My speedo is inaccurate for a variety of reasons (C6 trans replacing a 3-speed and 33" tires) but I still would like a reasonable estimate how fast I'm going down the road.

Considering that I have a 3.55 rear gearing in my 9" and a 300 I-6 engine, what formula would I use to calculate my speed based on engine RPM?

 

I'm not sure what others use or how realable mine formula is but i figure that at 55mph you would be running around 2100+ rpm.

 

Well, your trans has a 1:1 drive gear, so that's easy... let's say you're cruising down the highway in third gear at 2500 rpm.... the output shaft of the trans is also turning 2500 rpm. The rearend reduces that by 3.55 times, meaning your wheels are spinning 704 rpm... ( 2500 / 3.55 = 704.2 ) Now you just have to figure out how far your tires go each time. They're 33" tall, so the diameter is 103.6" ( 33 * 3.14 ), and that is how far they go every time they spin. If they turn 704 rpm at 104" each time, they are taking you 73216" per minute... ( 704.2 * 103.6 ). Now just convert that into miles (divide by 12, divide by 5280), which gives 1.16 miles/ minute. Take that times 60 for mph, and you get 69.3 mph. Now since you know the ratio of speed to rpm ( 69.3 : 2500 ), you can make a constant out of it by dividing by 69.3, which equals 1 mph per 36 rpm (in drive gear). Knowing that, you can get a series of useful numbers, like 55 mph = 1980 rpm and 65 = 2340 rpm....

 

Hey you can't take the tire size for granted. I have 31's but there like 29 inches tall.I figured that his are 31 inches or so. Since i am here again this is how i go about it. Lets says your tires are moving at 500 rpm. Take that times 3.55 and you get 1775 engine rpm. This can be done since the c6 3rd gear is 1 to 1. I guess that your ties are about 31 inches tall. So 31* 3.14 (pie) and you get 97.38 inches of circumference per revolution. Then take 97.38 (c) * 500 (wrpm) * 60 (minutes)=2921400. Then take that divided by 63360 (inches in a mile) and you get 46.107 mph. Then i take 46.107 / 5(mph) =9.22. Then 1775(engine rpm)/9.22=192.5(rpm per 5 mph). Last i take 11(since 55mph is broken into 5 mph incroments) *192.5 rpm =2117.5 (rpm at 55mph). If you had tach and your speedometer was right on it should be close but your speedo is propally off by 3 to 5 mph if you were to check. Its just a rough idea as you wanted. I think you should measure you tires for a more accurate prediction.

 

If you can get a hold of a handheld GPS.Put it up on the dash(or a place where it can acquire a good signal). That will give you real time speed.

Also if you are out in the highway, time yourself between mile markers while holding a constant rpm on level terrain. You can also use your odometer

Lets say it takes 63 seconds to go 1 mile.

3600 seconds in an hour
63 goes into 3600=57.14 times, so there is your speed

 

The one problem with an automatic is that the torque converter slips so you can't calculate engine rpm's from wheel revolutions. Engine Rpm's can be calculated for a manual as long as the clutch isn't slipping.

Hawkdriver has the best method above for calculating or determining speedometer accuracy.

 

Thanks ya fellas! But darn... thems fancy ea-kway-shuns is gonna be hard ta do on the fly

Ya dern'd math geniu-sez!

 

good point... just one more reason for me to drive a stick! :P

 

And rolling radius is from the ground to the center of the axle, not tire hight. And Ben deserves an A in any event for showing his work.

 

But the static radius to the center of the axle is not the same as the rolling radius at speed. Most of the technical writeups on the subject that I have seen say to use the diameter because it is closest to the rolling radius.

 

I stand educated. So this means the vehicle "lifts" somewhat as speed increases?

 

60mph =1mile a minute get a stopwatch and a hiway with mile marker signs check your tach simple

 

Ecuri-

There was an explanation on a tire company website, -I don't remember which one, but it was a major company. I may be all wet also, my memory ain't exactly as good as it once was...

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Here are some speedo calibration figures from my files:

Seconds to travel a measured mile:
==================================
Time= Actual road speed
50=72
51=71
52=69
53=68
54=67
55=65
56=64
57=63
58=62
59=61
60=60
61=59
62=58
63=57
64=56
65=55.4
66=54.5
67=54
68=53
69=42

I make no claims as to their accuracy, they came from a friend and I have not checked them.

 

Have you guys ever felt tires grow at speed? Back in the days...
A friend's Roadrunner with bias ply tires would lift the rear end, felt like about 2", at about 90 MPH. It was like getting an extra gear on the top end.