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Wondering if Spit has the right diff fitted?


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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.

 

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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.

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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

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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.

 

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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

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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.

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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?

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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.

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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

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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

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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? 

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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 👍

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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... 

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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. 

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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

IMG_20210417_134208_661.jpg

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