I have lifted my truck 6" with 35's and have not reprogramed the computer at all or new gears. But everytime i drive past one of those speed limit signs that displays how fast you are going it is exactly what my speedometer reads no matter what speed it is at. I used the calculator on 4lo.com and it calculated that with an actual speed of 60 my speedometer should be around 53 or so. What gives are our speedometers off that much from the factory. This has happened with four of the signs so I think they should be pretty accurate. I also been watching my speed better since I lifted the truck and noticed that I do closer to the speed limit to stay with the flow of traffic. People here don't drive that fast over the speed limit liike they do on the mainland so that is another reason I think the speed detectors are pretty accurate. And if our speedometer are off that much would that throw off our odometers also, and if my odometer is correct for the 35's I am getting worse gas milage than I thought.

 

I have a handheld GPS which I used to check my speedometer, and I found it accurate to within +-1 MPH. However, I agree that your actual speed should be higher than your indicated speed given that everything worked properly prior to your lift/larger tires.

Prior to your lift and larger tires being installed, did you have any indication that your speedometer was inaccurate? Alternatively, if you didn't install the lift and tires yourself, is there a chance your installer corrected the speedometer for you?

C-17 Pilot

 

I did all the work myself with a couple of friends. I don't know if was accurate before the lift or not.

 

I agree I also use a laptop with Streets and Trips 2005 W/GPS locator and my speedometer is always 2 mph ahead than what the GPS saids.

 

Okay, let's play along at home:

Old tires (OEM) = 31.9" Diameter
New tires = 33" Diameter

Circumference (the distance the tire will roll in one revolution) = π*D
Circumference old = 100.2168"
Circumference new = 103.673"

The difference between old and new = 3.455752"
The ratio difference = difference/old circumference = 3.455752/100.2168 = .03448

The percent difference = ratio difference*100 = 3.448%

That is the amount that your speedo should be off by.

3.448% of 40 mph = 1.3792 or the speedo will read 1.4 mph slow at 40mph.

 

So if the speedometer is off then so is the odometer? How would someone correct this?

 

my speedometer is off by 3mph...when the speed limit is 75, i need to go 78 per the truck dial to match... gps verified... i run my 35's at 595 rev/mile

 

Most of the time the ANALOG speedometer (what you look at) is 1-3mph faster than the digital PID speedomter of the truck. So you can't really rely on the analog gauges. If gearing or tire changes have been made.

 

When I lifted my truck I was off by 5+MPH. Used the Diablo Tuner and I am still off my gauge by 2mph but the radar and being paced by friend is always right on with what I am actually doing. I need to get in and readjust everything.

 

if you want to find out your digital speed before you start your truck press and hold in the trip reset button.. while holding this in turn your key to the on position your guages should all max out now start up your truck and use the reset button to scroll threw your menu this will tell you everything mph, rpms and all.