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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.
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.
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!!
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...
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.
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.
OK... I used the calculator on the Speedometer Gear Calculator and Charts - TCI® 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
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
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