Speedometer discrepancy

[Massman]

My 24 Lariat 2.7 reads 1 mph higher than I am actually traveling.

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[LDHunter]

Every car or truck I've owned in the last 30 years has reflected about 1-2mph faster than I was actually going. We have radar warning signs here in Florida and they reflect actual mph. I also have carefully used a Garmin GPS to read my actual MPH and they also confirm that all my vehicles' speedometers I've owned for a very long time show a bit faster than my actual speed.

I've come to the conclusion the manufacturers do that to avoid lawsuits from people getting speeding tickets and suing them.

 

[jrRaptor]

GPS and radar checked, my speedo is pretty much 1 mph over actual.

 

[scubajosh77]

So mine was good before the bigger tires. Used Forscan and it’s still off about 3mph. Speedo reads faster than I’m actually going. Need someone to help me math. 275/70R18 are ~2646 mm in circumference. How much and in what direction should I adjust to makeup the ~3mph discrepancy?

 

[scubajosh77]

So I threw it into ChatGPT. This is what I got back:


To correct the tire size setting in your truck’s computer, you need to adjust the circumference value so that the speedometer reads accurately.





Your truck thinks it’s going 50 mph when it’s actually going 48 mph, meaning the indicated speed is 4.17% too high (since 50/48 = 1.0417). This suggests the computer thinks the tires are smaller than they actually are.





Step 1: Calculate the Corrected Circumference





Since the circumference is currently set at 2646 mm, you need to increase it by 4.17% to correct the speed discrepancy:





2646 \times 1.0417 = 2757 \text{ mm}





Step 2: Update the Computer





Change the tire circumference setting to 2757 mm in your truck’s system. The method for doing this varies by vehicle, but it may involve using a tuner/programmer or a dealer service tool.





After adjusting the setting, verify accuracy by using GPS or a speedometer app while driving. If necessary, fine-tune the circumference setting further.

 

[Lutrev]

How much of this is tire pressure related ? does low pressure change the data ?

 

[scubajosh77]

How much of this is tire pressure related ? does low pressure change the data ?

No idea but I’m sure it could cause some variation but 3mph difference? Dunno. I’ve got mine at 35.

 

[Satex]

How much of this is tire pressure related ? does low pressure change the data ?

Interesting. You should do a measurement and answer it yourself. Measure the height and the horizontal diameters at some low pressure and then at the highest rated pressure.

Average the respective measurements to get a fairly accurate diameter. Pi × D = circumference. Get the percentage difference between the high and low pressure readings.

I think it will be difficult to get accurate enough measurements to really know that your comparing accurate differences. However, a 2% difference would account for a 1 mph error at 50mph.

You'll need something more precise than a tape measure. Do you know any rocket scientists?

 

[LDHunter]

Tire pressures vary widely due to heat and cold. Especially on hot summer pavements. I haven't noticed any measurable differences in actual MPH as reflected by my GPS checks therefore I don't think tire pressures have much effect unless maybe they're very low. They're not balloons. They're steel belted. Right?

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