[Croc Doc]

I have corrected my tire size on the canned Tow tune, and I have a question about the speedometer. I am using the CTS to monitor my speed, and my speedometer doesn’t match it. My speedometer is reading at least 2-3 mph faster than the CTS. Which one is right? I don’t want to get a ticket. If the CTS is accurate, then doesn’t this throw off my overall mileage?

Thanks.

[shotgun]

Ford designs and builds the speedometers to read 2-3 MPH fast at 60-70 MPH.

I'd say your speedometer is dead nuts on.

The best way to confirm is to drive a known distance (Google maps) or use a portable GPS. The longer the better, but at least 10 miles. Then use the following formula:

[Croc Doc]

It is 2-3 mph faster than the CTS shows from 0 - 80 (the fastest I gave gone). So is the CTS correct, or the factory speedometer?

[JackandJanet]

Actually shotgun, regardless of whether your speed is correct, your dash speedo always reads about 3% higher than the CS/CTS indication (because Ford built it that way). And, that percentage error works out to 2-3 mph high at 70-75 mph.

Now, the goal is to get the CS/CTS mph figure to be accurate, and then, the formula you posted is correct. So, the OP needs to check his CTS against an accurate measure, such as a GPS. Then, use the CTS value for "Truck Odometer" in the formula.

Then, with the CTS reading correctly, the dash speedo will STILL be 2-3 mph fast (which will help you not get a ticket) and, your dash digital odometer will be accurate.

- Jack

[shotgun]

Originally Posted by JackandJanet

Actually shotgun, regardless of whether your speed is correct, your dash speedo always reads about 3% higher than the CS/CTS indication (because Ford built it that way). And, that percentage error works out to 2-3 mph high at 70-75 mph.

Now, the goal is to get the CS/CTS mph figure to be accurate, and then, the formula you posted is correct. So, the OP needs to check his CTS against an accurate measure, such as a GPS. Then, use the CTS value for "Truck Odometer" in the formula.

Then, with the CTS reading correctly, the dash speedo will STILL be 2-3 mph fast (which will help you not get a ticket) and, your dash digital odometer will be accurate.

- Jack

Thanks Jack. That's what I was trying to say (and thanks for phrasing it a lot better than I did!).

[Croc Doc]

Originally Posted by JackandJanet

Actually shotgun, regardless of whether your speed is correct, your dash speedo always reads about 3% higher than the CS/CTS indication (because Ford built it that way). And, that percentage error works out to 2-3 mph high at 70-75 mph.

Now, the goal is to get the CS/CTS mph figure to be accurate, and then, the formula you posted is correct. So, the OP needs to check his CTS against an accurate measure, such as a GPS. Then, use the CTS value for "Truck Odometer" in the formula.

Then, with the CTS reading correctly, the dash speedo will STILL be 2-3 mph fast (which will help you not get a ticket) and, your dash digital odometer will be accurate.

- Jack

Okay. I was under the impression that once I put my new tire size into the CTS that it would work its magic and change my factory settings so that my speedometer and odometer would be correct. Just like if I changed the gearing….. Are you saying the CTS isn’t correct, and I need to check it per a GPS (that I don’t have)? All I want to know is: Is the CTS accurate? I understand now that Ford speedometer reads 3% higher. If the speed limit is 50, and I go 5 over the CTS will read 55 and my speedometer will read 58. This could be the difference in me getting a ticket or not.

Thanks again for the help.

[shotgun]

Originally Posted by Croc Doc

Okay. I was under the impression that once I put my new tire size into the CTS that it would work its magic and change my factory settings so that my speedometer and odometer would be correct. Just like if I changed the gearing….. Are you saying the CTS isn’t correct, and I need to check it per a GPS (that I don’t have)? All I want to know is: Is the CTS accurate? I understand now that Ford speedometer reads 3% higher. If the speed limit is 50, and I go 5 over the CTS will read 55 and my speedometer will read 58. This could be the difference in me getting a ticket or not.

Thanks again for the help.

If the tire size is correct, then both the CTS and your odometer will be correct. But your speedometer will read approximately 3% high.

You don't need a GPS to measure the distance. You can use Google maps. They have very accurate distance information.

[Croc Doc]

Thanks Shotgun.
I put the metric size (295/70/18) into the CTS. I would assume that it is as accurate as I can get. Do I need to double check it using Google maps then? If the CTS and my odometer are correct (as you are saying), then I will use the CTS to monitor my speed from now on.

Thanks again.

[shotgun]

Based on your first post

My speedometer is reading at least 2-3 mph faster than the CTS

I'd say your speedometer is as close as it's going to get (and your odometer is also accurate).

However, if you have an interstate with mile markers, you can easily double check your odometer and speedometer (60 secs for 1 mile at 60 MPH).

I usually keep my speedometer 2-3 MPH above the posted limit. I haven't had a speeding ticket since I lived in Germany (25 years ago). Yes, parts of the autobahn have speed limits...

[Croc Doc]

Thanks. The cops around here will give a ticket if you are going 6 mph over the posted limit, and they don't buy the "I have bigger tires than stock, and it is throwing off my speedometer" saying either. They will give a "fix it" ticket to have the speedometer corrected too.... This is why I wanted to make sure I have the facts straight.

Thanks.