simonjrwinter Posted May 21, 2011 Report Share Posted May 21, 2011 What RPM should I be doing at 70 MPH in O/D? The engine may be different but the drivetrain is all standard 1971 TR6 with standard wheels/tyres so my RV8 should be turning at the same speed as a six. Does anyone know what a car with a 3.45 diff should be doing at 70 as opposed to a 3.7 diff at the same speed? Thanks Simon Quote Link to post Share on other sites
david johnson Posted May 21, 2011 Report Share Posted May 21, 2011 Mines doing 2400 at 70 on 195s David Quote Link to post Share on other sites
simonjrwinter Posted May 21, 2011 Author Report Share Posted May 21, 2011 So 3,750rpm is a bit high then? Bloody hell, what diff have I got fitted? Actually that may not be exactly right, The car has a jaguar electronic tacho fitted so I'd like to get that checked first. Even so, I wonder if I do have a 3.7 diff fitted? Simon Quote Link to post Share on other sites
PJM Posted May 21, 2011 Report Share Posted May 21, 2011 Mine does about 2500 maybe a little less at 70 in overdrive top on 195's both speedo and tacho calibrated and appear to be accurate. Quote Link to post Share on other sites
simonjrwinter Posted May 21, 2011 Author Report Share Posted May 21, 2011 My speedo under reads by about 10% so The above 70 mph at 3750rpm is measured against GPS sat nav Simon Quote Link to post Share on other sites
Alec Pringle Posted May 21, 2011 Report Share Posted May 21, 2011 Checking the diff ratio is a 5 minute job which anyone can achieve - see link for a concise guide http://www.mgparts.co.nz/diff_ratios.html As for the tacho, again a 5 minute job which anyone can achieve - into your friendly local MoT bay and hook the car up to the emissions tester, which will give you a far more accurate reading than the average tacho . . . . compare your tacho, and compensate the dashboard reading as required. Cheers, Alec Quote Link to post Share on other sites
Peter Cobbold Posted May 21, 2011 Report Share Posted May 21, 2011 So 3,750rpm is a bit high then? Bloody hell, what diff have I got fitted? Actually that may not be exactly right, The car has a jaguar electronic tacho fitted so I'd like to get that checked first. Even so, I wonder if I do have a 3.7 diff fitted? Simon Measure ratio by jacking one rear wheel off ground, fit axle stand, turn prop shaft by hand one turn, measure rotation of elevated wheel. Speeds also are ratio specific- see dial lettering - PO might have fitted non-standard. Quote Link to post Share on other sites
Austin Branson Posted May 21, 2011 Report Share Posted May 21, 2011 Hi, I have an ex-California car, and I'm pretty confident it's a 3.7 diff - I do about 2700 rpm at 70 mph. Hope that helps. Austin Quote Link to post Share on other sites
Denis Posted May 21, 2011 Report Share Posted May 21, 2011 I don't have O/D but my original Triumph hand book says... 'Engine Speed at a road speed of 10mph O/D Top = 386 and 471. So times by 7 = 2,702 and 3,297. A hand written note I've added top speed once (when we were both young) at 5,750 RPM (down hill at night) at 21.21mph = 1000RPM = 121.96 mph. Quote Link to post Share on other sites
Ragtag Posted May 22, 2011 Report Share Posted May 22, 2011 The car has a jaguar electronic tacho fitted so I'd like to get that checked first. Is the tacho from a 6 cylinder or 8 cylinder Jag? If its from a 6 then your V8 would generate 133.333 rpm (signal)for every 100 rpm (actual) (8/6). So your indicated 3750rpm at 70mph would really be 2812. Just a thought Quote Link to post Share on other sites
simonjrwinter Posted May 22, 2011 Author Report Share Posted May 22, 2011 Is the tacho from a 6 cylinder or 8 cylinder Jag? If its from a 6 then your V8 would generate 133.333 rpm (signal)for every 100 rpm (actual) (8/6). So your indicated 3750rpm at 70mph would really be 2812. Just a thought Good point, I'll check. Thinking about it, there aren't many (any?) V8 Jags are there? only the daimler sp250 engined cars. Apart from the new cars of course. Quote Link to post Share on other sites
simonjrwinter Posted May 22, 2011 Author Report Share Posted May 22, 2011 (edited) Good point, I'll check. Thinking about it, there aren't many (any?) V8 Jags are there? only the daimler sp250 engined cars. Apart from the new cars of course. Once I'd got my glasses on! I could see that the tacho DOESN'T say "Jaguar" but Jaeger and it DOES say 8 cyl. I'll be looking for CP series tacho at some time in the future to convert so I can fill the hole properly. I'll try the wheel/prop thing today Simon Edited May 22, 2011 by simonjrwinter Quote Link to post Share on other sites
stuart Posted May 22, 2011 Report Share Posted May 22, 2011 Once I'd got my glasses on! I could see that the tacho DOESN'T say "Jaguar" but Jaeger and it DOES say 8 cyl. I'll be looking for CP series tacho at some time in the future to convert so I can fill the hole properly. I'll try the wheel/prop thing today Simon That is one from a Daimler V8. Stuart. Quote Link to post Share on other sites
88V8 Posted May 22, 2011 Report Share Posted May 22, 2011 18mph/1000rpm?? Sounds like no overdrive, or tacho wrong. Check diff ratio, o/d, tyres here http://www.mintylamb.co.uk/gearspeed/ Ivor Quote Link to post Share on other sites
dykins Posted May 22, 2011 Report Share Posted May 22, 2011 Once I'd got my glasses on! I could see that the tacho DOESN'T say "Jaguar" but Jaeger Regards Peter Quote Link to post Share on other sites
simonjrwinter Posted May 22, 2011 Author Report Share Posted May 22, 2011 Yes, getting old sucks! (but it beats the alternative.....) Quote Link to post Share on other sites
simonjrwinter Posted May 22, 2011 Author Report Share Posted May 22, 2011 OK checked the diff. One wheel up in the air, back wheel turned through two revolutions, the prop turns just under 3.5 times 3.45 diff? Simon Quote Link to post Share on other sites
88V8 Posted May 22, 2011 Report Share Posted May 22, 2011 OK checked the diff. One wheel up in the air, back wheel turned through two revolutions, the prop turns just under 3.5 times 3.45 diff? Simon Yup. Quote Link to post Share on other sites
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