JL8Jeff

My 88 4Runner 3.0 5 speed came stock with 4.10 gears according to the factory tag on the firewall. I'm running 32's and the previous owner had 30's on it when I got it. I've been assuming my speedometer is off around 10%. Today I was heading to the bank at lunch and as I came across a police car with the "Your speed" display I noticed that my speedometer said 34 mph and the police display said 34 mph. I thought, no way with these 32's unless the previous owner changed gears or the speedo gear but he would still have been wrong with his 30's. So I cruised down the next road at 45 mph which was reading 2200 rpm's on the tach. I ran that through a calculator and plugged in the 28" tire, 4.10 gears and 2200 rpm's and it said it should read 44.7 mph! The previous owner documented all the work he ever had done to the truck and I see no mention of gears or speedo calibration. So I'm kind of lost that my truck at 2200 rpm's with 32's and the 4.10 gears is exactly the same speed as it should be with the stock 28" tires. I'll jack it up and check the gear ratio but I'm kind of lost. This also means I'm probably not getting as good of gas mileage since I've been using that 10% correction factor. What are you guys reading on your tach at 45 mph in D or 4th gear which is 1:1? It looks like the previous owner changed gears or had the speedo cable gear changed.

CynicX

those This is your speed radar machines are usually about 3-5mph incorrect. Not only that but if you wanted an exact speed from a radar calcuation you need to be heading directly at the front of it on a collision course. The greater the angle the less it will say you are going.

Not only that but your speedo at 35 mph being 10 percent off is going to read 32.5 mph. Not a very big difference esp since your reading it from a needle that isnt that accurate to begin with considering its 17 years old. No offense about the age of the vehicle I'm just saying things over time arent quite as accurate as they once were.

Another thing is even brand new my speedo is 1-2mph faster then it says. I followed a friend down the highway going on a vacation and if I set my cruise at 60mph and he set his cruise at 60mph I would slowly catch up. I had to set it around 58 mph to pace him. So just by there general nature the speedos are absolutely perfect.

davidhaile

(cough) I'm the designer of the most popular radar unit in those "Your Speed" road signs. They are far more accurate than they need to be (0.01%) with a solid and continuous vehicle speed. It is basically the same unit that is in police cars, and speeding tickets can only be enforced if the readings are proven accurate. There is no "offset" built into road sign readings. This is an Excellent way to calibrate your speedometer! The only problem is they are displaying the strongest return off the oncoming vehicles and that may not be the closest one to the sign. Do it during a time when you are the only vehicle on the road, keep your speed steady for a few seconds, and what is displayed is guaranteed 100% accurate. This is more accurate than going to a speedometer calibration shop. Most signs are pointed straight down the road so angle of approach is not critical.

UKMyers

The error is negligible until you get up around 60mph or so. My Tundra is 10% off and I can't tell until I get to at least 55-60. Get a GPS and head out on the hwy and compare it to get some accurate readings.

RMA

Sorry to hijack, David since you are one of the designers can you give me a hint or clue on how to beat the radar , maybe some kind of inside info only you guys know!!!

UKMyers

Psssss... I know the secret... DONT SPEED!

davidhaile

People ask me all the time. The real truth is that traffic cops use radars 8 hrs/day, 5 days/week, 250+ days a year. If they want to catch you, you don't have a chance. In my opinion, radar detectors are only useful on long stretches of rarely-used highway where it may find a cop over a hill trying to tag someone else. There are too many false signals in any city.

Also - in my own city, even though cops use radar, the radar is only there to confirm the cop's own estimate of your speed. They intentionally do that so the court case is your word vs. theirs with the judge in the middle to know who is telling the truth.

The best advice that I have is for people to realize when the cop comes to your window and asks, "Do you know how fast you were going?" Your answer is part of your record. Don't say "maybe 42" when it could have been 35.

UKMyers

I have a friend who is an ex Highway Patrol and he was saying before they even gave him his gun he had to be able to consistently judge an oncoming cars speed with his eyes and be within 5mph.

To stay on topic have you ran your truck with a GPS yet?