The_Doc Posted October 27, 2015 Report Share Posted October 27, 2015 (edited) Hi All Since I seem to get sensible and good answers I thought I'd just keep asking questions to get help with the rebuild of my Tr4A (solid axle). When considering the differential, there seems to be two on offer, a 3.7:1 and a 4.1:1. What are peoples opinions on this. A quick and crude calculation: In direct drive (1:1gear ratio) with the wheel diameter = 0.645 m at 4000 RPM, this should result in a speed of 80 mph for a 3.7:1 differential. This is very rough as it doesn't account for resistance. Is there any benefit for going for a slower diff (i.e. the 4.1:1)? Or if you are purely considering a direct drive ratio, and thus no overdrive (!), is the 3.7:1 the best way to go to maximise top speed? It seems like the 3.7:1 will increase top speed but the 4.1:1 will give you better acceleration. Thanks Adam Edited October 27, 2015 by The_Doc Quote Link to post Share on other sites
iain Posted October 27, 2015 Report Share Posted October 27, 2015 My car has a 4.1:1 diff.....great acceleration but would be horrible without O/D. O/D gives 22 mph/1000 rpm with this set up. I guess a 3.7 with O/D would give 25 mph/1000rpm. In day to day driving the 4.1 will out perform the 3.7 but not so nice if you want to do a motorway journey at any speed. Quote Link to post Share on other sites
mike3md Posted October 27, 2015 Report Share Posted October 27, 2015 Adam, My 4 has a 4.1: 1 diff, with overdrive. Last year I had the overdrive rebuilt to give a 28% reduction, and am very happy with the blend of good acceleration with long legs for cruising. It does bring o/d 3rd and direct 4th close together, but I haven't found it an issue. You can check rpm/mph on the Mintylamb website. Mike Quote Link to post Share on other sites
Mark1965TR4aBRG Posted October 28, 2015 Report Share Posted October 28, 2015 Mine is 3.45:1 and I have OD wired to work in all four gears (with a fail safe in case I look for reverse without disengaging and lunching the OD unit), so that's an 8 speed TR!! With a mildly tuned engine and standard flywheel acceleration is good and especially if you want to cruise 'at speed' it's superb. I've a new engine going in with 89mm pistons and a lightweight flywheel etc etc, so performance will be up and so will my acceleration. The OD upshifts are to me what the TR is all about! Also worthy of note is touring tank range. Quote Link to post Share on other sites
BlueTR3A-5EKT Posted October 28, 2015 Report Share Posted October 28, 2015 Adam, My 4 has a 4.1: 1 diff, with overdrive. Last year I had the overdrive rebuilt to give a 28% reduction, and am very happy with the blend of good acceleration with long legs for cruising. It does bring o/d 3rd and direct 4th close together, but I haven't found it an issue. You can check rpm/mph on the Mintylamb website. Mike Mike I like the idea of 28% overdrive with the 4.1 ratio. - My brother ran a 4.1 with Dol Sprint J type box in a TR2 for many years, and that was quite acceptable round town. I am still striving to decide what ratio to build into my replacement axle ( I bought the car in 1975 needing an axle rebuild) My shelf NOS axle ratio choices are 3.45, 3.7. 3.9, & 4.1 I look forward to reading other hints or advice. Minty Lamb link http://www.mintylamb.co.uk/gearspeed/ Cheers Peter W Quote Link to post Share on other sites
stuart Posted October 29, 2015 Report Share Posted October 29, 2015 Also worthy of note is touring tank range. Mark what mileage can you do on a tank full? Stuart. Quote Link to post Share on other sites
Mark1965TR4aBRG Posted October 30, 2015 Report Share Posted October 30, 2015 (edited) ..depends how fast I'm going.. I've never actually bothered to check Stuart, but cruising at lower rev's should use less fuel and it's more sympathetic mechanically too. 70mph in OD is close to 2,850 rpm and my speedo is calibrated so pretty accurate. Edited October 30, 2015 by Mark1965TR4aBRG Quote Link to post Share on other sites
mike3md Posted October 30, 2015 Report Share Posted October 30, 2015 Mark, According to MintyLamb a 3.45 diff with 22% o/d = 2640rpm @ 70mph, sounds like your tach is as inaccurate as mine!! Mike Quote Link to post Share on other sites
Mark1965TR4aBRG Posted October 30, 2015 Report Share Posted October 30, 2015 (edited) Quite probably Mike - hadn't considered that - all part of the character. This might be a daft question but is 22% the norm then? Edited October 30, 2015 by Mark1965TR4aBRG Quote Link to post Share on other sites
BlueTR3A-5EKT Posted October 30, 2015 Report Share Posted October 30, 2015 Standard TR spec A type was 22% ratio. Remember it all goes to pot when you mess with the tyre sizes too. Peter W Quote Link to post Share on other sites
Mark1965TR4aBRG Posted October 30, 2015 Report Share Posted October 30, 2015 Every day is a school day!! Quote Link to post Share on other sites
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