Tire ?
Tuned
Joined: Jul 2003
Posts: 455
Likes: 3
From: Berkeley
measure the actual mounted diameter of the tires. rated size is one thing, but actual mounted diameter will determine your final drive.
that said, assuming the rated sizes are exact, the diameters would be
15 + ((.75 x 235)mm x 2 / 25.4mm/in) ~= 28.9"
vs.
15 + ((.70 x 235)mm x 2 / 25.4mm/in) ~= 27.9"
1" smaller diameter will result in about 1/29 or ~3% higher reading on your speedo for the same rpms. 3% isn't that far off, and at least you'll read high, but it would bother me if i knew about it.
but measure, to be sure. and if you need to, change out the speedo gear to compensate.
also, consider that smaller tires will have the corresponding shift in yourtoruqe curve. ie: if your peak torque (most efficiency) is at 2000 rpm , that's now at 3% less mph than before the tire swap.
ymmv, literally. :-)
that said, assuming the rated sizes are exact, the diameters would be
15 + ((.75 x 235)mm x 2 / 25.4mm/in) ~= 28.9"
vs.
15 + ((.70 x 235)mm x 2 / 25.4mm/in) ~= 27.9"
1" smaller diameter will result in about 1/29 or ~3% higher reading on your speedo for the same rpms. 3% isn't that far off, and at least you'll read high, but it would bother me if i knew about it.
but measure, to be sure. and if you need to, change out the speedo gear to compensate.
also, consider that smaller tires will have the corresponding shift in yourtoruqe curve. ie: if your peak torque (most efficiency) is at 2000 rpm , that's now at 3% less mph than before the tire swap.
ymmv, literally. :-)
