2.76
Re: Help in Iowa
- Thread starter hemisedan
- Start date
-
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
- Joined
- Jan 19, 2014
- Messages
- 24,952
- Reaction score
- 12,303
Here's the formula; but wait;
First you need the circumference of the tire; you have to actually measure it. You can calculate it, but because of the Pi factor (Pi is 3.1416) in the equation, any error is multiplied by that stinking Pi
For example;
a 195/75-13 maths out to (195/25.4x.75x2) +13= 25 inches tall, and the circumference would be (pi x diameter), or 25x3.1416=78.54, by the math. But because of the way the tire is built, and the several possible rim widths that could be used,and the manufacture's rounding up/down the branded size to the nearest 5mm, the physical tire diameter could vary, I'll guess ~.25 inch in either direction. When you multiply that error by Pi/3.1416, then your circumference will also be out by that tripled amount. Hey now yur up to an error of 5 or 6% .......... and that is ~one tooth on the speed-o gear. So if you do the math and order a speed-O gear that you think you need, install it, and find out it is outside the 2 to 3% error margin..... then you get a pita messy do-over.
So either take the tire off and measure it, or do a roll-out, or live with the % error.
To do a roll out, I put a mark on the center tread with a sidewall marking grease pencil, and roll the car, in a straight line,lol,until I get two marks on the concrete floor,then measure the center to centers. No math required. If the grease pencil don't work for you, figure something out.
Ok so suppose your tire measures 80" roll-out. Here's the Formula;
MPH = (rpm x Rollout)/ (1056 x R1xR2)
where R1 and R2 are your gear ratios in the rearend and trans. The trans is usually 1.00
Here's an example; using 2.94s
65= (unknown rpm x 80)/1056 x2.94 x1.00 simplifying,lets call the unknown Q, and we get
65= Qx80/3105, and re-arranging we get
65 x 3105 = 80Q, again
65 x 3105/80=Q,and
Q=2523 rpm.....This would be at zero slip, like with a manual trans.
But with an automatic, the trans could be slipping 2,3,4 or even 5%, depending on road conditions, 5% would be 126rpm, so your Speed-O could be reading 2523+126=2649.. But if you got a good tailwind, or going downhill,your Speed-O might be reading 100plus fewer rpm; say 2522-123=2397. And if you had a high compression torque beast, the Speed-O might be bang-on. So the range would be 2649 to 2397, or written thus; 2523 +/-126rpm
Please don't hate me,lol.
First you need the circumference of the tire; you have to actually measure it. You can calculate it, but because of the Pi factor (Pi is 3.1416) in the equation, any error is multiplied by that stinking Pi
For example;
a 195/75-13 maths out to (195/25.4x.75x2) +13= 25 inches tall, and the circumference would be (pi x diameter), or 25x3.1416=78.54, by the math. But because of the way the tire is built, and the several possible rim widths that could be used,and the manufacture's rounding up/down the branded size to the nearest 5mm, the physical tire diameter could vary, I'll guess ~.25 inch in either direction. When you multiply that error by Pi/3.1416, then your circumference will also be out by that tripled amount. Hey now yur up to an error of 5 or 6% .......... and that is ~one tooth on the speed-o gear. So if you do the math and order a speed-O gear that you think you need, install it, and find out it is outside the 2 to 3% error margin..... then you get a pita messy do-over.
So either take the tire off and measure it, or do a roll-out, or live with the % error.
To do a roll out, I put a mark on the center tread with a sidewall marking grease pencil, and roll the car, in a straight line,lol,until I get two marks on the concrete floor,then measure the center to centers. No math required. If the grease pencil don't work for you, figure something out.
Ok so suppose your tire measures 80" roll-out. Here's the Formula;
MPH = (rpm x Rollout)/ (1056 x R1xR2)
where R1 and R2 are your gear ratios in the rearend and trans. The trans is usually 1.00
Here's an example; using 2.94s
65= (unknown rpm x 80)/1056 x2.94 x1.00 simplifying,lets call the unknown Q, and we get
65= Qx80/3105, and re-arranging we get
65 x 3105 = 80Q, again
65 x 3105/80=Q,and
Q=2523 rpm.....This would be at zero slip, like with a manual trans.
But with an automatic, the trans could be slipping 2,3,4 or even 5%, depending on road conditions, 5% would be 126rpm, so your Speed-O could be reading 2523+126=2649.. But if you got a good tailwind, or going downhill,your Speed-O might be reading 100plus fewer rpm; say 2522-123=2397. And if you had a high compression torque beast, the Speed-O might be bang-on. So the range would be 2649 to 2397, or written thus; 2523 +/-126rpm
Please don't hate me,lol.
Last edited:
-
