Manufacturers give tyre diameters to assist in deciding whether a tyre will have adequate clearance for a particular application, not for the purposes of gearing calculations.
The manufacturer also produces technical data sheets giving the rolling circumfrence and tyre revolutions per mile. For a NEW Toyo T1-S 215x40xZR17 this is 869 revs per mile and 1841 mm. This compares very well with my figure of 1838 to 1840mm. Bear in mind that as the tyre wears the circumfrence will reduce so I am happy to accept the accuracy of my figures.
I hope this helps.
If Ron has a 2.5 litre with plenty torque and low down then I am sure he will be happy with a 3.9 diff ratio. As I already said the 3.9 is on the tall side for a 2 litre, even a fairly torquey one simply because the spool point is higher and there may not be as much torque.

 

I wish this thread had started a couple of weeks earlier. I have just replaced the gearbox and rear diff in my 94 wrx with a P1 gearbox and rear diff, i have just calculated my max mph at 7000 rpm in each gear and it's alot lower than I had hoped, less than 150 mph in 5th, not much good for Scooby shootout top speed

I replaced my wrx box because it was too low geared, it had an RA gearset and 4.11:1 diffs giving it lower gearing than an RA which has 3.9:1 and a top speed of under 112 mph in 5th, most likely the lowest geared Impreza in the UK.

If I had seen this I wouldn't of bought the P1 rear diff and kept my old 4.11:1 diff and swapped the P1 front diff for my old WRX 4.11:1 diff while the box was out as the gearing looks spot on for my 341 bhp and 320 lbs/ft 2.0 litre.

Looks like I will be taking the box out yet again, 5th time now

 

For decent top speed and relaxed cruising you need a box with a .728 top gear. The other common top is .825 but that will be too low, I would suggest. With 4.11 diffs that gives about 136 mph at 7k rpm from memory. With the .728 you can choose your diff from the info I posted previously but on a 2 litre with reasonable power I feel that 4.11 diffs are as high as you want to go and even 4.44s will be a big improvement over your existing set up.

 

Harvey he should already have 4.44's if he has a P1 set....................think he meant that he feels its too low geared on these??

the other thing to bear in mind when working out theoretical gearing is you hardly ever achieve these max speeds due to aerodynamics!! sometimes better to have the car slightly under-geared and then allow it to pull easier up to its top speed.....

alyn

 

P1 = 4.44s with .738 5th so on 16" tyres that's near enough 21 mph/1000 RPM.
Rev limit??? Is it 7k or 8K????
Other 5th gears are .825 where top speed is about 16 @7k and a .9 something probably .972 (35:36) which is on some of the type Rs and it is an **** of a gear for road work.

 

Harvey, I have a 7,000 rev limit and 4.44:1 diffs now from P1.

Gear Original box P1 box 4.44s P1 box 4.11s P1 box 3.9


1st @ 7k 30mph 34.16mph 36.89mph 38.92mph

2nd @ 7k 45mph 57.47mph 62.09mph 65.45mph

3rd @ 7K 61mph 83.44mph 90.16mph 94.99mph

4th @ 7k 84mph 111.3mph 120.19mph 127.26mph

5th @ 7k 117mph 146.58mph 158.34mph 166.81mph

Original box readings were taken off the speedo that I knew was 10% over reading, other readings are using Harvey's diff / box calculations.

The extra 1000rpm limit on the 4.44:1 diffs make the standard gearing more usable,

1st = 39.04, 2nd = 65.68, 3rd = 95.36, 4th = 127.2mph, 5th = 167.52.

These figures are nearly the same as changing to 3.9 diffs but retaining the lower rpm figure of 7k rather than the P1's 8k. This would explain why John Banks thought this was the ideal gearing on a 2.5 with a 7k rev limit.

Colin.