speed and milage

A large thread is available on this subject at http://www.poi-factory.com/node/6695

edit: also thread http://www.poi-factory.com/node/17170

It covers the subject ad nauseum !

I've noticed this too and brought it up to my maintenance guy.

He said that it most likely was the tire size, since I'd bought and upgrade package that increased my size from 16 to 17, and from 205s to 215s... makes almost a 5 kph difference on my van.

I've driven by those signs that tell you your speed and the GPS is never more/less than 1 kph difference.

My Tom Tom shows MPH 2 - 3 slower than my speedo also.

Thanks for the link it was very helpfull

Upgrading on a new car, from the factory should not affest this. The speedometer should be corrected by the factory. If you changed after you got the car, is another matter.

Factory installed 205s on my Kia Rondo results in the speedo reading about 5-6 kph high compared to the Garmin. My snows are 215s and the speedo is almost correct. Only about 2 kph error

From what I've been told most car manufactures set their speedometers to read about two MPH fast. I guess the theory is it will help you not to speed. My Impala is right with the GPS and my wife's Pontiac Vibe reads about two MPH slow.

Actually the Fed Regulations on the cars allow the speedometer to be fast, but forbid the speedometer to read "slow" out of the box. Most manufacturers opt to make the speedometer a touch fast to stay in compliance with the Federal Regs.

my Ford Ranger speedometer also reads about 2 mph faster than my nuvi 200. I figured Ford programmed it that way to help keep people from going over the limit. (But when you like to drive 65 mph on almost all open highways, that doesn't really matter, does it?)

This was something I was aware of before... many newer cars/vans/crossovers will read right on whereas trucks, SUVs and larger vehicles will read within 2 mph faster/slower.
I have a 04 Dodge Durango and with the police radar signs showing your speed, those always read 2 mph under what my speedometer did. This was also verified with my new GPS unit. Anything over 35mph is always 2mph off (GPS would show 50 mph, my speedometer would show 52mph).
Strange part is that it doesn't matter if I am doing 40 or 80mph, it is ALWAYS 2 mph off. Normally if there is a difference it is minor or non-existent at slower speeds (under 20mph) and can be 5mph or more difference at faster speeds, say around 75mph...
I know for trucks and SUVs this has been common for at least 10 years.

My GMC Jimmy is 2mph slower than my garmin

A friend has a Morris Minor, with a Toyota transmission and rear end. The speedo is quite confused!

Back in the days when my "new" GPS was the 45XL, we spent part of a morning going up and down Foothill Expressway near work, building up conversion tables, one of which is still attached to the dash, and somewhat faded.