So, I got my mickey thompson MTX tires for my 4runner today...they're 265/75/r16...my old tires were 265/70/r16...I heard that when you go to a bigger tire size, your speedometer readings will be off?!?!?! if so how off? will it show that I am going faster than i actually am(ex: IF I'm going 40 Im actually going 45)? or slower than I actually am(ex: If Im going 45 im actually going 40)?

 

From the looks of it you've basically gone wider, depending on tire manufacturer the difference will be slim to none. The taller the tire the longer the revolution, making your speedo show a slower speed.

 

The way to check your speedo is to find 2 mile markers along the highway and drive at exactly 60MPH between them and time it. If it takes 1 minuint to go the 1 mile than the speedo is right if not then do it again until you figure out how far off it is and just keep it in mind from then on.

 

will the difference in readings be drastic though???like say +/- 5mph?

 

I would not think it would be much with that tire size change but i don't really know what your new tires size is. Your stock ones were 30" if these are say 31" then it could be a pretty good differnce.

Either way you should just make a quick pass on the highway with a stopwatch and get an idea as to how far off it is.

 

It is possible, but with that small of difference in tire size should not be too bad. My Tacoma came stock with 265/70/16's, I am now running 285/75's and it is off by approximately 5 MPH at 55 on the speedo but I am actually going 60 according to my GPS. It is much worse in my '81 with 33's, 55 MPH indicated it more like 66 mph...

 

--I'm guessing the GPS is a more accurate way to determine speed??

 

It is another way, not more a accurate per se.

The time between the mile markers is the best way to get a good reading as there is nothing that will mess that up other than you mesing up. The GPS can be off by a lot if it is a cheap one or has bad reception but it works.

 

ohhhh....one more thing...isn't there a way to reprogram the speedometer to the new tire size???

 

The GPS works for me. Safer for me since I am not try to see the stop watch and mile post at the same time. Just push reset on the GPS and it shows the speed over any distance or time frame...Guess my GPS would be considered a mid Range...

Try timing it see how far off it is but to answer you since, I think that you can get new gears or speed sensors from Summit Racing or Superlift...

 

with my 33's i would be doing a speedo indicated speed of 60 but doing an actual speed of 72.

btw all commercially available GPS are derated and made less accurate by the military and are only acurate to with in about 1 to 3 yards depending. US milatary owns the GPS satilites.

 

It is easier but as i said they are not always accurate. I have had a few and they were all off by some. The cheapest one was off by almost 10mph and the best one was off by about 2 - 5. They have gotten better though but a cop won't take "the GPS said i was going..." better off knowing exactly how far off it is. but to each there own.

 

With just the width change 75's to 70's if any difference at all the most could be 0.25 per hr + or -. Wider tires are typically slightly smaller in height, so you could be going 64.75 mph when it reads 65. Now if you went taller 265 to 275 then it would be a slower speedometer reading than your actual speed. If your still worried which you certainly shouldn't be, GPS it.

 

I went to this site with a metric-inches tire converter...and it said that 265/70/r16 are 30.6 and 265/75/r16 are 31.6 so my new tires are in fact taller?

 

Yeah that sounds about right. the middle number is not the wigth but the perctage of the hight that it is wide.

 

I'm about to order the speed sensor- recalibrator....before I do....A car shop can't recalibrate it for me?It's kinda hard to let go of $200....

 

A lot of guys just live with the speedo being off. Once you learn how far off it is just keep it in mind. And it depends on the truck, sometime the shops can fix it and sometimes they can't you can ask around. but i would see how far off it is first, if it is only a little then it may not be worth it.

 

They are taller not wider. You got about an inch more in diameter, about 3-4% more travel per rotation (31.6" diameter on the 265/75s vs. 30.6" on the stock tires). You will be a couple of miles faster than the spedometer shows with your new tires (if they are true to size). Not something I would worry about.

edit see others got it correct too.

 

can i get help from other people who increased the tire size of their 4runners?

 

From 265/70/16's to 265/75/16's is a 3.4% speed difference. When the speedometer says 60mph, you're going 62. Also consider that if you're going to more aggressive tires from street/bald tires, you'll probably gain a little more height than just the sizes say.