Thread: speed-o help please

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hey guys i am running a th350 tranny in my 53 ford f100 with a 10 bolt rear ,,now i checked it and the drive shaft turns 3 times to the tires 2 times .. i am running a 255 tire in the rear ,, i am tring to get my speed-o right right now i am running the white gear inside tranny ,, can someone add to what i need to do please

thanks

did a test and i am off about 8 mph

thanks

Start by finding your gear ratio. Posi and non posi have different ways of calculating.


How to find your differential gear ratio without stamps, tags, or tear down - YouTube

After you have an exact gear ratio, place your truck down and measure from the floor to the center of your hub. This will be a true 1/2 diameter of your wheel. Get back with this info and we can find an on line calculator for the gear sizes.

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i think the gear ratio is 3.08 tire is 28.5 from the ground to top, non posi,, i am going to do it again by the video to double check

Tire size from bottom to top is irrelevant. Remember, it's flat on the bottom.

Size from ground to center x 2 = the actual rolling diameter.

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If you want the true diameter of a tire, use a cloth or fiberglass tape to find the circumference of the tire, then divide by 3.14159. The circumference is what the roadway sees, not the loaded diameter.

Use a little math to find the proper speedo gears for your TH350.....
http://www.tciauto.com/tc/speedomete...alculator/#350

Originally Posted by techinspector1

If you want the true diameter of a tire, use a cloth or fiberglass tape to find the circumference of the tire, then divide by 3.14159. The circumference is what the roadway sees, not the loaded diameter.

Use a little math to find the proper speedo gears for your TH350.....
Speedometer Gear Calculator and Charts - TCI® Auto

I agree, Tech. Inflation pressure (and the size of the flat spot) doesn't affect tire speed. You want the actual tire diameter, and your approach of measured circumference divided by pi is more accurate than trying to measure the diameter.

For a true reading, try this. Mark your tire and your floor/driveway. Roll the truck, not the wheel alone, and roll it one revolution and mark the floor/driveway. Then measure between marks and divide by pi (3.1416) for actual rolling diameter.

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Although tire size and gear ratio play a part in what the speedometer reads, it is not necessarly the information you really need to correct/calibrate the speedometer.

The two things you need are:

the NUMBER of teeth on the speedometer driven gear (the color really doesn't help that much as I have seen the same color gear with different tooth counts).

The MPH the speedometer is off.......but that in and of itself is not enough. On the old analog speedometers the innacuracy will be off by a percentage, in other words if the speedometer is off by 5 MPH when driving 30 MPH it will be off by 10 MPH when driving 60 MPH. You need to provided the actual readings. I.E. at a vehicle speed of 70 MPH the speeedometer reads 62 MPH.

If you provie the 2 pieces of information I can figure out what speedometer gear you need.

i will take the driven gear out and count the teeth ,, and i will drive it at 30 and check and at 60 and check ,, ill get back with you

thanks

If you looked at Tech's post #6 above and opened the link you've seen that on the TH350 there are eight different drive gears, which are internal to the tranny tailhousing, and then seventeen different driven gears, which come out easily. You need to know which drive gear you have, too, unless Mike has some way of calculating that it doesn't come into play. On that link if you simply plug in the drive gear tooth count, the axle ratio and the tire diameter calculated from the circumference it will tell you which driven gear you need with no hassles and eliminating any guesswork.

Hi Roger, you are correct about the drive and driven gear, the drive gear (the one internal to the transmission) will only come into play if a driven gear is not available in the proper tooth count.

I am aware of the calculators you are talking about, however there are two componets in the speedometer itself that can render them inacurate. One is the return spring that returns the needle to 0 MPH and the other is the stregnth of the magnets. Both the spring or the magnets can get weaker over the years an it is not unusual for the the driven gear to need to be changed by a tooth or 2 compared to what the chart says especially if the speeometer is old.

The other variable is the bran of tire being used. I have personally seen the same size tire made by different companies be 1/2 to 1" different in height.

I'm not trying to be mysterious about how I figure out what speedometer gear to use, it's just that I have tried to write out the process in the past and in written form it gets complicate and hard to follow. It's easier to actually do than to explain.

Norm all I really need is how much the speeometer is slow or fast at 60 MPH....the higher the vehicle speed (within reason) the more accurate the calculation.

Mike,
I understand that writing out an explanation can get messy for a lot of things like this. Of course, like the other Mike (34_40) pointed out not too long ago, the alternative is to buy one of the new GPS speedometers, or have the old one converted to a GPS unit and eliminate the tranny interface and error concerns. They're pricey, though.

i did some checking ,, it don't make sense ,, i am off 10 at 30 mph and 10 at 60 mph i thought it would be different

thanks please help ,,
i have 19 tooth in there now ,, i do have a 20 but it was still off with that one as well

Like Tech said in his post above, get the information needed and use the tool - Speedometer Gear Calculator and Charts - TCI® Auto

It's not really difficult, and the first thing is to get the right gear in the tranny based on the tire size & rear end gears. Then if you're still off you know it's the speedometer, and you may have to get it reconditioned.