Series revs

[LandyAndy]

Hi Series Pilots
Can any of you tell me how many revs your rigs do at 100ks with 4.7 diffs and 7.50 tyres.In the cattledog from 4 wheeldrives melbourne state 3850rpm.
Ive just changed to 8.25 x 16 tyres and fitted a tacho,trying to work out a rough speedo correction.
Thanks
Andrew

[Phoenix]

:? I would have thought it was the same, as a 8.25R16 is wider than a 7.50R16, not larger (isn't it ?)

can't tell you what I do, no tacho & a holden motor.

[LandyAndy]

Hi Phoenix
Mine is a holden as well,they still rev the same as the gearing is unchanged with a motor change.
8.25 tyres are at least 3 or 4 inches taller than a 7.50,and a good 1 to 2 inches wider.
Andrew

[UncleHo]

Hi All style_emoticons/<#EMO_DIR#>/smile.gif
3850 sounds like it could be for a 2.6ltre six, a bit low for a 4cyl they're up around 4k as their SAFE listed max is 4250rpm and they self distruct at between 5100--and 5520rpm (that was done under test conditions in the late Sixties, tha explosion and resultant hemorrhage was spectatualar, all over the test track.) As to the Question, on 8.25 x16 a safe bet would be to assume that the speedo (if accruate in the first place ) would read 10 klm slow IE 90 klms indicated==100klms actual if in miles i would use the same figures, for a more positive calc, measure circumferance of 8.25x16 and that of NEW 7.50x16 then calc the % diff and hence your answer (never could understand how to use a calcuator, weren't invented when I went to school but then I rode a dinosaur to school)
Chers Rocket

[Hellspawn]

Hi Andrew
My holden does somewhere around the 3400 in overdrive at 100 with 15in 31 x 10.5 tyres and 4k in fourth at 100.
My suggestion would find a quiet road with a friend in their car and do some comparisons.

[LandyAndy]

Hi Guys
Thanks for the input.
It turns out the speedo is reading correctly with the 8.25 tyres,checked it today on the trip against the other guys using their GPS.When the needle decided not to flicker it was pretty well right.The oddometer was cheked against the roadside distance markes and was also correct.
Thanks
Andrew

[Bushie]

Formula runs along these lines

Kph/1000rpm = .........60000.....................
...............gear ratio x trans ratio xdiff x tyre revs/km


A 750R16 is 409 revs/km
A 205R16 is 448 revs/km

Just need to find the revs/km of 825s

I suppose you could work it out by measuring the radius of the tyre under load calculate the circumference from that figure then your revolutions per Km is 100000/the circumference in cm


Bushie