Speedo off?

[Raudermaster]

I was going through a speedtrap today, my speedo read 30mph, and on the speedtrap it read 28mph. So, it got me thinking. I turned around, went through the same one, but now my speedo was reading 40, and the speedtrap said I was doing 38. What the hell could be causing this? My old thread was asking if I had a tune when it was stock because of the speed limit I was able to reach and the grunt off the line, but then I got a tune from Lidio and definately no tune at all compared to his. It's just irritating I can't find a logical explanation on why my speedo is off 2mph, OR if the speedtrap was messed up. I mean, it's very unlikely the previous owner before me put in gears other than 3.55's. Although, my rear diff cover has black stuff all over it, so I was wondering if it could have been serviced at the Ford Dealer. But if they were replaced, could they have put in smaller, or bigger gears?

[cyclopsram]

The standard deviation from actual speed in most conventional speedometers and/or odometers can range up to 5%. Heck tire wear can cause enough to make the differences you are seeing.. Who is to say that the slow down machine you went thru is not calibrated to read high so you will slow down...in the old mechanical shaft driven speedos, temperature and friction in the shaft would cause big errors. Drive your car a hundred miles on a highway using the milemarkers, nonstop no turns no rest stops. then see how far off your odometer is... A 30 inch tire which wears 1/4 inch on the radius from new to used takes a half inch off the diameter which is almost 2 percent of the new size..and you will think you are going faster than you really are when the tires are worn. In the old MoPar cruisers in the 70s there was a little tag in the glove box which gave the "certified calibration of speedometer head" limits at various speeds and if I remember correctly the police head just barely met the 2 percent error range. In my opinion, you have nothing to worry about. Remember that the radar unit you went thru may or may not have been accurate!!!

[whoskal]

A guardrail can throw a radar gun up to 5mph off.

[Paul T. Casey]

Scared me for a minute, at first I thought some old fat hairy guy at the beach removed his swim trunks. :rolleyes: 2mph off on the roadside speed sign is about right. As stated before, tires, factory error, poor calibration on the radar trap, all could affect the speedometer. Most LEO's realize this error, which is why you will usually not get a ticket for 5 mph over the posted speed.

[SergntMac]

I use a Scangauge (I) to monitor my speed through the EEC/OBD II port. My dash speedo is 2 MPH faster at all speeds than the OBD II data, but I have 255/45-18 tires on the rear.

[Marauderman]

I use a Scangauge (I) to monitor my speed through the EEC/OBD II port. My dash speedo is 2 MPH faster at all speeds than the OBD II data, but I have 255/45-18 tires on the rear.

Gee........just wandering....with the tune of my rear set with my 410's and me using 295/45/18 's on the rear----whata'ya think ???????...I'm told that with the tune set to address the gears its correct----never thought of the different tire size as creating a diff---duh!!! Think there is significant amount with mine Sarge.....seems I'm always "going faster" on the speedo than what it feels.....any comments folks.........Tom..........

[TAF]

.....seems I'm always "going faster" on the speedo than what it feels.....any comments folks.........Tom..........

That's 'cause speed.....
is your middle name....
is your friend....
it's that drug you just can't kick....
it's that feeling you got the first time your Dad let you sit up front and said "Hold On, Son"....
it's what wakes you up, and makes you "smell the coffee"....
it's....it's.....it's....

Ah Tom...just get your car over here to Team Ford and they can calculate it for you on the Dyno.

[KillJoy]

I have heard that our speedo's read 2mph slow intentionally from the factory.

KillJoy

[Hotrauder]

I have heard that our speedo's read 2mph slow intentionally from the factory.

KillJoy

that is to make sure that the UAW guys get to work on time.:rofl: Dennis

[Hotrauder]

Oh, and put your speedo back on! Everyone cleared the pool already. Dennis:lol:

[SergntMac]

Gee........just wandering....with the tune of my rear set with my 410's and me using 295/45/18 's on the rear----whata'ya think ?Tom..........Based on tire size alone, I think you would be within actual speed specs of the OEM tire. 65 MPH on the speedo = 64.64 actual MPH from your new rear tire size. Check for yourself, Tom, here?

http://www.discounttire.com/dtcs/infoTireMath.dos