Speedometer ratio

[Big Dutch]

How do I calculate the correct ratio off for a ratio adaptor using the speedometer and a gps without knowing the ratio or tire size?

Where can find a calculator to do this?

I shouldn't need any more info., but no one has this.

Let me give you my problem. The speedometer reads 96 and I am going 60 using 2 gps units to check the speed. What percentage ratio adaptor do I need?

I know that the driven gears do not give me enough play and it would be cheaper to buy a ratio adaptor than play with the gears and gear housings, which would also have to be changed.

Total for driven gears and gear housings playing around would be $120 and probably not correct. Ratio adaptor is $70.

I'm not a junkyard person and would like to buy online.

Thanks, Big Dutch

[geezer#99]

Look here.
Ratio adaptors, calculators, gears etc.
To get down to 60mph from your indicated 96 you need to slow your speedo 37.5%.
http://www.transmissioncenter.net/sp...n_______va.htm

[Big Dutch]

Used it and didn't work. Is there an engineer or someone who knows a calculator.

[geezer#99]

I used a calculator.
Your speedo reads 36 mph high which means you need to slow it down 37.5%
37.5% of 96 is 36.
96-36 is 60.
What more do you need?

[Keith Seymore]

Originally Posted by Big Dutch

Used it and didn't work. Is there an engineer or someone who knows a calculator.

Originally Posted by geezer#99

I used a calculator.
Your speedo reads 36 mph high which means you need to slow it down 37.5%
37.5% of 96 is 36.
96-36 is 60.
What more do you need?

This is correct.

Big Dutch: It is a straight up ratio of one number related to another.

The fact that you are calculating vehicle speeds is immaterial. The math would be the same if it was a compression ratio, a cookie recipe or comparing your net earnings to your gross earnings.

The link geezer provided is the one I always use.

K

[MidLifer]

Cheapest is not always best. If this were me I would do the calculation using your final drive ratio and your tire size and buy the right gears for your trans.

Did your truck used to have monster tires on it? That reading sounds way off. The GPSes are right, not doubting those.

[Keith Seymore]

Originally Posted by MidLifer

Cheapest is not always best. If this were me I would do the calculation using your final drive ratio and your tire size and buy the right gears for your trans.

Did your truck used to have monster tires on it? That reading sounds way off. The GPSes are right, not doubting those.

I always prefer a speedo adapter when I can get away with it.

There are too many opportunities for error in the calculations, with variation in tire rolling radius vs overall diameter, differences between actual vs published tire data, unknown rear axle ratio and/or perhaps a standard speedo gearset is not available for his monster tire. The ratio calculation and use of an adapter cuts right to the chase.

My last correction (as a result of a trans swap) I was able to find the adapter I needed on one of my shelves.

K

[Ironhorse]

Subscribed to this, good info here.

Keith, PM sent for some assistance/tech questions.