205/60r13 rear tyres. Also standard 4 speed.
 
I dont do much long distance driving at higher speeds so I dont mind doing extra revs. Also I do a bit of track work and hillclimbs where I want the extra revs at a given speed.
 
Car is much more lively with 3.77 than 3.54. It is noticably faster. Will be instaling a 3.9 with lsd (not sure if 3.9 is accurate but there-abouts) soon to give better track performance and no doubt will be screaming when cruising at 100km/h.
 
A shorter diff ratio will no doubt compromise economy and high speed cruising comfort. Even though my car is daily driven I feel its much more important for it to be more responsive on the street and quicker at the track than save a couple extra $$$ when I fill up.
 
Ultimately it depends what you want the car to do
 
Cheers,
Ryan

it accelerates slower and you want to go from a 4.125 to a 3.54 diff?  Thats backwards man, it will be significantly slower on acceleration with a 3.54

Gdub
it accelerates slower and you want to go from a 4.125 to a 3.54 diff?  Thats backwards man, it will be significantly slower on acceleration with a 3.54

Agreed. I thought the whole idea of most people changing to a  5speed was so one could bump up rear gearing but retain some level of cruise with 5th gear.
Wheel weight is something I really notice going from 13" wheel to a 15" wheel. Were 1/4 mile times on exact same setup with Gearbox being the only change?
 

It might accelerate quicker with lower gearing, but 1st and 2nd run out of puff super fast, it only starts to get decent speed in 3rd! The key to a quick 1/4mile is getting out of the hole as fast as you can. Which means a good take off and less gear changes at low speed.
 

Rhys, yeah mate.All the same, different gearbox

From memory when i had my pinto/t9 5 speed combo i was a running a 3.9 diff. 1st was quiet short, but then 2nd was quiet long and usable. Would just get to 100kmh revving the tits off it. Was a bit wilder engine than your running by the sounds though..

I would try a 3.9 diff first (with the 5 speed) which will let you use the torque a bit more. Being a pinto around 130hp it won't have wild top end cams in it like mine did, but more nice torque so the slightly longer ratio will lend itself to this.
 
Weight of the box won't have much if anything to do with it.. 10L of fuel weighs the same as the difference of the 2 boxes.

According to Gearcalc, 100kph in second with 3.89 diff gears means you would be revving at 7500RPM.
Does that sound right to you?