Hello,
I have a 1970 F350 custom with a 390 and C6 motor and transmission.
My rear gears are 4.10, Tires are 8.00 x 16.5 Circumference is 90.5 inches
The gear I have now on the cable in the transmission is 18 teeth.
The speedometer is way to fast!!
Any help would be greatly appreciated,

Thanks from Germany
Bob O'Neal

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Hello, there may be something on this site that will help you.
Tire Size Calculator
Wayne

 

Very Nice Truck! Hoping the chart, "whomrig", sent You will answer your question. Do you know what the original tire size was when the truck was built? Have you run, deciphered the Warranty plate info, the original tire info may be in/on there? 390/ C6, your doing or factory set up??? My question pertains to what I did for my F-250, with 4.10 gears. The truck was built with 650-16 tires. First new tire set I bought were 700-16, Bias Ply, cold flat spots drove me nuts, Speedo Off. I then went 750R-16 Radials, speedo WAY OFF! With the Ford Master Parts manual, I chose the correct gear for 750-16 tire with 4.10 gears. Tested it with a GPS in the truck, My speedo was dead on at all speeds.

So the tires you have are smaller in Diameter/Circumference than what the truck was built with. GUESSING came with 750-16 Bias tires. Looks like you were after a Lowered Appearance when you did your truck over.

These tire specs below are new Bias ply. The dimensions vary some by Mfgs.
750-16 = 31.60 dia, 99.22 cir
700-16 = 30.40 dia, 95.47 cir
650-16 = 39.90 dia, 93.88 cir
600-16 = 28.50 dia, 89.50 cir. This is the closest to your 90.5 cir tire. The 600-16 is said to vary the most in dia between Mfgs. from 28.00/87.92 to 28.99/91.02, I chose 89.50 as a mid range for this size. 600-16 is a before and after WWII era tire size. I Don't know if C6 was the original Auto transmission in a 70, F-350. Your tire size not from that 70 era truck, maybe a gear from a ford passenger car, with a tire close to you circumference might be a perfect match. If you answer questions, I'd search a bit farther for you. Cap

 

Cap, Thanks for the response!!
As I am in Germany and is the truck. To get the truck registered here the Motor vehicle department requires documentation of everything including the factory tire size.
The truck still has 16.5 wheels and the 800 is the size that is reported in the papers. I had to take off the 285/70 x16.5 tires and put these 800 x16.5 tires on. (Bias ply poop).
I have not really done anything to change the appearance other than wax, since I bought him in 2014. I know that originally he was a manual transmission most likely a 4 speed as the transmission tunnel cover was replaced. The drive that goes into the transmission is not like a car, as it is all metal and the holder threads into the transmission. One of my customers made me a decal to cover the Miles per hour with Kilometers per hour , looks factory. but my goal is to get it more accurate. I have attached a few pictures,
Thanks in advance
Bob O'Neal

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Don't mean to leave you hanging. Yes the 800-16.5 was an option as well as the C6. Not knowing what the C6, You have (came out of) OR if 370-1 was the original rear axle ratio from the factory, there are a # of variations in the Drive Gear on the output shaft in the tail stock, could be 6,7,8,9 teeth. Your 18 tooth gear is the DRIVEN GEAR. Would like to try find your exact proper gear, but can't seem to do with out the transmission ID, a tag on the transmission bolted near the shift lever would help ID what it came from.

This might cut the chase. Have or, can you drive the truck with a GPS aboard to get the actual Speed, MPH of the truck. Say 60 MPH, GPS and note what speedometer reads and record it. After a few visited site's, the consensus IS, if speedo is faster than the actual speed, to slow the speedo down, you need more teeth. 20,21, 22 teeth ???? For 6 to 8 dollars for most gears, could you try one, say 21 teeth. Then do the the road test again, note the improvement, of closer or to much and now the speedo slower than actual speed. IF CLOSER, do a percentage calculation between the differences the 3 more teeth provided compared to the 18 teeth you have. You should be able to see if 1 or 2 either side of the 21 will be right for you. I KNOW it's trial and error, but may be the best way to do it

ONE NOTE, some have made a Driven Gear change that was not in a range to connect properly with the Drive Gear or just couldn't bring close enough to make speedo accurate. They had to get correct Drive Gear and Driven Gear for their situation after changing tire size and or rear axle ratio. Cap

 

In Canada, speedometers became metric in 1977. They had been 100 mph, then went to a dual scale 80/130. Perhaps you can make a sticker which matches the speed, instead of making the speed match the scale? It is unlikely that truck will reach the 160 Km/H of your present sticker, and a lower scale will be easier to read.
This is assuming you are comfortable with the gearing and only trying to change the speedometer. Also that the needle is not so far off as to be off the scale.
Also, your odometer will remain almost meaningless.

 

There's a partial Ford part number on this 18T DRIVEN gear: 1T 71 A 18

'Course I deleted the wrong pic, should be the next pic.

Translates to 01T-17271-A

The 19T DRIVEN gear is 01T-17271-B // The 20T DRIVEN gear is 01T-17271-C

If the speedo is reading too fast, you have to slow it down by adding teeth to the DRIVEN gear

These gears have been obsolete for years, were used on myriad different trucks and are getting very hard to find NOS (New Old Stock).

I have a friend who lives in Germany, he has a 1968 F100 2WD Styleside pickup (it's red), he bought it in the US then had it "shipped across the pond." He works as a quality control inspector at the VW factory in Wolfsburg. Met him on another Ford truck site, he was looking for many NOS obsolete parts and I found him everything. Since then, he's vacationed in California twice and I've met him twice. I'll see if I can dig up his email address.

 

Hi, and thanks for the response..
I made contact with Deluxe Speedometer & Cable Service Inc in Denver Colorado and gave them the recorded speed at 30mph & 60mph and they sent me a adapter the that corrected the issue.
Now reads exactly correct.
Greetings from Germany. Bob O'Neal