i just upsized from a 31" tire to a 33" tire. anyone know how bad this will throw off my speedometer? or is there a way i can calculate it myself?

 

Use this to put in your full size of each tire

Tire Size Calc

 

a mile or 2 at the most..increases with speed

 

It should be about 3 mph difference.

 

*not trying to hijack* but i think, hopefully, the op would like the answer to this also,
Is the speed reference taken before the differentials? Would changing the gears "fix" the spedo?

 

The OP didn't give any info on his truck, so we can't answer how to fix his speedo calibration. If he has a 99 or newer F-150, he can recalibrate with one of the aftermarket computer tuners.
In theory, you could recalibrate the speedo by changing the gears in the differentials but that would depend on what gears are available. The gears he needs may not be available for his truck.

 

the dealer can also recalibrate your computer, or you can just use a hand held gps and take ride and see what the difference is at various different speeds.

 

i had 255/70/16's on my truck and put 295/75/16's and at 70 mph its about 11mph off....yes ELEVEN.....if i put cruise on 64 its actually doing 75.....my Garmin GPS says about 77 but its a little off i think

 

people that say its off by X mph are not giving accurate description.

its would be off by a percentage. If your 31" tires are factory, then the 33s would be 93% of the 31s. so if you are actually doing 20, the display would show 20x.93=18.6 If you're doing 75 the gauge would read 75x.93=69.75. So at 20 its off by 1mph, and at 75 its off by 5.

So its really inaccurate to say "its off by 2mph"

 

Yeah its definitely going to increase with speed but at the only place it really matters, (highway), mine is off by atleast ten.... around town doing 30-45 its off anywhere from 4-6mph but thats not a huge deal cuz you dont really look at ur speedo to much around town anyway

 

Mines around 16% off which isn't too big of a deal around where I live I just go by what my speedo says and haven't got been stopped.

 

Thanks for the link!

 

Example, going from 265/70/17 to 285/70/17 Goodyears actually brought my speedo closer to the actual mph, speedo says 69, gps says 70. That's going basically what you're asking, 31 to a 33 inch tire.

 

I'm in need of some information as well. I have a 2004 f150.. I had like 265s (31s)on my truck. I now have 315s (35s) my father said it should only be a few miles off the speedometer. but yesterday I got pulled over on my road (25mph) in which i was doing 30. Cop says I was doing 47. Now going that slow isnt going to be a 17mph difference. Is the cop full of ****. Or do I need to do somthing.

 

Divide the new tire diameter by the old size diameter and multiply this fraction by the old speedometer reading at a given speed and that will be the new speed. A GPS is good for checking real speed. Or you can time the vehicle between mile markers on the interstate. A tuner will allow you to adjust the tire diameter in the computer and correct the speedometer.

So, 35/31 = 1.129
1.129 x 30 = 33.87

So, if your speedometer was correct before, you were going about 34 mph. How did the cop clock your speed? It's impossible to accurately judge speed by just following someone unless it's done over a very long distance.