Diff ratios

[cambelt]

A question for the maths guru,s. If my tailshaft is doing 1000rpm
with a 3.26 diff ratio with the standard
supra wheels 225/50/16 ( I think thats the standard supra tyre for 16 in)
how much extra RPM is the tailshaft going to do if i use a 3.58 diff with 205/65/15,s ( these shoud be bigger than the supra tyres).
Ps , i did find norbies search on the diff ratio calculator but i cant seem to open it
Dave

[cambelt]

So far i have found a few ratio calculators on google, but whats other suggestions.
Dave

[river]

Hi,

The size of your wheels/tyres are not going to make any difference to the speed of the tailshaft or diff.

That is, a ratio of X:1 will spin at X:1 ratio whether you have 205, 265 or no wheels.

I think what you are after is the difference in circumference between the tyres, so you can work out how much more (or less) rolling distance each revolution of the tye makes, then use this on your rpm calculations to determine to road speed?

seeyuzz
river

[cambelt]

Yeah thats what im trying to say , i,ll get a few outside circumferances on a couple of tyres first.
Dave

[wilbo666]

Ignoreth my previous drivel

Cheers
Wilbo

[Norbie]

http://akb.norbie.net/Public/Gearing.aspx

This will answer all your questions.

[cambelt]

I like the % way .
Thanks wilbo
Dave

[Rcubed]

225/50 R16 -> 1983.6mm Circumference
205/65 R15 -> 2034.2mm Circumference

+2.55% difference in tyres.

-8.38% in diff (If I got it right?)

So about -5.83% in overall ratio, so you will do 5.83% more engine revs at any given speed?

Cheers
Wilbo

no you multiply, not add
225/50R16 diameter is 2*225*50%+16*25.4=631.4mm
205/65R15 is 2*205*65+15*25.4=647.5mm
ratio is 631.4/647.5=0.975=
-2.5% tailshaft speed at const speed
ratio of circumference is the same

diff ratios are 3.58/3.26=1.098 =
+9.8%

(100-2.5)%*(100+9.8)%=0.975*1.098= 1.07=
+7.0%
tailshaft speed [and therefore engine] will be 7% higher at same speed

[wilbo666]

no you multiply, not add
225/50R16 diameter is 2*225*50%+16*25.4=631.4mm
205/65R15 is 2*205*65+15*25.4=647.5mm
ratio is 631.4/647.5=0.975=
-2.5% tailshaft speed at const speed
ratio of circumference is the same

diff ratios are 3.58/3.26=1.098 =
+9.8%

(100-2.5)%*(100+9.8)%=0.975*1.098= 1.07=
+7.0%
tailshaft speed [and therefore engine] will be 7% higher at same speed

Yep this is right, ignore my drivel!

Happy to be corrected

Cheers
Wilbo