speed odometer difference with larger tires

does anyone know how much my speedo my be off between factory 245/78/16 to a 285/75/16 I get a little worried in 15mph school zone

 

Use the tire calculator.

Frank

 

I think I heard its only 1-2% off, which puts it at the "right" speed. I know my car (and I've read this here as well) it is a few mph above the actual speed.

 

It can be a little off. I use my truck for work and check mileage with my gps and it is always higher than the odometer. That being said i would be careful in 15 mph zones but if you are right at 15 probably fine, just don't speed the little bit over you normally do

 

Use the tire calculator, they'll help some. If you're stepping up from 245's to 285's that's a huge difference and definitely not just a 2% increase...

 

GPS is the most accurate way to tell.

 

My truck came with 245's now with 285's im about 9% off according to GPS

 

I think you heard wrong

 

+1

I went to 275/70/17 and I'm off by about 5.5 miles for every 100 traveled. I found that my speedo was pretty much the same to my nav before the tires. Now when I am going 67 my nav shows 70 which matches up with the tire size calculator.

For $200 you can buy this and it will correct your speedo.

http://www.hypertech.com/products-results.aspx

Or

http://www.4wheelparts.com/Performance-Parts/Speedometer-Odometer-Recalibration-Device.aspx?t_c=74&t_s=33&t_pt=6297&t_pl=2708&t_pn=HYP730117

 

^^ I was just going to recommend that

Great product!

 

Gps is a great idea my wife has a portable in her edge i may have to borrow for a couple days thanks guys

 

^I installed a hypertech unit a couple months ago. Easy install and calibration of the speedometer was not too difficult. First time in six years that my speedometer has been dead on.

 

Have an 09 with scangaugeII. Have tested with the SG, with GPS, and against mile markers on multiple 100 mile + trips. Stock tires 245/75/16.

Speedometer reads 2 mph high (62 on dash = 60 on SG and GPS). Speedometer reads higher than the road-side portable radar trailers by about the same.

Odometer is about 1% low (dash odometer 100 miles, SG, GPS, and mile markers show 101.0 to 101.4 miles).

Word on the street is manufacturers deliberately have speedometer read 1-2 mph high cause one of them (or more) got sued over speeding tickets claiming they were going faster than speedometer indicated.

Folks that install taller tires and get the speedometer more accurate may end up with less accurate odometer.

 

That is good to know I plan on getting one. Where did you purchase?

 

eBay

 

I have that size tires and at 40 my mph isn't even 1 off, maybe like half. That's using GPS and a civic beside me with a digital dash. At 70 its about 2 off. I've had none of the problems that ive read in this thread.

My 95 single cab is a different story though. With those same size tires my speedo is off 7 mph at 40! Haha its terrible