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chrisyork said:
The low profile tyres bit is the key. If the rolling diameter is small enough to pull the wide bit of the tyre back from the pinch point all is well. BUT you then have a horrendously undergeared car - the P6 is undergeared with the standard rolling diameter tyre!

Chris
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Couldn't you then just up the ratios in the 'box? Either that or have a very quick P6...
 
EccentricRichard said:
Couldn't you then just up the ratios in the 'box? Either that or have a very quick P6...
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Unless you're a complete masochist, I think having a taller final drive ratio would be the only 'straightforward' ( :roll: ) solution for that one. You'd need a properly good transmission specialist on your side with proven facilities to make up a new crown and pinion gear. At least then you could retain the original gear spacing. And put it this way, how would you 'up the gearing' of 4th gear in the P6 manual 'box when it doesn't have any cogs? It's just a dog clutch that locks the input shaft to the output giving a 1:1 direct drive. The gears themselves are in neutral at that point, hence why you can't hear any gear whine in top.

Michael
 
You'll have to excuse E R - it's hard to understand gears when you haven't yet passed your test
Most of us know you could put in a 5 speed box but if you then use very low profile tyres you're back to square one
 
DaveHerns said:
You'll have to excuse E R - it's hard to understand gears when you haven't yet passed your test
Most of us know you could put in a 5 speed box but if you then use very low profile tyres you're back to square one
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Actually, understanding of gearing has nothing to do with having passed one's test, it's about mechanical knowledge - and I'd forgotten that 4th in the 4-speed is 1:1. Sounds like a 5spd conversion would be the way to go - I don't know what the standard ratios in an LT77 are, but I do know that the often-used Toyota 'box has a decidedly overdriven top.
 
The LT77 ratios are very close to the standard 4-speed, but 4th is still direct with an overdriven 5th gearset. Whichever way you look at it you can't escape the 1:1 ratio of 4th. You could turn 4th into 3rd, but if you're going that far it would be as much effort to completely reengineer the transmission. Again, sorting the diff ratio remains the easier option.
For the record, the vast majority of manual boxes in old British cars have a direct 4th, as it's far simpler, more reliable and cheaper than having an addition gear set for the sake of a minor deviation in ratio.
I think this design approach fairly conclusively explains the British motor industry's reluctance to develop 5-speed transmission until the mid-70s, and the widespread use of bolt-on overdrive units as a result. Europe was adopting 5-speed from the early 60s.

But we digress.....
 
So the only easy way out is finding another differential, which is a rather expensive option.
Incidentally, you might have noticed my P6 has an automatic gearbox ;)
 
And you aren't going too get a diff ratio to compensate for low ratio tyres
Only hope is Harvey P6's add-on overdrive
 
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