The Ford Barn

Tomjc · 09-01-2014, 07:41 AM

What gear do I need for my 54 Merc withT85 and 4:10 rear. My speedometer says I am running 100 mph thru town.

Motorhead6 · 09-01-2014, 02:27 PM

Do some math.

Divide the original new gear ratio (4.10) by the original gear ratio (say, 3.55) to get the difference in ratios. Now, multiply the speedometer gear you have by this product to get the approximate number of teeth that you should have. 4.10/3.55=1.155 If the gear you have is 17, then 17X1.155=19.635. You need either a 19 or 20 tooth gear. Fortunately they are cheap so you can buy more than one and try one then the other. Using the GPS, one can verify your speed for accuracy.

Alternate method.

Using your GPS, drive at a constant indicated speed, say 50 mph. Using the GPS, determine the actual speed you are driving. Now divide the indicated speed by the actual speed and this is your multiplier. Multiply the number of teeth on your speedometer gear by the determined multiplier and this will give you the approximate number of teeth you need. As above, buy a couple and try one then the other.

Example. Indicated speed is 50mph, actual speed by the GPS is 42 mph. Say you have a 17 tooth speedometer gear. 50/42=1.19; 1.19X17=20.23. Buy a 20 tooth gear and try it; should be close.

Remember, the original speedometers in these cars were never highly accurate. You can't buy a 20 1/2 tooth gear so get as close as you can.

ford3 · 09-01-2014, 09:15 PM

first you really need to know how fast you are going, get a friend to drive 50 mph in front of you and see what your speedo reads, one tooth more on gear will slow speedo down about 3 to 4 mph, I bet you need a gear with at least 2 more teeth, maybe 3

deucemac · 09-02-2014, 12:31 AM

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I used to do speedo cals for a living using a Clayton chassis dyno at both Ford ans L/M dealers. one tooth change will get 2-2 1/2 mph change Ford made 15 -21 tooth gears. Output shaft teeth can vary by car. I once had a Merc that was the owner's demo and could not get it to read anywhere near right. Turned out it had an F250 output shaft in the C6. He would go down the street reading 60+ and actual speed was about 42! Once the trans guy changed it, everything was great. Keep in mind that ANY mechanical speedo is correct at only one speed You could be slow below that point and fast above or visa versa. When you think you are close, have someone on a dyno do the final cal and get a calibration card so you know just what speed you are actually doing at any indicated speed.

09-01-2014, 07:41 AM	#1
Tomjc Member Join Date: Jul 2012 Location: Burlington, Iowa Posts: 63	Speedometer gear What gear do I need for my 54 Merc withT85 and 4:10 rear. My speedometer says I am running 100 mph thru town.

09-01-2014, 02:27 PM	#2
Motorhead6 Senior Member Join Date: Jul 2012 Location: Orange Park, Fl Posts: 406	Re: Speedometer gear Do some math. Divide the original new gear ratio (4.10) by the original gear ratio (say, 3.55) to get the difference in ratios. Now, multiply the speedometer gear you have by this product to get the approximate number of teeth that you should have. 4.10/3.55=1.155 If the gear you have is 17, then 17X1.155=19.635. You need either a 19 or 20 tooth gear. Fortunately they are cheap so you can buy more than one and try one then the other. Using the GPS, one can verify your speed for accuracy. Alternate method. Using your GPS, drive at a constant indicated speed, say 50 mph. Using the GPS, determine the actual speed you are driving. Now divide the indicated speed by the actual speed and this is your multiplier. Multiply the number of teeth on your speedometer gear by the determined multiplier and this will give you the approximate number of teeth you need. As above, buy a couple and try one then the other. Example. Indicated speed is 50mph, actual speed by the GPS is 42 mph. Say you have a 17 tooth speedometer gear. 50/42=1.19; 1.19X17=20.23. Buy a 20 tooth gear and try it; should be close. Remember, the original speedometers in these cars were never highly accurate. You can't buy a 20 1/2 tooth gear so get as close as you can.

09-01-2014, 09:15 PM	#3
ford3 Senior Member Join Date: Oct 2012 Location: oroville calif. Posts: 1,453	Re: Speedometer gear first you really need to know how fast you are going, get a friend to drive 50 mph in front of you and see what your speedo reads, one tooth more on gear will slow speedo down about 3 to 4 mph, I bet you need a gear with at least 2 more teeth, maybe 3

09-02-2014, 12:31 AM	#4
deucemac Senior Member Join Date: May 2010 Location: Tehachapi, Ca. Posts: 208	Re: Speedometer gear Sponsored Links (Register now to hide all advertisements) I used to do speedo cals for a living using a Clayton chassis dyno at both Ford ans L/M dealers. one tooth change will get 2-2 1/2 mph change Ford made 15 -21 tooth gears. Output shaft teeth can vary by car. I once had a Merc that was the owner's demo and could not get it to read anywhere near right. Turned out it had an F250 output shaft in the C6. He would go down the street reading 60+ and actual speed was about 42! Once the trans guy changed it, everything was great. Keep in mind that ANY mechanical speedo is correct at only one speed You could be slow below that point and fast above or visa versa. When you think you are close, have someone on a dyno do the final cal and get a calibration card so you know just what speed you are actually doing at any indicated speed.