With standard gearing and original tyres the SP is geared at 20.6 mph/1000 rpm, so at 70 mph the engine speed is 3400 rpm.
With your 4.11 axle ratio Josh, 70 mph in top will be achieved at 3900 rpm.
The Laycock 'J' type overdrive (the 'J' is the easiest to fit in an SP) is available in a number of ratios but that recommended (by Overdrive Repair Services, the Laycock specialists) is 28%. For a given engine speed a 28% unit will produce a 28% increase in roadspeed or, for a given roadspeed, a (1 - 1/1.28) = 22% reduction in engine speed. So a 28% unit would reduce your engine speed at 70 mph in top to 3040 rpm.
Similarly a 25% unit would reduce your engine speed at 70 mph in top to 3120 rpm.

Another possible approach (depending on the availability of A-H parts) would be to swap your Crown Wheel and Pinion for an alternative ratio. A-H axles were originally available in 4.11, 3.91 and 3.54 ratios. If a 3.54 could be found this would bring your overall gearing back to almost the 3.58 standard SP.

It just occurred to me that the post '66 MGB rear axle would likely be a good swap.. whilst the standard ratio for these is 3.9:1, there are alternatives.. MGC came with 3.77:1, 3.31:! ( likely ideal for the SP 250) as well as 3.08:1...which was used in Both MGC and MGB V8..alternative ratio crown wheel and pinion sets are available.. this is also basically the same design as the original SP 250 axle..and another version was used in volvo cars up to the point they went to independent car end.. its basically a US 'Dana corp' unit made under license in Uk..( the V8 saloons uses the larger '4HA" unit which corresponds to 'Dana 44' in USA.. the MGB and Triumph TRs use a smaller and lighter unit.. made by 'Salisbury' then 'GKN' .. but now under the corporate banner of US Dana corp.. I can confirm that this is a perfectly adequate rear axle for the SP 250.. and would be willing to bet its close and may well bolt right in .. track width on an MGB measured from centre of rear tyres is approx 49.5" on a step wheel car i just measured.. the spoke wheel/splined hub versions carry a narrower rear axle, to compensate for the different 'offset' of wire wheels..as I recall by approx 7/8" narrower per side ..bolt pattern for steel wheels on the MGB is 4 1/2" and 4 x 1/2' studs..

4.11 is all out racing stuff. Even 3.9 fells low on the road. I think 3.7sh with a five speed is spot on or back to the 3.58ish if still with the four speed.
One of mine has a jag 4ha with a 3.75 in it and even that is bloody annoying on the open road with the std. box. The difference between 2800 with the std 3.58 and 3400 ish rpm with a 3.75 starts to get on the nerves after a while.
regards Steve

I like the 4.11 as it is very quick off the line and can keep up (and even beat) my Sunbeam Tiger. The only issue I have is highway cruising - the reason for my asking about OD. I re-red the article given to me by Wilf but am still confused a bit.... I read that there is a difference in Laycock type J units -some being fitted to Triumphs, MGs and the likes and other to Volvo’s ( and not to get those Volvo type) . I think that I would want to find o unit with the largest reduction as with the 4.11 it will be reving the highest possible and I think even with the most reduction, I will still be in the sweet spot (in terms of acceleration and torque. Which brings me to my question- how do you tell what the reduction percentage is? I only found 1 person on the web who says that the reduction percentage is given by the 1st two digits of the part number ... so that a part starting with 22/ , 25/, or a 27/ part number would mean 22%, 25% and 27% respectively (actually I may be slightly off on the definition- a 22% actually means it will go 22% faster given the same rpm- which for some mathematical reason reduces the rpms something like 18%). Can anyone verify how to tell the %? Even Most sellers do not know this information and I don’t think a 22% would be adequate for me so I want to be sure of what I get. Thanks

Im unsure what ratios are viable for the J type overdrive.. those for the 'A' type (as used on the jag moss box , and big healeys) come in several options.. some quite tall .. the best source for info n the J type, would likely be 'gear vendors' in El Cajon Ca USA.. they make all manner of conversions for this unit and they have re-enginerd them to take significant torque outputs..

Just a thought-- The lower diff ratio will get you there quicker of course but how long will the org. first gear last if used with enthusiasm ? Are the replacement parts nowadays any better ( longer life) than the org -- which wasn't long.
steve

I think Vortex in his post above explained everything about the various ratios of the J type OD very clearly indeed.

The J Type 28 is the recommended one for the SP and it works very well regardless of diff ratio.

Bear in mind that you are not fitting an OD to increase your speed but to make for more comfortable and relaxed cruising.

An A spec with a 22% overdrive was reputed to have attained over 140mph in the 70's, though I have no first hand knowledge of this, though rumour said the car was from the Birmingham area.

So how does one know what the % is that they are buying? from the part number? sellers don't seem to know what they are selling (other than what car it came from and if it worked or not.

Also, does anyone know if Overdrive Repair Services is still in business. I e mailed them asking the same questions as discussed here as well as about stocking the modified main shafts (based on the articles that Wilf had provided me), but have yet to have a response.

The only way to tell the % of the J Type OD is to look at the serial number on the attached plate. The numbers 22, 25 or 28 are marked either at the beginning or end of the serial number, I can't recall which.
Overdrive Repair Services are still advertising in the mags - give them a phone, +441142482632. They are very helpful.