mkgarrison5

hey guys few questions. i just had some 265/75/16's firestone AT's put on my truck. the previous ones were 245/75/16's which are 1 inch shorter and 1 inch thinner than the 265/75's.. my question is how does this affect my speedometer? also gas mileage? i can tell i am taking a small hit on gas mileage.. let me know what you guys think? btw the tires are great! much much much better than the BF's i had on before

tc

Well, for the same indicated speed on the speedo, you are going faster than you were before. For the same indicated distance on the odometer, you are going farther than before. THIS DOES NOT INDICATE THAT YOUR SPEEDO IS WRONG.

IMHO, the speedos are off by enough from the factory, that a (relatively) small change like this may make it MORE accurate. The only way to know is GPS or stopwatch between mile markers.

People are going to post up with all kinds of calculations about the tire circumference yadyada. Waste of time, unless you want to compare the mileage with the new tires against the old. And REALLY to be accurate enough there, you really need the actual measurements of your old tires, which you probably don't have. The whole system is fraught with so much error that it's kinda pointless to use the numbers for anything more than a passing curiousity.

mkgarrison5

you know i felt like i was going faster. esp on the highway going 70mph compared to my old crap tires.. so in reality i may be going 73mph now though it says i am going 70 right?? about gas mileage, it will drop a litte bc its a larger heavier tire correct?

4Crawler

iTrader: (3)

On-line calculator to figure out the changes:
http://www.4crawler.com/4x4/4LoCalc.shtml

If you have not corrected the speedo/odo for the change, your odo will read slow so you'll need to adjust MPG for actual miles traveled. And since your speedo is reading slow, you may end up driving faster which also kills MPG, so account for that as well. Whether you end up with better, worse or the same MPG with the tire change will depend on many factors, so hard to say what yours will do.

mkgarrison5

it says i am 2.7 mph over at 70mph interesting

drguitarum2005

2.7 miles per hour at 70 is nothing, its miniscule at city speeds and whatnot so i'd say you're fine. i'm 6 mph off at 70 and ive never had any speeding problems or anything. down at city speeds (30), im still close enough to 30 that its ok

flyfishexpert

I recently replaced my 245/75R16 Cooper Discoverer ATRs with 265/75R16 Cooper Discoverer S/Ts. At 75mph and the 245's I my speedometer was reading 1-2mph slower than I was actually traveling - verified using gps on several occasions. Now with the 265's I run about 1mph faster than what my speedometer reads - again verified using gps. I can not tell you about the change in mpg as I have only had them for less than a week, and thanks to carpooling, I haven’t filled up yet. I will post some numbers later on.

tc

Note:

That means you are going 2.7 MPH faster than you were with the old tires when the speedo said 70. Whether you are truly going 65, 70, or 75 you don't know - and yes, there can be that much error in your speedo.

mkgarrison5

well from what little i know, when i pass those police speed checks (the ones mounted to a trailor) that post your speed when going by my speedo was dead on when i had the old tires on..so i ASSume this 2.7 or so more mph is close to being right.. again i am ASSuming... either way thanks man, its good input to know