Speedo off

[JD_countryboy]

I have a question about my speedo not reading accuratly. When my speedo reads 41mph, i was clocked by a roadside radar at 35 mph. Last Sunday i was drivin down the interstate at 72 mph, a car passed me and i could read its huge digital odometer stating 67 mph. It was passing me slightly, not blowin my doors off. 245/75/16 tires and no mods that i can see. Any ideas what would cause this? VSS bad? as far as milage by the odometer, its off just a bit also. reads a bit high according to mile markers along highway. Thanks

[grancito]

I think that the large difference in MPG that people get is due to speedo error, they are normally made to read a little high so that you don't get speeding tickets, around 2%, but yours is 17% high. My Chev is 17% high but the GMC is only 1% high if it had standard tires. Is the odometer off that much? If not it is a gage problem.

[firetruck]

The only thing I can think, is that the vssb is calibrated for different tires. Go figure.

[grancito]

The calibration can be changed in the ECM, someone posted how to make it, resistor added between two pins, or between connector and pin.

[JD_countryboy]

ya, the odometer will read a bit high also. as compared to the mile markers along I 75. it takes a few miles to see it but it does.

[Sundance]

Just checked mine last night coming home. It is a little over 9% off. Reads 56 MPH and clocks at 60 MPH. Done with stopwatch......

I decided to check the odometer against the mile markers. Over a 30 mile course my odometer only registered a bit over 27 miles. So I have to adjust my MPG figures accordingly. Kinda a relief as I now get a "correct" 18 1/2 MPG when tooling down the road at 60 MPH (speedo showing 56).

[1999GMC]

Mine is off by 4 MPH. A buddy was following me and my speedo said 60 and his was 64. So last couple of trips I've taken the GPS and it is reading 64 when speedo is reading 60.

[Oldsmoke]

A GM tech. recalibrated my speedo by reprogramming the ECM when I changed from 245 to 265 tires. :confused:

[JD_countryboy]

well, unfortunatly glad to see mines not the only one off. Now to fix it cheap. Maybe i'll add taller tires. wanted more ground clearance anyway.

[Sundance]

I could be wrong but won't taller tires make your problem worse??

I just live with my speedo error. Once you have it figured out anyway.

A easy and reliable way to test your speedo is to set your cruise at exactly 60 MPH (1 mile per minute) and use a stopwatch (most cell phones have them) and time yourself using mile markers. If the speedo is right on your stop watch will show one minute. If it is coming in at less than 1 minute then you're going faster than 60 MPH, longer time equals slower.

[Chevylover]

You're right !

taller tires than stock -> speedo shows lower than real speed !

smaller tires than stock -> speedo shows more than the real speed !


Cu,
Sven

[JD_countryboy]

my speedo reads higher than what i'm actually going, so if i add taller tires, i will travel farther per revolution of tire and and should be closer to what speed my speedo reads.

[ChevyCowboy]

ya that should work i just put new tires on my 92 350 gaser. it had 26575r16 i put on 24575R16, my brother is a deputy sheriff and one night i had him clock me with the 265s still on, when my speedo read 25 he clocked me at 29 when i saw 55 he saw 60 and on time i got pulled over my speedo said 80 and the cop said 91 but then again the speedo pegs out up there:D . so any way i put the smaller tires on and i have yet to have him clock me but if i drive with the speedo 5 under the limmit i get passed like an old grama so im fairly convinced that the tires where what messed up my readings

[njdevi11]

If the gears have been changed you will read a different speed. The speed is measured at the t-case/trans output not the wheel hubs.

If you swapped 3.73's for 4.10's your speedo will read 73mph and you will be going 66MPH.

are you the original owner of the truck?

[torque454]

245 75 16 is whats on my truck stock. 265s would have been too tall.

[94K30]

when I got my truck, it had 235/85/16's on it. when I went to NH with those still on it, I checked the odometer against the mile markers over 40 miles, it was only off 4/10ths of a mile. (about 1% under) I just replaced those tires, with 265/75/16's and they are almost same hight , 31.9 inches vs. 32 inches, so I figure that mine is pretty darn close to true......

[JD_countryboy]

Hey thanks for that 3.73/ 4.10 chang in rear end ratio change njdevi11. I am not the origainal owner of the truck, and actaully never gave that a thought. Now that i think of that, if the previous owner did change the rear, wonder about the front? YIKES that would be a broken driveline of some sort waitin to happen. But my front axle isn't engaging at this time, so if it snows or im goin offroad, i just take my other truck. What is the easyiest way to check the actaull rear end ratio, and front, other trhan taking rear diff cover off and counting teeth? Front diff is 2 halves, no cover, whats an easy way to check that gear ratio? Thanks.

[grancito]

Mark the tire, mark the drive shaft, move one tire turn and count the drive shaft turns. To check the front against the back, put it in 4wd and see if it suffers tire scuffing/drive chain wind up, do that on soft ground, tells you if they are different.