gear ratio

[bill]

I enjoyed last weeks thread regarding the optimum rear ratio for big and small blocks, close and wide ratio trannies. Of course, confusion set in after the third post. So let me ask, if my '70 is turning 3,000 rpm in 4th at 70 mph with 225x70 tires, whats the rear? I'm guessing its a 3.36. Thanks.

P.S. Also noticed that this coming weekends Car and Driver TV show will feature the Z06.

[Gerry Davis]

Re: gear ratio

Assuming your tire height is 27" -> 3.44, so 3.36 is probably right. You can use the calculator at the web page below.

Gerry


Rear end ratio calculator

[Redondo Duke NCRS #22045]

Axle ratio calculation

If you know the RPM at 60 MPH it's just a matter of dividing this number by the tire revs/mile figure. The result is the axle ratio. In this case we have to adjust back to 60 by multiplying by 60/70 or

3000/760(60/70) = 3.383.

looks like a 3.36, and your speedo and tach are right on the money.

Duke

[Gerry Davis]

Re: Axle ratio calculation

Duke,

Where does the 760 revs/mile figure come from? For a 225x70x15 I think thats 736 revs/mile. Or am I missing something?

Gerry #33803

[Redondo Duke NCRS #22045]

Tire size and revs/mile

The dimensions and rev/mi figure will vary slightly depending on the manufacturer and model line. Standards are promulgated by the Tire and Rim Association, and industry trade group, and the range of acceptable values is usually within about two percent for a specific size. I got the 760 rev/mi figure from an old GY catalog when they offered the Gatorback in that size. What is the exact make and model of your 225/70-15. I have data for other tires, but 736 is off the mark for a 225/70-15. Also, as in past discussions, I remind you that the rev/mile figure cannot be calculated from the specified OD because tires are not rigid bodies, and the actual measured rev/mile figure as published by the manufacturer is based on testing a loaded tire. Deformation in the tire increases the actual revs/mile figure two to three percent above the "rigid body" revs/mile calculated from pi D squared over 4. It sounds like you got the 736 figure this way.

Duke

[Gerry Davis]

Re: Tire size and revs/mile

Yep, thanks for setting me straight. No wonder I could never really get the rear-end ratio of my car ('69 215x70x15) I assumed my speedo/tach were inaccurate. I guess the rev/mile also changes as you change speed so its best to measure at the same speed the tests were performed.

Gerry #33803

[Redondo Duke NCRS #22045]

Oops, sorry, wrong formula

I meant to say pi times D, the circumference of a circle, not pi D squared over four, which is the area. If we use your example with a diameter of 27 inches, the "rigid body" revs per mile figure is (5280 x 12)/ (3.1416 x 27) or 747. A 27 inch steel railroad wheel would run 747 revs per mile without slippage, but a pneumatic tire will deform and increase the number by about 2.5 percent on average, so a 27" radial tire will run about 1.025(747) or about 765 revs/mile. BTW both the old 7.75-15 and current 215/70R-15 are typically speced at 775 revs per mile. The old 6.70-15 is 760 revs/mile, which is also the spec for many 225/70R-15s.

If an inflated pneumatic tire is tested without load it will yield pretty close to the rigid body number, but revs per mile data specified by manufacturers is always based on a loaded tire, and usually a specific speed. I'm not sure what the load standard is, but it may be max load at max pressure. Goodyear specifies the speed as 50 MPH. Of course, at a given pressure, if we reduce the load the rev per mile figure will get closer to the rigid body number. The point here is that we can only look at published data as approximations, and real world conditions and production tolerances may result in variations up to two percent. Another thing to remember is that revs/mile increase slightly tires wear, and a tire worn to the 2/32s inch legal minimum might show up to two percent higher revs per mile that a when new. This is also why you want the rear tires within one percent static circumference. Otherwise the car might pull and the positraction clutches can wear abnormally fast.

Duke

[Redondo Duke NCRS #22045]

speed and torque effect

The drive tires are always slipping, but the slippage at normal cruising speed is so slight that it's at the noise level. Also, tires expand with speed due to the centrifugal force. This was a real issue with bias ply tires, but radials have so much rigidity in the belt package that growth with speed is, again, usually at the noise level. At the other extreme are the slicks used by Top Fuel and Funny Cars. The tires are designed to grow a good thirty percent at peak speed, which effectively gives the cars an infinately variable transmission with a 30 percent spread between "low" and "high".

Duke

[Craig S.]

Re: Plus

Thx Jack .... I haven't bought a GM rebuilt yet, but, the units from Auto Zone or Pep Boys they had a whopping $6 core charge so I have elected to keep the old units anyway for spare parts. Maybe GM gets a heftier core charge...I am sure they do a better rebuild, unless they vend it out to someone else. Do you know if they use the ceramic seal on the GM rebuilt units? Craig