Im finally getting into figuring why my speedo reads high and thanks to help from the forum, reailze that it is the obvious ... my differential has been changed. Looking back at my files, I see that the PO "thought" that it had been changed to an Austin Healey as he said it was changed to a stronger rear end. I am 90% certain that it is as he described as I've looked at pics for comparison. I checked the ratio by spinning the rear wheel while up (leaving the other on the ground) 2 revolutions and counting the revs of the drive shaft (prop shaft). THe shaft went around 4 1/8 times leading me to believe that I have a 4.10.

Finally my questions:
(1) Did all the Darts have a 3.58 or were there differences between the series (I have a B Spec).
(2) When having the Speedo Calibrated, do you just look at that gear ratio? I found a calculator on line that given the new and old gear ratio AS WELL AS new and old tire sizes, calculates the Actual speed and % diff. As a matter of interest I calculated both with and without tire size differences (Old bias Ply 26.5" diameter vs Radial at 25.43") and this is what I found:

Speedo Reading--Actual speed Calculated with Tire and Gear change --Actual speed Calculated with Only Gear Change (old tire)

20 -- 16.7 -- 17.4
40 -- 33.4 -- 34.8
60 -- 50.2 -- 52.3
70 -- 58.5 -- 61.0
75 -- 62.7 -- 65.3
80 -- 66.9 -- 69.7
90 -- 75.2 -- 78.4
100 -- 83.6 -- 87.1
% Diff -- -16.4% -- -12.9%

So, It would seem that I want to have my speedo calibrated to show 16.4% less ...if that is possible.

Finally, As I understand it, the Higher the gear number, the lower the gear which would be slower top end but quicker off the line and conversely the lower the gear number, the higher the gear anhas a faster top end, but a bit slower getting there (and easier cruising) ...
any thought or experience with different rear ends (other than not having disc brakes)Looking forward to answers and thoughts on this (and going out and testing the math).
Josh

No comments here. Confirming 3.58 for all A, B and C specs? Op opinions on how a 3.58 feels/compares to the 4.1?
Josh

I can´t believe a AH 100/6 diff fits inbetween the axle tubes of a SP250. The axle casing is completely different. The diff of my SP250 was changed by a crude American pre-owner to a TR3 one, half axles modified which weakened them not strengthened.
Axle ratio was then 1:3.7.
With a 1:4.1 diff you may not need the first gear, top speed at 70 miles/hour?
Sorry, better to find a propper rear axle. Meanwhile very hard to find.
Or if you find a skilled mechanic to fit an available 7HA Salisbury diff between the original tubes. 1:3.58 crownwheel & pinion are available from different sources.

Josef Eckert
Königswinter/Germany

Thanks for the input. I think the entire rear end was used as I seem to have the AH drum brakes as well . I was told, that in the US, this was often done as the rearends in the AH were stronger. You are correct that I dont really need 1st and that at (actual) 70MPH seems to be getting up there ...Ill check the RPMs if I can take it out this weekend. Thanks,
Josh

the BMC C series rear axle is a very tough and durable unit.. the huge drum brakes make it heavy though .. the common ratio for the big healeys was 3.9:1... or 3.5 without overdrive ( rare.. most had o/d fitted) .the early cars ( 100/4 and 100/6) had a higher ratio overdrive and once use the 4.1 diff.. this may be the axle you have... the non overdrive diff closely matches the SP250 ratio.. you can get a disc brake set up to fit the C series axle.. a good machinist can make some adaptors to take the standard SP250 ( or jag Mk2/ Daimler v8 2 1/2 litre) dunlop rear brakes.. personally I prefer the design of the BMC C series axle.. the diff is very easy to remove/reinstall and requires no special tools to do so.. the salisbury rear axle as used on the SP 250 is very close in design to the TR2/3/4.. as is the morgan axle.. esentially a US 'dana' design..( salisbury became GKN.. and I believe it in now under the banner of 'Dana' once again) come to think of it the 1960s volvos also used this axle.. as did the later MGB and the 2 years only MGC.. which offered 3.08, ( early cars) 3.31( auto box) and 3.77( later cars with overdrive) ratios..
Id think the best 'alternative' ( and close in appearance to original) would be a post '67 MGB axle.. which is 3.9:1.. 3.08 crown wheel and pinion sets are readily available for these axles.. they come set up for spoke wheels ( different widths for steel wheel and spoke wheel..the step wheels one is wider.) and both very strong and are cheap as chips.. as well as being easy to adapt to disc brakes..

I’m wondering if I can, or should change the internals of mine to the 3.9 or 3.5 as I am ok with the drum brakes and after putting so much work into the car didn’t want to take the whole back end apart again and put the discs on ... if I could even find the discs. I don’t plan on driving at top speed or near it but having some lower revs on the freeways might be nice (I do not have overdrive)

While my 4.1 rear end gets me off the line nicely, cruising on the freeway at 70mph to 80 mph feels like it is pushing the revs pretty good. Considering a laycock J overdrive, I was told that the proper (best reduction) is found on the ones with a part number starting with a 25- .... And not to use the more common ones made for the Volvo (which started with a 26- or a 27-. I found this recent listing for a Laycock J but as you can see it clearly says it came from an '80 Volvo, but starts with a 25- . Also, it says it uses atf and to research what oil to use. As I know nothing about this, I'm looking for some advice here - will this one be a good fit/ reduce the revs enough?

Thanks, Josh

https://www.ebay.com/itm/VOLVO-240-OE-J ... SwxJlayILM

Don't know about the gearing but the J type will run on a multitude of oils.
Volvo advised ATF other manufacturers advised much thicker oils.
The ones used on SP's run extremely happily with SAE 30 or 20W50 or what ever engine oil you generally use.

3.72 ish is a great fit with the tq. curve and a five speed @ .8 through to .86 ( toyota or T5 box). Why load a mtr. that like to rev ? It realy depends on the cruise speed and the keep it at 2800 to 3000 rpm at that speed.
Will tune the SP to the rebuilt stainless SU Manifold along with checking the advance curve --- not on the Dyno but from real road driving experience . I'd found that tuning to max power is not the same as what is needed for pulling from 2500 in third on local twisty roads

steve

Big Col - good to know about the oil

Steve- I am confused on the gearing. I have a 4.11 rear end and the revs seem high cruising at 70-80 mph. Do you think this is the right unit for me?

I read that the units that start with a 25/ are correct for our cars (with a std rear end) and were not the ones in the Volvo’s (with less gear reduction) ... but this seller says it came from a 1980 Volvo... thoughts?