tjs Posted February 14, 2012 Report Share Posted February 14, 2012 Hi folks I am convince I have the wrong diff in my car mainly because it ir revs waaay too high at 65-70mph (with OD in 4th). In fact I can barely drive it at that because its just uncomfortable. Does anyone know what diff ratio I should have in a 71 PI? I know the gbox is original. Quote Link to post Share on other sites
stuart Posted February 14, 2012 Report Share Posted February 14, 2012 3.45 Originally. Sounds as if you may even have a 4.1 fitted Stuart. Quote Link to post Share on other sites
Alec Pringle Posted February 14, 2012 Report Share Posted February 14, 2012 3.45 was the PI norm, 3.7 for carb cars. PI cars had a theoretical option of 3.7 or 4.1, but that was mainly for competition homologation, whether any cars ever left the factory so equipped is a different matter ! 40 years on, who knows how many replacements the car has had, or what is in there now . . . . Jack up one rear wheel, leave the other on the ground, gearbox in neutral. Rotate the wheel exactly two rotations, and count how many rotations the propshaft makes - or to avoid error, rotate the wheel 10 times and divide the number of propshaft revolutions by 5. The number of prop revolutions is your diff ratio, 3.45 or 3.7 or 4.1. Cheers, Alec Quote Link to post Share on other sites
Walkerwek1958 Posted February 20, 2012 Report Share Posted February 20, 2012 3.45 for PI cars, 3.7 for US spec carb cars. However, the US cars used 185 x 15 tyres whereas the UK used 165 x 15 (both approx 80 profile, normal for late 60s). This had the effect of negating the lower gearing, to give identical mph/1000revs figures for both. Always struck me as a strange thing to, but that's Triumph! As the previous owner of VUX 15K (Andy Smith's current car), which had a 3.7 diff when I bought it in 2000, I always felt the 3.7 suited the car nicely. Matched with 185/70 tyres (same rolling radius as 165 originals) the gearing was high enough for me and the slight acceleration improvement was worthwhile. Andy's now altered the car back to the 3.45 ratio. Only problem I had was the speedo and mileometer over-reading by around 7%.....but I could live with that. Paul Paul Walker Quote Link to post Share on other sites
JJohn Posted February 21, 2012 Report Share Posted February 21, 2012 My ex-US 6 has the 3.7 diff and I agree with Paul that, with the 185-70 tyres and overdrive, it suits the car well. The sharp acceleration is very welcome and at 70 mph in O/D top the engine is turning over at 2700 RPM, quite a relaxing ride. I had the speedo calibrated and, checking against the SatNav, it appears remarkably accurate never varying more than 2 MPH. Jim Quote Link to post Share on other sites
tjs Posted February 21, 2012 Author Report Share Posted February 21, 2012 Jealous. So im on a 4,11. When is Christmas again? Alex Quote Link to post Share on other sites
Andrew Smith Posted February 21, 2012 Report Share Posted February 21, 2012 .......As the previous owner of VUX 15K (Andy Smith's current car), which had a 3.7 diff when I bought it in 2000, I always felt the 3.7 suited the car nicely. Matched with 185/70 tyres (same rolling radius as 165 originals) the gearing was high enough for me and the slight acceleration improvement was worthwhile. Andy's now altered the car back to the 3.45 ratio. Only problem I had was the speedo and mileometer over-reading by around 7%.....but I could live with that. As Paul says, VUX (my third TR6) had a US spec 3.7:1 diff fitted which it had slight clunk on take up, and I didn’t feel it matched the o/d unit so decided to fit a UK spec 3.45:1. But regardless of the clunk I would have changed it for the reason I stated – Pete Cox found me a good used unit and rebuilt it checking all the tolerance, adjusting where required and fitted new bearings and oil seals. The speedo is now pretty much spot on and I feel the engine drives more comfortably with the 3.45 fitted, and on the round trip to Harrogate last year returned 31 mpg , but VUX maybe looses some slight on acceleration over the 3.7. Cheers Andrew Quote Link to post Share on other sites
Andrew Smith Posted February 21, 2012 Report Share Posted February 21, 2012 .......As the previous owner of VUX 15K (Andy Smith's current car), which had a 3.7 diff when I bought it in 2000, I always felt the 3.7 suited the car nicely. Matched with 185/70 tyres (same rolling radius as 165 originals) the gearing was high enough for me and the slight acceleration improvement was worthwhile. Andy's now altered the car back to the 3.45 ratio. Only problem I had was the speedo and mileometer over-reading by around 7%.....but I could live with that. As Paul says, VUX (my third TR6) had a US spec 3.7:1 diff fitted which it had slight clunk on take up, and I didn’t feel it matched the o/d unit so decided to fit a UK spec 3.45:1. But regardless of the clunk I would have changed it for the reason I stated – Pete Cox found me a good used unit and rebuilt it checking all the tolerance, adjusting where required and fitted new bearings and oil seals. The speedo is now pretty much spot on and I feel the engine drives more comfortably with the 3.45 fitted, and on the round trip to Harrogate last year returned 31 mpg , but VUX maybe looses some slight on acceleration over the 3.7. Cheers Andrew Quote Link to post Share on other sites
Andrew Smith Posted February 21, 2012 Report Share Posted February 21, 2012 (edited) .......As the previous owner of VUX 15K (Andy Smith's current car), which had a 3.7 diff when I bought it in 2000, I always felt the 3.7 suited the car nicely. Matched with 185/70 tyres (same rolling radius as 165 originals) the gearing was high enough for me and the slight acceleration improvement was worthwhile. Andy's now altered the car back to the 3.45 ratio. Only problem I had was the speedo and mileometer over-reading by around 7%.....but I could live with that. As Paul says, VUX (my third TR6) had a US spec 3.7:1 diff fitted which it had slight clunk on take up, and I didn’t feel it matched the o/d unit so decided to fit a UK spec 3.45:1. But regardless of the clunk I would have changed it for the reason I stated – Pete Cox found me a good used unit and rebuilt it checking all the tolerances, adjusting where required and fitted new bearings and oil seals. The speedo is now pretty much spot on and I feel the engine drives more comfortably with the 3.45 fitted, and on the round trip to Harrogate last year returned 31 mpg , but VUX maybe loses some slight acceleration over the 3.7. Cheers Andrew Edited February 21, 2012 by Andrew Smith Quote Link to post Share on other sites
RonA Posted February 21, 2012 Report Share Posted February 21, 2012 I've also got a 3.7 diff with overdrive and if I was to do it again I would go for the 3.45 one, I think it's a better ratio for a car that is used mainly for touring but neither my back nor my wallet would like to change it at present. My speedo is so far out I use small marks on the speedo glass to indicate the correct speed, I suppose the first time I get nicked I'll do something about it RonA Quote Link to post Share on other sites
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