Speedometer/odometer help - Ford Truck Enthusiasts Forums

Notices
1973 - 1979 F-100 & Larger F-Series Trucks Discuss the Dentsides Ford Truck

Speedometer/odometer help

  #1  
Old 01-23-2016, 04:50 PM
BrushPopper's Avatar
BrushPopper
BrushPopper is offline
Freshman User
Thread Starter
Join Date: Nov 2011
Location: High country of Arizona
Posts: 32
BrushPopper is starting off with a positive reputation.
Speedometer/odometer help

I have a '73 F-250 4-speed manual 4wd 390, with a dana spicer 60 rear end. I was having problems with the odometer reading about 1.75 niles in a 30 mile stretch, but the speedo seemed to be working, so I swapped out the speedo/odometer from a '76 F-150.. the odometer is working now, but everything is a little bit off.

In a 1 mile stretch I first marked the odometer and it read that I went 1 and 1/10 of a mile. Then I marked the seconds it took me to run a mile at 60 mph and it took about 70 seconds.

Think I need to change the speedo gear in the tranny but not sure how to calculate to get the right one. I am running Mud Claw 32X11.50R15LT tires. rear gear ratio is 4.10.
 
  #2  
Old 01-23-2016, 04:56 PM
rodbender
rodbender is offline
Junior User
Join Date: Jul 2004
Location: Central Illinois
Posts: 99
rodbender is starting off with a positive reputation.
  #3  
Old 01-23-2016, 04:59 PM
BrushPopper's Avatar
BrushPopper
BrushPopper is offline
Freshman User
Thread Starter
Join Date: Nov 2011
Location: High country of Arizona
Posts: 32
BrushPopper is starting off with a positive reputation.
Hey Rodbender, thanks for the kwick response but where do i get the # of Drive Teeth? for the formula
 
  #4  
Old 01-23-2016, 05:33 PM
rodbender
rodbender is offline
Junior User
Join Date: Jul 2004
Location: Central Illinois
Posts: 99
rodbender is starting off with a positive reputation.
The drive teeth will be on the output shaft of the transmission. What I did was take the speedo drive out and get a paint marker and a small flashlight. Put a dab of paint on one tooth (they're made at an pretty good angle so be careful). Then, rotate the trans tail shaft (in neutral) while counting the teeth. For instance, the black drive gears are 8 teeth.
 
  #5  
Old 01-24-2016, 07:27 AM
scottscott
scottscott is offline
Elder User
Join Date: Dec 2007
Location: Wildwood Pa
Posts: 614
scottscott has a good reputation on FTE.scottscott has a good reputation on FTE.
The best thing to do is drive your truck at the MPH you want the speedo to be most accurate, let's say 60 MPH. Have a GPS in the truck, & see how far off you are at that speed. Let's say speedo shows 60 MPH, GPS shows 72 MPH. That is roughly a 20% error, speedo reading slow. Pull your driven gear, count the teeth. Let's say it has 20 teeth. You need a gear with 20% less teeth to get the speedo close to accurate at 60 MPH. 20 X .8 = 16 teeth.

You do the opposite, add teeth, if the speedo is reading faster than actual.

If you can't get a driven gear that will correct the error, then you may need to change the drive gear. I've never had to do that, I guess I've been lucky.

I've heard that anything under a 5% error is considered very good. I know the truck I drive at work, a 2013, is about 10% off at 60 MPH, but it becomes slightly more accurate as the speed increases. Maybe at 100 its dead accurate.

I'm not the greatest at math, so be sure to double check my calculations!!
 
  #6  
Old 01-24-2016, 10:11 AM
NumberDummy's Avatar
NumberDummy
NumberDummy is offline
Ford Parts Specialist
Join Date: Nov 2006
Location: Simi Valley, CA
Posts: 78,574
NumberDummy has a superb reputationNumberDummy has a superb reputationNumberDummy has a superb reputationNumberDummy has a superb reputationNumberDummy has a superb reputationNumberDummy has a superb reputationNumberDummy has a superb reputationNumberDummy has a superb reputationNumberDummy has a superb reputationNumberDummy has a superb reputationNumberDummy has a superb reputation
Speedometer DRIVE gear located on output shaft of transmission.

Speedometer DRIVEN gear located at end of cable, 16 thru 21 tooth nylon gears are available.

It's much easier to change the DRIVEN gear than the DRIVE gear.
 
  #7  
Old 01-24-2016, 10:32 AM
BrushPopper's Avatar
BrushPopper
BrushPopper is offline
Freshman User
Thread Starter
Join Date: Nov 2011
Location: High country of Arizona
Posts: 32
BrushPopper is starting off with a positive reputation.
Thanks NumberDummy, but not looking to replace the drive gear, just was not sure where it was.. for the calculation...so i can get a replacement driven gear, for some reason I was thinking the number of splines on the drive shaft coming out the rear of the tranny... duh...
 
  #8  
Old 01-24-2016, 10:34 AM
BrushPopper's Avatar
BrushPopper
BrushPopper is offline
Freshman User
Thread Starter
Join Date: Nov 2011
Location: High country of Arizona
Posts: 32
BrushPopper is starting off with a positive reputation.
scottscott, I do not have a GPS, so I am going to do the calculations mentioned on the website Rodbender posted
 
  #9  
Old 01-24-2016, 11:52 AM
scottscott
scottscott is offline
Elder User
Join Date: Dec 2007
Location: Wildwood Pa
Posts: 614
scottscott has a good reputation on FTE.scottscott has a good reputation on FTE.
Using your time to run 1 mile while running 60 MPH shows your speedo is slow by roughly 15%, if my calculations are correct. That assumes your timing is spot on.

Let's say you check your driven gear & it has 19 teeth. 19 X .85 = 16.15 tooth gear needed. You'll use .85 as the multiplier on your driven gear.

However, your odometer error looks to be the opposite direction, it is too fast. Might be hard to get it dialed in without another speedo head or adjusting the delicate spring on the speedo needle shaft. You can see it behind the large plastic front on the speedo head. If you have a drill that goes to 1000 RPM, you can spin the speedo at 1000 RPM, speedo should read 60 MPH.

I like using the GPS to get actual speed, it is quick & easy. We have radar trailers along the roadside in my area, those work well for getting actual speed as well.
 
  #10  
Old 01-24-2016, 12:16 PM
dlburch
dlburch is offline
Elder User
Join Date: Jul 2005
Location: Overland Park, KS
Posts: 934
dlburch has a good reputation on FTE.dlburch has a good reputation on FTE.
you can also install one of the many speedometer apps on your smartphone if you have one...
 
  #11  
Old 01-25-2016, 01:26 PM
NumberDummy's Avatar
NumberDummy
NumberDummy is offline
Ford Parts Specialist
Join Date: Nov 2006
Location: Simi Valley, CA
Posts: 78,574
NumberDummy has a superb reputationNumberDummy has a superb reputationNumberDummy has a superb reputationNumberDummy has a superb reputationNumberDummy has a superb reputationNumberDummy has a superb reputationNumberDummy has a superb reputationNumberDummy has a superb reputationNumberDummy has a superb reputationNumberDummy has a superb reputationNumberDummy has a superb reputation
Originally Posted by BrushPopper View Post
Thanks NumberDummy, but not looking to replace the drive gear, just was not sure where it was.. for the calculation...so i can get a replacement driven gear, for some reason I was thinking the number of splines on the drive shaft coming out the rear of the tranny... duh...
Number of teeth of DRIVEN gear based on rear axle ratio, trans type and tire size.
 
  #12  
Old 01-26-2016, 11:56 AM
BrushPopper's Avatar
BrushPopper
BrushPopper is offline
Freshman User
Thread Starter
Join Date: Nov 2011
Location: High country of Arizona
Posts: 32
BrushPopper is starting off with a positive reputation.
OK... I used the calculator on the Speedometer Gear Calculator and Charts - TCIŽ Auto site and I get 18.4742286 as the answer, so should I go with the 18 tooth or 19 tooth gear? Almost ready to order one of each and just see which one works better, but would rather know ahead of time so I only order the one I need
 
  #13  
Old 01-26-2016, 03:23 PM
scottscott
scottscott is offline
Elder User
Join Date: Dec 2007
Location: Wildwood Pa
Posts: 614
scottscott has a good reputation on FTE.scottscott has a good reputation on FTE.
Hard to say.... You'll probably have to try a few gears & see which one gets you close.

Have you pulled your current driven gear to get a tooth count?? I'm curious as to what it is.

These are hard to pass up at the price. I have personally used these, they work great. I don't believe they are NOS OEM as the ad states. http://www.ebay.com/itm/SPEEDO-GEARS...mtr&rmvSB=true
 
  #14  
Old 01-26-2016, 05:53 PM
BrushPopper's Avatar
BrushPopper
BrushPopper is offline
Freshman User
Thread Starter
Join Date: Nov 2011
Location: High country of Arizona
Posts: 32
BrushPopper is starting off with a positive reputation.
Thanks for the link... price is about the same for all 5 as just the 2 I was going to get...And the one installed now is a 17 tooth


 
  #15  
Old 02-05-2016, 03:12 AM
BrushPopper's Avatar
BrushPopper
BrushPopper is offline
Freshman User
Thread Starter
Join Date: Nov 2011
Location: High country of Arizona
Posts: 32
BrushPopper is starting off with a positive reputation.
Just a quick follow-up, I did buy the ones you suggested Scottscott, put in the 18, still about 10 mph off, put in the 19 still off by about 5 mph... finally put in the 20 tooth gear...wheeew, now when my speedo says I am running at 60, my phone app says 58... close enuff for me... just confused about the calculation stating i needed 18.47 but had to go all the way to a 20


 
Related Topics
Thread
Thread Starter
Forum
Replies
Last Post
wisk
1987 - 1996 F150 & Larger F-Series Trucks
3
05-10-2017 12:35 AM
Matt Joines
Electrical Systems/Wiring
5
01-07-2017 01:55 PM
Robs89wag
New Member Introductions
0
01-04-2017 10:36 PM
Abner
1968-2013 Full Size Vans
4
07-25-2016 09:36 PM
fftball
1987 - 1996 F150 & Larger F-Series Trucks
7
01-24-2016 06:55 PM


Thread Tools
Search this Thread
Quick Reply: Speedometer/odometer help


Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service

© 2018 MH Sub I, LLC dba Internet Brands

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.