The speedo reads to 85, that is true. But the needle will actually go way past that. And I know for a fact that the needle in my 78 150 4x4 will rap all the way around. I can't remember if the one in my 86 150 would do the same or not.

 

Lots of speculation in this thread.

The correct answer is tire speed rating and driveshaft critical speed / harmonics.

Whipping of the driveshaft and subsequent pitching it out the bottom of the truck, and a tire blowout in a truck designed to go faster than the tire speed rating are both items an intelligent design would avoid, hence speed limited by the vehicle electronics

 

Finn, then why did my '01 F250 have a limiter of 94mph but my '05 F250 would go 110mph? Never got it faster because like Raider50 posted "handling got a little sketchy"

I think the OP asked a curiously dumb question being that he already knew the answer. Love this forum !!

 

i think it is a safety thing. passenger and taxi crown vics are limited to 95. police package crown vics will do 137 mph. and that 137 number was confirmed by 2 different GPS units on the Connecticut turnpike back in 2005. my car and 2 Connecticut state police cars, all topped out at the rev limiter doing 137 mph.

 

What was the gear ratio, overdrive ratio, and tire speed rating of the '01 and the '05? Were they the same? Did both have the same wheelbase and driveshaft length?

Medium trucks are often limited by driveshaft harmonics, as were many rear wheel drive cars and light trucks..

There's lots to this fad of actually engineering a design.

Math is your friend.

Just checked Tire Rack: 01 had 16" and '05 had 17 or 18".

Some 01 had Q speed rating: all 17 and 18" tires are R or S rated in a cursory search.

The other issue with the EB speed limiter may have to do with converter cooling. Di engine technology may not be compatible with the strategy of richening up the mixture at high speeds to fuel cool the converters that normal gasoline engines use. (Based on discussions w/ Ford Engine Development engineers a number of years ago).

 

Why not? You asked for a reason that, more than likely, does not exist.

 

OP sped off after his last post on 10/3/13. This ain't even a sleeping dog--it's a dead horse.