cliff.b Posted April 16, 2021 Report Share Posted April 16, 2021 As explained on other parts of the forum, just bought a largely restored but non running Spitfire 1500 this week and have now sorted the engine and brakes sufficiently to make a proper test drive. All surprisingly good, I'm pleased to say, but at 50MPH it was reporting 3000 revs which was higher than I was expecting. Could anyone confirm what I should be seeing? There is no overdrive fitted and the rear tyres on the car when purchased are brand new 185/70 13's which I'm thinking should be fairly close circumference to those originally fitted. Any thoughts appreciated. Link to comment Share on other sites More sharing options...
johny Posted April 16, 2021 Report Share Posted April 16, 2021 This site is very useful for calculating rev/speed with your cars details (you can try other diff ratios until the results match your readings): https://richyrichracing.com/tools/gear-ratio-speed-calculator-for-standard-triumph-drivelines/ Link to comment Share on other sites More sharing options...
cliff.b Posted April 16, 2021 Author Report Share Posted April 16, 2021 25 minutes ago, johny said: This site is very useful for calculating rev/speed with your cars details (you can try other diff ratios until the results match your readings): https://richyrichracing.com/tools/gear-ratio-speed-calculator-for-standard-triumph-drivelines/ Thanks Link to comment Share on other sites More sharing options...
thescrapman Posted April 16, 2021 Report Share Posted April 16, 2021 3000 should be nearer 60mph. but is that say the speed o or rev counter are accurate. get a sat nav or GPS speedo on a smartphone. Link to comment Share on other sites More sharing options...
GrahamB Posted April 16, 2021 Report Share Posted April 16, 2021 My handbook states 18.5 mph/100 revs in non overdrive 4th gear with 155/80/13 tyres. Checked with a GPS speed indicator which confirmed that my speedo reads 10% low. I found that the 175 tyres on 4.5J rims were too wide leading to excessive side wall flexing and handing improved on reverting back to 155 section tyres. Link to comment Share on other sites More sharing options...
cliff.b Posted April 16, 2021 Author Report Share Posted April 16, 2021 15 minutes ago, thescrapman said: 3000 should be nearer 60mph. but is that say the speed o or rev counter are accurate. get a sat nav or GPS speedo on a smartphone. Yes, good point. I have an app on my phone so will test it out Link to comment Share on other sites More sharing options...
cliff.b Posted April 16, 2021 Author Report Share Posted April 16, 2021 7 minutes ago, GrahamB said: My handbook states 18.5 mph/100 revs in non overdrive 4th gear with 155/80/13 tyres. Checked with a GPS speed indicator which confirmed that my speedo reads 10% low. I found that the 175 tyres on 4.5J rims were too wide leading to excessive side wall flexing and handing improved on reverting back to 155 section tyres. I think the 18.5 is what I was expecting, giving about 55 at 3000 RPM. My Speedo was showing just under 50 but as suggested, one of those might not be accurate. I will see how the speedo compares with GPS. Link to comment Share on other sites More sharing options...
Pete Lewis Posted April 17, 2021 Report Share Posted April 17, 2021 speedo will always read faster they have too used to be 10% + 4mph so at 60 it can read 70 its right they cannot read slow then the rev counter has a tolerance , they are an indication not that accurate Pete Link to comment Share on other sites More sharing options...
cliff.b Posted April 17, 2021 Author Report Share Posted April 17, 2021 5 minutes ago, Pete Lewis said: speedo will always read faster they have too used to be 10% + 4mph so at 60 it can read 70 its right they cannot read slow then the rev counter has a tolerance , they are an indication not that accurate Pete Yes, I will test the speedo against GPS. On my test drives I keep going for 5th gear LOL. Guess I will have to think about fitting an overdrive Link to comment Share on other sites More sharing options...
Mathew Posted April 17, 2021 Report Share Posted April 17, 2021 Look at the diff, the prefix will 99 percent of the time tell you the ratio (unless someone changed the crown wheel and pinon) mk4 spit 3.89 , 1500 3.63. I put a mk4 in a 1500 for quicker acceleration. Link to comment Share on other sites More sharing options...
cliff.b Posted April 17, 2021 Author Report Share Posted April 17, 2021 9 minutes ago, Mathew said: Look at the diff, the prefix will 99 percent of the time tell you the ratio (unless someone changed the crown wheel and pinon) mk4 spit 3.89 , 1500 3.63. I put a mk4 in a 1500 for quicker acceleration. Can I see that when it's fitted? If so, where should I be looking? Link to comment Share on other sites More sharing options...
Mathew Posted April 17, 2021 Report Share Posted April 17, 2021 Just now, cliff.b said: Can I see that when it's fitted? If so, where should I be looking? Should be stamped on the lower side. FH or FM. Top of my head FH is 3.89. I could be wrong. There are other stamps, we can cross that bridge when we come to it. Link to comment Share on other sites More sharing options...
cliff.b Posted April 17, 2021 Author Report Share Posted April 17, 2021 15 minutes ago, Mathew said: Should be stamped on the lower side. FH or FM. Top of my head FH is 3.89. I could be wrong. There are other stamps, we can cross that bridge when we come to it. Ok, thanks. I have some work to do in the rear brakes do will check then Link to comment Share on other sites More sharing options...
johny Posted April 17, 2021 Report Share Posted April 17, 2021 1500 Spit should be 3.63 which is FR Link to comment Share on other sites More sharing options...
cliff.b Posted April 17, 2021 Author Report Share Posted April 17, 2021 Tried to read what was written on the diff but painted & couldn't make it out. However, I just went for a spin with a GPS speed app on my phone. At 3000 rpm the Speedo was reading 48 but the GPS said 55. So I'm happy I have the correct diff in the car but now have another issue to resolve lol Link to comment Share on other sites More sharing options...
cliff.b Posted April 17, 2021 Author Report Share Posted April 17, 2021 20 minutes ago, cliff.b said: Tried to read what was written on the diff but painted & couldn't make it out. However, I just went for a spin with a GPS speed app on my phone. At 3000 rpm the Speedo was reading 48 but the GPS said 55. So I'm happy I have the correct diff in the car but now have another issue to resolve lol Hmm, could that mean I have a gearbox from an earlier car with the Speedo drive set up for a higher ratio diff? Link to comment Share on other sites More sharing options...
johny Posted April 17, 2021 Report Share Posted April 17, 2021 I think we've agreed on here that the variation for diff ratios isnt corrected in the gearbox but in the actual speedo. The correct one for yours should show 1000 (revs/mile) on the face... Link to comment Share on other sites More sharing options...
cliff.b Posted April 17, 2021 Author Report Share Posted April 17, 2021 6 minutes ago, johny said: I think we've agreed on here that the variation for diff ratios isnt corrected in the gearbox but in the actual speedo. The correct one for yours should show 1000 (revs/mile) on the face... If that's the case then good news as far easier to resolve 👍 Link to comment Share on other sites More sharing options...
johny Posted April 17, 2021 Report Share Posted April 17, 2021 I think you need to identify the diff either by cleaning off the painted serial number or if that isnt possible by counting the actual wheel turns. To do the latter, with the car raised, you have to accurately mark the position of both wheels and diff input flange then lock one of the wheels solidly. An assistant has to turn the free wheel exactly two revolutions while you count the turns of the input flange and mark where it ends up. Your flange will have rotated 3 times plus some which you have to work out is 0.27, 0.63 or 0.89 of a turn... Link to comment Share on other sites More sharing options...
Nick Jones Posted April 17, 2021 Report Share Posted April 17, 2021 On the later cars, the ones with 1000tpm speedos the calibration was set by the drive gear..... and Dolly 1500s had 3.89 diffs..... Nick Link to comment Share on other sites More sharing options...
cliff.b Posted April 18, 2021 Author Report Share Posted April 18, 2021 16 hours ago, johny said: I think you need to identify the diff either by cleaning off the painted serial number or if that isnt possible by counting the actual wheel turns. To do the latter, with the car raised, you have to accurately mark the position of both wheels and diff input flange then lock one of the wheels solidly. An assistant has to turn the free wheel exactly two revolutions while you count the turns of the input flange and mark where it ends up. Your flange will have rotated 3 times plus some which you have to work out is 0.27, 0.63 or 0.89 of a turn... I will have another look next time I jack it up. Link to comment Share on other sites More sharing options...
cliff.b Posted April 18, 2021 Author Report Share Posted April 18, 2021 21 minutes ago, cliff.b said: I will have another look next time I jack it up. I took a picture but couldn't make anything out. After making it brighter it is a bit more legible but still not sure what I am seeing Link to comment Share on other sites More sharing options...
Colin Lindsay Posted April 18, 2021 Report Share Posted April 18, 2021 The diff serial number is along the rib, dead centre. As you can see you may need to clean a lot of grime of first. Link to comment Share on other sites More sharing options...
Pete Lewis Posted April 18, 2021 Report Share Posted April 18, 2021 its "hand stamped" in the long flat protuberance underside of the diff...the lowest part of the front casing not a casting number sorry Colin got in quick thats brilliant Link to comment Share on other sites More sharing options...
johny Posted April 18, 2021 Report Share Posted April 18, 2021 Cliff, I would seriously recommend downloading a free manual from that site as this information is common to many cars and would help you learn a lot..... Link to comment Share on other sites More sharing options...
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