Little Confusion Here...

AzDragonLord · February 16, 2011

OK... My 2009 Sierra came with factory 245/70R17 tires, and I've been considering getting a larger tire...say 265/70R17, but I'm beginning to second-guess myself.

My speedo now shows 1-2 MPH slower than actual, as measured by Garmin. Would the larger tire make it read correct or make it worse? How would changing the tire size affect the antilock brakes and stability system calibrations?

Thanx!

stlavsa · February 16, 2011

OK... My 2009 Sierra came with factory 245/70R17 tires, and I've been considering getting a larger tire...say 265/70R17, but I'm beginning to second-guess myself.

My speedo now shows 1-2 MPH slower than actual, as measured by Garmin. Would the larger tire make it read correct or make it worse? How would changing the tire size affect the antilock brakes and stability system calibrations?

Thanx!

IF you went to the 265 your truck will ride at the same speed as your garmin. You can swap up to a 265 no problem. The tire will only be approximately 1.5 centimeters greater in overall diameter, hardly any noticeable difference... You can get away with up to like a 285 width if u want to put a level kit on the truck and it wont make much of a difference in gearing..

AzDragonLord · February 16, 2011

Thanx for setting me straight again!

502ci · February 17, 2011

My speedo now shows 1-2 MPH slower than actual, as measured by Garmin. Would the larger tire make it read correct or make it worse?

If you are actually going 60 mph and your speedo is showing you 59 or 58 mph, putting on that 265/70/17 tire will make your speedometer turn another 2 mph slower. Meaning the speedometer will show around 56 mph when you are actually going 60 mph.

AzDragonLord · February 17, 2011

Hmmm.....according to this Tire Calculator, you're right, the speedo would show about 2MPH slower with the larger size. It's so easy to get this bass-ackwards!

http://www.discounttire.com/dtcs/infoTireMath.dos

kawtoy_77 · February 17, 2011

Yes the speedo on the truck would show slower, but you would actually be going faster than what the speedo says. RPMs at 60mph on your Garmin would be slightly lower. If you got better fuel mileage out of it; that could be a toss-up. Gaining rotational mass (hurting braking, and acceleration) vs. Lower Rpm's (better fuel economy at speed) would be your two factors.

stlavsa · February 17, 2011

Hmmm.....according to this Tire Calculator, you're right, the speedo would show about 2MPH slower with the larger size. It's so easy to get this bass-ackwards!
http://www.discounttire.com/dtcs/infoTireMath.dos

whoops ya it definitely brings the speedo down a couple MPH.. Thats my mistake saying that it will correct it with the 265s. I was somehow tricked myself in to thinking his speedo was reading high because of the smaller diamter tires and that the bigger ones would correct it.... Thats wrong it will definitely make the speedo read even slower by another mph or two.

mrbojangles · February 19, 2011

Does anyone know how accurate or the margin of error, speedo's are in the first place? Mine seems to be approx 10% lower than what my GPS tells me and as far as I know, the tyres are the stock ones.

AzDragonLord · March 8, 2011

OK... My 2009 Sierra came with factory 245/70R17 tires, and I've been considering getting a larger tire...say 265/70R17, but I'm beginning to second-guess myself.

My speedo now shows 1-2 MPH slower than actual, as measured by Garmin. Would the larger tire make it read correct or make it worse? How would changing the tire size affect the antilock brakes and stability system calibrations?

Thanx!

I goofed this up! I MEANT to say the speedo show 1-2 MPH FASTER than actual speed as measured by Garmin. Installing larger tires should lower my speedo since it's turning less RPMs for any given speed...right?

This tire calculator confirms this theory...

http://www.csgnetwork.com/tireinfo4calc.html

stlavsa · March 8, 2011

OK... My 2009 Sierra came with factory 245/70R17 tires, and I've been considering getting a larger tire...say 265/70R17, but I'm beginning to second-guess myself.

My speedo now shows 1-2 MPH slower than actual, as measured by Garmin. Would the larger tire make it read correct or make it worse? How would changing the tire size affect the antilock brakes and stability system calibrations?

Thanx!

I goofed this up! I MEANT to say the speedo show 1-2 MPH FASTER than actual speed as measured by Garmin. Installing larger tires should lower my speedo since it's turning less RPMs for any given speed...right?

This tire calculator confirms this theory...

http://www.csgnetwork.com/tireinfo4calc.html

AHHH i had a feeling that was the case.. which is why i put that the 265s would put your right in the accurate range.. if u went to a 285 you will be over just as much as you were under before..

AzDragonLord · March 9, 2011

AHHH i had a feeling that was the case.. which is why i put that the 265s would put your right in the accurate range.. if u went to a 285 you will be over just as much as you were under before..

Yea...I hate when I have a mental-enema!