Hi guys. Brought my new F150 home this week, and discovered the speedo was reading 3 mph faster than I was actually traveling...at 70mph. (Checked against the Garmin GPS running on the dash). Double checked it against the wife's '08 Explorer and a couple of buddies rigs as well. I had the dealer swap the 6 ply's for Hankook ATM's before I left the lot, but with the same size tire. The Hankook's appear bigger (just more aggresive), but if so, the speedo result would be the opposite. Any ideas?

 

Almost all of the trucks are fast by 2-3 mph at 65. Ford builds them that way. You will find lots of references if you use the Advanced Search. Occasionally, somone will say thiers is spot on, but it is rare.

 

Thanks Bluejay. I'll put 35K on a year, and knowing where it set the cruise to avoid tickets becomes important.

 

The wife's new 09 Explorer is fast by 2-3 all factory according to the Garmin nav...

 

i have the same problem. kinda sucks, cuz in all honesty i thought the f150s were slow enough anyway, and then i found out it was even slower than i thought lol

 

I have two Garmins and they disagree with eachother by 3mph. One of mine reads 1mph fast, the other 3mph fast.

Sorry to put things in check, but they're not accurate. I've used each of these Garmins in several different cars with the same results.

 

Last week I passed one of those speed indicators .I checked my speedometer and it read 3 mph high at 40 mph. That's on my '09 but my '03 seemed to be right on.
75 mph seems to be ok on I-84 in NY as far as the police are concerned.

 

It's time to go back to basics here guys. Screw the GPS, they're not accurate enough nor is the speedo in your truck.

All you need is a highway with mile markers, a stop watch and a calculator. Run a mile at your speed of choice and do this simple calculation

MPH = 3600 / (# of seconds to travel 1 mile)

Remember your speedo is not that accurate and the difference between it's displayed speed and actual speed will vary as a function of speed. So don't just assume if your speedo is off by 5 at 60 it will be off by that much at 70. It might be, but it might be a little different.

 

I solved my issue by getting big mud tires and a level kit...now I run about 1-2 MPH slow up to 45-50MPH and then close to 4-5MPH slow at 70MPH

 

I run right at 2mph slower - using factory 20" Perelli tires.

 

X2!! You can go even farther, though. Back in the 60s and 70s, I would do these calculations then calculate the error as a percent of indicated (speedo) speed for a % of speedo error, which then could be used at any speed. Repeated cross-checking against highway mile markers proved this to be very accurate. And those were gear-driven speedos with more potential for variance. This could be an invaluable method on vehicles with a non-standard tire circumference.

 

mines not based on gps. i checked mine with one of those things that flashes your speed when you drive by. idk how accurate they are

 

Ya, the good ole days.....

 

I think they read fast to error on the side of safety. Mine shows 2mph faster than GPS.

 

Speedos are rated at +/- of some percent. In order to make sure it's not ever going to read too low than actual (speeding ticket time), they make sure the + side of the error is at or under the real speed.

Example.

A speedo is rated for +2% to -2% error (4% span).
In practice, it's going to be +0% to -4% error (4% span).