marki Posted September 11, 2023 Report Share Posted September 11, 2023 Hi All. Just checking a few things out on my newly purchased 4a, can someone confirm the RPM at 70mph in 4th OD. cheersMark Quote Link to post Share on other sites
RobH Posted September 11, 2023 Report Share Posted September 11, 2023 (edited) Revs depend on a number of factors - what tyre size what diff ratio. 4.1 or 3.7 (normally 4.1 on overdrive cars but not always) what overdrive ratio. ( the standard A type is 22% but a 28% is also possible ) Edited September 11, 2023 by RobH Quote Link to post Share on other sites
John Morrison Posted September 11, 2023 Report Share Posted September 11, 2023 I run 165 x 15 XAS, 22% ration, and 70mph is about 3100 John. Quote Link to post Share on other sites
marki Posted September 11, 2023 Author Report Share Posted September 11, 2023 Thanks guys Quote Link to post Share on other sites
ctc77965o Posted September 11, 2023 Report Share Posted September 11, 2023 Something around 3200rpm Quote Link to post Share on other sites
keith1948 Posted September 12, 2023 Report Share Posted September 12, 2023 (edited) Hi Mark Just deleted my reply because I think my calculations were wrong. Maths eh Keith Edited September 12, 2023 by keith1948 not sure I have given correct info Quote Link to post Share on other sites
BlueTR3A-5EKT Posted September 12, 2023 Report Share Posted September 12, 2023 Do we not like Minty Lamb then? http://mintylamb.co.uk/gearspeed/ Quote Link to post Share on other sites
ianc Posted September 13, 2023 Report Share Posted September 13, 2023 (edited) Thanks, Peter. Minty Lamp's figures agree with the original tests of TRs from the 1950s and 1960s, and what I have always experienced in my TR2 and in 4VC, both of which I have driven on the tacho, rather than my wobbly speedo (which is in front of the navigator on 4VC, so not easily read other than for the mileage figure when filling the tank). Additional thoughts: What makes the sums so simple on a TR with 3.7 axle, 165/80 tyres and A-Type (22%) overdrive, is that the mph/1000 rpm are (apart from 1st gear) in multiples of 5 mph: 10, 15 20 and 25 (just under) in 2nd, 3rd, 4th and O/D 4th. At 5000 rpm, one has speeds of 50, 75, 100 and 124 mph. Overdrive 2nd, an exceedingly useful gear as it slots halfway between 2nd and 3rd, means one can exceed 60 mph at 5000 with only one change of gear (1st to 2nd). Ian Cornish Edited September 13, 2023 by ianc Quote Link to post Share on other sites
keith1948 Posted September 13, 2023 Report Share Posted September 13, 2023 (edited) 17 hours ago, BlueTR3A-5EKT said: Do we not like Minty Lamb then? http://mintylamb.co.uk/gearspeed/ I had forgotten about Minty Lamb - thanks for reminding me. Mark you should be able to input your car data onto Minty Lamb to see what your revs should be at 70 in OD. Problem on my 4A is that the speedo is the 1184 one as fitted in 1965. Looking at the Moss catalogue with 165/80 tyres it looks like I should have the 1152. My speedo reads about 10% high. I have 3.7:1 diff and A type overdrive. The odometer also reads wrong. At 60mph in OD the rev counter reads just under approx 2400 which is roughly double 1184. That doesn't really mean a lot since the speedo needle can be 'adjusted' to this figure. According to a tyre cross reference chart I have found, the 165/80 tyres do 795 revs per mile whereas the closest equivalent to the original cross ply would be 165/75 (815 revs/mile) or 175/70 (819 revs per mile). However more revs per mile to my mind would mean the speedo would read higher still. I could get the speedo recalibrated I guess but is there an option to fit a slightly different tyre size to bring it back to 1965 reading. It must have been correct once. I'm getting brain fatigue trying to figure out why my speedo/odometer is high. I have learned to live with it but would be better if it was more accurate. Could I use Minty Lamb and set the speedo to read exactly 60 at 2410 revs. At the moment it will read above 60 at these revs. Any help appreciated please Keith Edited September 13, 2023 by keith1948 Quote Link to post Share on other sites
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