I have a 2006 H3 Adventure model. When I got new tires for it, I pretty much left it to the shop I worked next to to come up with a replacement set that fit my budget. Over the past few weeks, I couldn't help but notice when I'm going the speed limit and I run in to one of those speed warning signs that show your actual speed and compared to my speed, it always showed I was going a little faster on my speedo.

I was doing a brake job on it and I noticed that the shop had put on 265's rather than 285's that should've been on it. Too late to exchange them now but would that size difference throw off the speedo? And for those who go to 315's are y'all experiencing the same ?

 

Smaller diameter also adds extra miles to the odometer.
Compare sizes here:::>>>Tire Calculator

 

Anny change in tires size can make your speedo read incorrectly. Been like that since speedos and tires were invented.

 

Best to go bigger, reduces stress on the odo. When I purchased my '98 Blazer(2000) it had 205's on it (4wdZR2)? Took it to my shop and installed 31/10.5's. Daily driver and took many multi states road vacations with the family. I sold it in '12 with under 50k miles on it, NEVER tampered with the odometer. The buyer was impressed after a 1hr TEST drive, gave me well over KBB/NADA.

 

Yeah tyre size, especially diameter, makes a difference all right. I was a little bit annoyed when I realised my H3 has 265/65/17's on it. These are actually smaller diameter than the standard 265/75/16's, but I just don't have the cash right now to go bigger, and the current tyres are nearly new so hard to justify the expense anyway. Although with unleaded pushing $2 per litre (around AUD7.80 per US gallon) it might be worth doing. Interested (and a little envious) to see the size difference for Hummerz old Blazer above - state legislation here in Queensland says we can't go any bigger than 26mm over original diameter, and this is enforced so not only would I be voiding my insurance if I put some decent sized tyres on it but I would likely cop a fine as well. The joys of living in an over-regulated country .

 

Smaller tire diameter, affects the speedometer and odometer, NOT fuel economy.

 

Great story, but I find the "facts" to be askew.

If that is a photo of the 98 Blazer you speak of it IS a Zr2 and came from the factory with 31X10.5/15LT.
So you putting on 31/10.5's as you stated would have done nothing to the speedo or odometer because that is what it was set up for stock.

Even if it wasn't a ZR2, ALL 4x4 98 Blazers came with P235/70R15 tires. Significantly larger than the 205s you posted.

 

you forgot gearing, with a smaller tire he will pushing higher highway rpms for a given speed, but he will get better acceleration off the line with less throttle. Depending on his city/highway split he may be better or worse (better if mostly city/worse for highway)

 

2dr came with 205's too. I purchased a dealer demo with low miles, my buddy worked there and let me know it was available to me cheap($15k/msrp$25k). Dealer swapped tires for demo(oem 235's)
You are correct about size to speedometer/odometer programmed, except the fact that if I kept driving on the tires that I purchased it with, the odometer reading would have been higher when I sold it. I bought a 2dr/4wd/off road suspension/emerald green blazer as is, since price was low and I worked at a tire shop(installed new tires, sold the used tires)
Still a huge variance from stock to 31's:

 

Good point: Larger tires decrease your fuel economy because they are heavier, while smaller tires increase fuel efficiency. Bigger tires also have a higher rolling resistance than smaller tires which means they require more resistance and effort to get them rolling. However, if you do a lot of freeway driving at high speeds, having larger tires can help increase your vehicle’s fuel efficiency.