265/70/17

Shouldn’t this size throw off my Speedo?

I used GPS today on highway and it’s spot on.

 

If I recall, the speedo on the 2nd gen is off with stock tire sizes. Going with a 265/75R16 or 265/70R17 actually corrects it.

 

That tire size will produce a 3 percent deviation from stock. Therefore 62mph will read 60, 103 will read 100, IF the speedo was accurate.

3 percent is within the error of a typical speedo from what I've seen, and my truck is in a very similar neighborhood as yours. Mine very slightly over reported on stock tires, it was dead on with 31.5s, and might be under reporting by <1% now that I'm on 32s. I've had old cars that were off by as much as 10 percent, stock, but most modern cars seem to be doing better than <5% these days. Still, that particular gauge isn't exactly a lab grade instrument.

 

I wouldn’t say lab grade, but it is pretty much spot on with how Toyota wants it to be. In stock form, the stock tires read 3.6% fast. Ends up 2-3 mph off at higher speeds like 60+. The odometer however is exactly right and reports exact mileage.

So when you step up by an inch (265/65 to 265/70 for example which is also a 3.6% change), the speedometer all of a sudden becomes exactly right while the odometer is now shorted by the same amount.

with the 255/75s you have, the odometer is shorted by 5% while the speed is slow by around 1.5%. Which at speeds we drive means around 1mph off. When I had the 255s I was showing 79 while going 80.

In other words what you found is exactly right and how mine is too. Some Canadians say theirs isn’t the same as ours, and that the stock tires have the right speed for them. I have no way to confirm or deny that myself, just something I was told and found interesting.

 

It's impossible for manufacturers to have the speedometer exactly correct all the time because your tires get smaller as they wear down. Somewhere around 3 mph is about as close as they can get it. This is more apparent with trucks with all terrain or off road tires than on cars because there is more tread depth when new. Most truck tires start with about 1/2" of tread. When you wear that down to 1/8" your tires are now almost 1" smaller than when new.

Your 265/70/17's are about 1" taller than the stock size. When your current tires are near the end of their usable life they will be about 1" shorter, your speedometer when checked with a GPS will show you about 2-3 mph faster than the GPS then.

It's off by a percentage. At 90 mph you'll be different by about 3 mph. At 60 you'll be different by about 2 mph and below 50 only a bit over 1 mph. Not enough to worry about.

Years ago most trucks and SUV's came from the factory with 28-29" tires. When people upsized to 32-33" tires that was a significant difference that threw speedometers off by a lot. Today most trucks come with 31"-32" tires. Unless you upsize to 35" or larger tires there isn't enough difference to worry about.

Use this website: Your odometer will be off too by about the same percentages.

My son works electrical construction. He travels to job sites for months at a time until that project is finished then moves to another site. A few years ago he asked my why his odometer suddenly changed. For 4 months it had been exactly a 100 mile round trip commute to work. Suddenly it was only 95 miles. I pointed out that he had recently put new tires on his car. Even though he went back to the same size they were enough larger to change his odometer by 5%.

 

Even with brand new stock size tires the speedo starts out reading fast and reads faster as the stock tires wear down.

they could make the speedometer more accurate if they wanted to, I think they leave some room in case anyone wants to upsize their tires a bit and not read slow.

 

This has been my experience as well. Although I never checked with the stock tire size, my speedo is spot on with 265/70/17s

 

No.

Because speedos are set 3mph higher from factory to make people think they are speeding.
Bigger tires corrected that.