RPM & Speedometer Readings

[Mike L.]

This question have come up before but one more time. What should your tach and spedo read in 4th gear with the following. 375hp, 4.11 axle, 670x15 tires, at 50 mph?

[Dick W.]

Re: RPM & Speedometer Readings

This question have come up before but one more time. What should your tach and spedo read in 4th gear with the following. 375hp, 4.11 axle, 670x15 tires, at 50 mph?

Going to or coming from Cold Stone Ice Cream

[Michael W.]

Re: RPM & Speedometer Readings

I'd guess your speedo should read ummm, let's see, uhhh - 50?

[Wayne M.]

Re: RPM & Speedometer Readings

This question have come up before but one more time. What should your tach and spedo read in 4th gear with the following. 375hp, 4.11 axle, 670x15 tires, at 50 mph?

There are a couple of calculators out there -- you plug in your tire size and gear ratios (if 1st, 2nd and 3rd RPM and speed are desired). Maybe someone on the DB knows the link.

But in the 1965 showroom brochure, they have this neat chart and a graph that gives MPH per hour per 1000 RPM in final drive (4th). Although it's for 7.75 rubber, the 6.70's have to be close.

I'll do the math for 50 mph (your request). With 4.11's tach reads 2604 RPM [say 2600]. The speedo should read 50 mph if calibrated correctly .

[Michael W.]

Re: RPM & Speedometer Readings

I get 2520 rpm, that's using a tire diameter of 27.42 in.

[Duke W.]

Re: RPM & Speedometer Readings

At 60 MPH in fourth gear engine RPM is tire revs/mile times axle ratio - an easy relationship to remember - and you can linearly proportion to any other speed.

760 x 4.11 = 3124 at 60 MPH.

(50/60) x 3124 = 2603.

Tire revs per mile are specified for each size in a narrow range by The Tire and Rim Association, a tire industry trade group.

Most manufactures list them for each size in their catalogs and they are usually specified at max load and cold pressure for the tire, which is very close for typical actual load and pressure, but the value typically increases about two percent when the tread is down to the wear grooves

The nominal values I use for vintage tire sizes are from old catalogs I have that date to the early seventies.

6.70-15: 760
7.75-15: 775
F70-15: 775
GR70-15: 760
225/70R-15: 760
255/60R-15: 775

Actual revs per mile values that are measured by tire manufacturers are a few percent greater than what is computed for a rigid body due to the fact that pneumatic tires deflect under load as opposed to a steel railroad wheel, whose deflection is so small that is can effectively be considered a rigid body.

Duke

[Joe C.]

Re: RPM & Speedometer Readings

This question have come up before but one more time. What should your tach and spedo read in 4th gear with the following. 375hp, 4.11 axle, 670x15 tires, at 50 mph?

Depends on the air pressure in the tires.

[Michael W.]

Re: RPM & Speedometer Readings

I get 2520 rpm, that's using a tire diameter of 27.42 in.

6.70-15: 760

I used 736 from this website

http://www.carnut.com/specs/tires.html

hence the difference in predictions.

[Duke W.]

Re: RPM & Speedometer Readings

I recommend you delete that page from your browser. They apparently just computed the "rigid body" revs per mile, which shows you how much they know about tires.

Typical measured loaded revs/mile figures by the tire manufacturers are two to three percent greater than the rigid body revs/mile calculation.

Duke