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MPH based on RPM?

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Old 07-12-2011, 01:25 AM
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MPH based on RPM?

Speedo is inop, just got a tach from the junk yard. Have been just going with the flow of traffic, hoping not to get pulled over for speeding.
I have the the inline 6 with the 4 speed manual overdrive with 3.08 rear end and stock tires. Buying a speedo would be a good idea, but the gear inside the trans is stripped.

Can anyone tell me what speed they are going and what RPM and gear?

Any help would be appreciated.
 
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Old 07-12-2011, 01:47 AM
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Is all that is making the speedo inop. the stripped gear? Are you talking about the plastic gear at the end of the speedo cable where it goes into the tail houseing of the tranny? It is cheap part at dealership or junk yard, just need to know how many teeth on the gear. Give or take you should be running 2143 rpms in 3rd at 60 mph and 1607 in OD.
 
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Old 07-12-2011, 01:59 AM
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The whole speedo cable was replaced, brand new gear on the cable, cluster was swapped with a known working cluster.

The RPMs sound about right though. Last night I found out that the new Honda Civics have massive digital speedo displays that can be seen day or night, too bad that they accelerate a bit faster than I can... Thanks for the #'s.
 
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Old 07-12-2011, 02:24 AM
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It's not too much work replacing the speedometer drive gear in the tranny on a 2wd.

You should be able to remove the tailshaft and put the new gear on.


Did you make sure you got the right speedometer cable gear with the correct pitch for your application? They have both Left handed pitch and right handed pitch on the speedo gears.
 
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Old 07-12-2011, 05:25 AM
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Old 07-13-2011, 01:46 AM
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I guess adding in that it is a TOD/T170FT would help. The gear calc is mainly for the tremec... but thanks anyways.
 
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Old 07-13-2011, 02:00 AM
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Need your tire size.

OD is probably .78, depending on which one you have. Do you have an iron side shift, or aluminum top shift, or the SROD, and was it out of a truck or is it the OEM trans?

Toploader 4 Speed Transmissions

Now to calculate mph at a particular rpm. It may be easier to just know the mph per 1000 rpms. If you have:

1000 rpms
30 inch tires
.78 high, 1:1 3rd
3.08 axle

You will have 37 mph per 1000 rpms in 4th and 29 mph per 1000 in 3rd.

You can plug in what you like here:

Novak Conversions - Gearing & Gearing Math for Jeeps

Yes I know there is a calculator on FTE but it does not allow the transmission variable.

Note the above is for 30 inch tires, you need to figure out what yours are, and there is a tire size to inch diameter converter on that site as well.
 
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Old 07-13-2011, 07:40 AM
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Using the calc i posted with 1:1 you get (with a 235/75R15 Tire)
Engine RPM: 2151.04945336627
Vehicle Speed: 60
Read End Gear: 3.08

now take that RPM and multiply by your OD ratio (.78) you get 1678 RPM @ 60MPH

Tremec or not use 1:1 ratio then multiply answer by your OD and you'll get your RPM at whatever speed you enter.

If solving for an auto with NON Lockup converter you will need to add 200-500RPM for converter slip.
 
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Old 07-13-2011, 01:06 PM
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It's very simple thing to check. Get on the interstate and get someone to time how much it takes to go from mile marker to mile marker. If it takes less than a minute, slow down a little bit and try again. If it takes longer than a minute, speed up. When you get it to a minute, you are going 60 mph. That will get you started marking the tach for speed, though this will only be good in high gear.
 
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Old 07-14-2011, 12:59 PM
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You could borrow a gps from someone if you dont have one.
 
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Old 07-19-2011, 12:28 PM
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bull.nosed.doit,

I've been out of town and while trying to "catch up", I just read your thread. My 1986 F150 with 4.9L, 4 speed OD [SROD], 3.08 rear gears and original sized tires [215/75 X 15] does as follows in O.D.: 1000 rpm = 35 mph [going downhill]; 1500 rpm = 53 mph; 2000 rpm = 70 mph.

By the way, welcome to the forum!
 
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Old 07-21-2011, 03:05 PM
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Maybe no interest as no response, but on my way to work this morning, same truck as previously described and determined the following speeds at 2000 rpm: 1st=17 mph; 2nd=28 mph; 3rd=55 mph.
 
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Old 07-23-2011, 10:04 PM
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On behalf of bull.nosed.dolt...thanks a bunch! This is pretty much what he's looking for. He happens to be out of town at the moment on his honeymoon.


Thanks again!
 
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Old 07-28-2011, 12:27 AM
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Yeah, thanks for all the info.

By the way, on my honeymoon in the Caribbean, about half of all the trucks that I saw were 80-96 Fords (the other half was still Fords, just the most recent F-250's, oh, and like 2 chevys and a dodge,) doubtfully with anything other motor than the 300 based on the landscape. Anything else would crap out with the stop and go, ups and downs and minimally maintained roads after that much time. Probably would be my choice if I make the move after deployment.

Just another testament to the Fords...
 
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