my 92 suburban was 5 mph off as well, 245/75/16 tires stock,went to 265/75/16 and it was spot on.next set of tires i put on was 285/75/16, at the time i had the upper hand before the warranty expired and also picked up 3 mpg (per gps miles) on the highway..


i think they do this so we have to buy a new vehicle before it time to buy.

For radar to be close to accurate you must be headed on a path EXACTLY at the gun in all 3 planes so it should read slower then actual speed.
GPS will produce a better accuracy comparison for speedo testing IMO

This is absolutely correct. There are a few things that come in to play, everything from proper calibration to tire and wheel size, even vehicle size as well as proportion or location within whats known as the radar cone are all factors. Beware of the lazer radar, dead on accurate, and can target a single car within a pack...
Also beware they (radar boars) are constantly calibrated and infact in construction and school zones mainly, we will write tix using them, yeah you can put up an argument in court, but who has time for that shhhhhhht.
Hopes this helps.....if you must speed do it Midnights in Miami, I hate writing tix now, lmao.

Thanks guys. Good info on the radar. Makes perfect sense since it's nothing more than a Doppler signature reading, meaning it's non directional (distance only), and thus it traveling in a direction not directly towards or away from the source would yield less than actual travel speed. I'm going to give my GPS a go one of these days to see how it compares.

And the Honda note was interesting as well. I looked it up and you're right Mr3D, they did get busted on that a few years back. WHile I was digging, I found that Nissan was found guilty of doing the same.

most cars are 3-8% over under on speedos on the average and most radar guns are the same. Thats why in most cases you wont get pulled over for doing anything less than 10 over, because you can fight it. Also factor in your wheels, tires etc. all make a difference.

I personally know this is true about cars being off. I had a 06 Monte Carlo SS. I got pulled over and given a ticket doing 83 in a 70. Well I had my cruise set on 79-80. So I got out the GPS and sure as chit the car was reading 3-4 slower than what I was really going. So i take it in to dealer because it was less than a year old and with like 5k miles on it. They tested it and said there is nothing they can do unless its off more than 10% at 55mph or higher. Well I was only like 4-5% off. So come to think of it its because the SS had bigger rims than tha regular Monte. Take in mind my SS had stock tires and rims.

I have cops friends who all say kind of the same thing. Ten and under they arent going to bother unless your in residential or by schools.

The roadside radar signs are not calibrated on a regular basis.