Speed Limit
Post Fiend
Joined: Sep 2004
Posts: 5,231
Likes: 1
From: Near Fresno, California
If you will post the diameter of your tire I will figure your rpm for you. With 29 inch tires it should be around 2800 RPM at 60 MPH.
If you want to be sure, jack up one wheel and count the number of times the driveshaft turns when you rotate the wheel one turn. Should be 3.92 times (about 4 times, as close as you can estimate.)
btw, it sounds like the tach is set for 6 cyl operation - sometimes there is a switch on the back, sometimes you have to change the wires. It should say so in the directions.
If you want to be sure, jack up one wheel and count the number of times the driveshaft turns when you rotate the wheel one turn. Should be 3.92 times (about 4 times, as close as you can estimate.)
btw, it sounds like the tach is set for 6 cyl operation - sometimes there is a switch on the back, sometimes you have to change the wires. It should say so in the directions.
Last edited by WillyB; May 28, 2006 at 11:56 PM.
Fleet Owner
Joined: Dec 2000
Posts: 25,351
Likes: 5,374
From: Central PA
What I did a couple of years ago to check my speedo was this: when on an interstate or the PA Turnpike I checked my odometer when I passed a mile marker. After some 20 miles my speedo was off only about one half mile, to the high side. My odometer read more than the actual mileage. I assumed then if my odometer was pretty accurate then my speedo should be pretty close, but a bit faster than my actual speed.
Is this correct logic?
abe
Is this correct logic?
abe
Thread Starter
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Senior User
Joined: May 2006
Posts: 255
Likes: 0
My tires are just over 27". I did the math myself and came out with the same answer as the online calculator. The tach is adjustable for 4, 6, and 8 cyl. It is set at 8 as it should be and when I switch it to 6 or 4 the RPM indication increases as to be expected. Based on my math I figure that either the rear gearing would have to be between 7:1 and 8:1 which is impossible, or the trans ratio in 3rd is 2:1 or that Tach just isnt compatible with my engine for some reason. Maybe it has something to do with how the cylinders fire that causes a false reading on the tach (about double what it should read). Has anyone heard of The trans gearing being anything lower than 1:1 in 3rd? I think without OD it is pretty common to have a 1:1 ratio in top gear. Thanks,
Mike
Mike
Post Fiend
Joined: Sep 2004
Posts: 5,231
Likes: 1
From: Near Fresno, California
We used to try to cruise those engines around 2250 RPM, and hold them below 3K at all times. Hot rodders would turn them faster, but they upgraded the heck out of them.
It sounds to me like you have a 50 MPH truck until such a time as you change the rear end ratio.
And, trannys with less than 1 to 1 output are called underdrives, and are very special. I have never heard of one stock in a pickup.
It sounds to me like you have a 50 MPH truck until such a time as you change the rear end ratio.
And, trannys with less than 1 to 1 output are called underdrives, and are very special. I have never heard of one stock in a pickup.
Postmaster
Joined: Apr 2000
Posts: 3,890
Likes: 9
From: La Verne, California
I chequed my Speedo in a Rather simplistic fashion.
I drove through a School Zone that has those Electronic Speed Meters that shows you how Fast you are drivining through the S.Z. & to Slow Down if you are going faster than 25mph.
My Speedo was showing that I was travelling @ 22mph & the S.Z.M. was showing that I was traveling @ 27mph, so my Speedo shows that I'm travelling 5mph Slower than I actually am going.
If I'm not mistaken, that means the faster I'm travelling the worse the gap becomes between my actual speed & what my speedo is showing. Yes or No?
I drove through a School Zone that has those Electronic Speed Meters that shows you how Fast you are drivining through the S.Z. & to Slow Down if you are going faster than 25mph.
My Speedo was showing that I was travelling @ 22mph & the S.Z.M. was showing that I was traveling @ 27mph, so my Speedo shows that I'm travelling 5mph Slower than I actually am going.
If I'm not mistaken, that means the faster I'm travelling the worse the gap becomes between my actual speed & what my speedo is showing. Yes or No?
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Post Fiend
Joined: Sep 2004
Posts: 5,231
Likes: 1
From: Near Fresno, California
Colonel, if the problem is the wrong speedo gear, then the error will be a pecentage. In your case you would be traveling 1.227 times faster than indicated. If you read 60 MPH, then you would be going 73.6 MPH.
You could do it in your head if you considered you are off by about 25% - which would be 15 MPH at 60 (1/4 of 60) You would then add the 15 to the 60 for a speed of 75 - not real accurate, but good enough to avoid a ticket.
That is assuming you indicated 5 MPH slow at 22 MPH - I would want to verify that. You can not get a different speedo gear?
And Chris, Ford had a 4 speed column shift in their early econoline vans - never seen one in a pickup. They were a pain, not very popular, and there was no chance you would think 3rd was high gear.
You could do it in your head if you considered you are off by about 25% - which would be 15 MPH at 60 (1/4 of 60) You would then add the 15 to the 60 for a speed of 75 - not real accurate, but good enough to avoid a ticket.
That is assuming you indicated 5 MPH slow at 22 MPH - I would want to verify that. You can not get a different speedo gear?
And Chris, Ford had a 4 speed column shift in their early econoline vans - never seen one in a pickup. They were a pain, not very popular, and there was no chance you would think 3rd was high gear.
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