Just thought I'd post this for those that care and for archive searches.

I'm running 265/75 tires - Firestone Destination AT FWIW. I tested the speedometer and odometer with a GPS to see if it's different from the stock 265/70 tires. The GPS is a Garmin Nuvi 760. The 4Runner is a 2002 Limited with an automatic transmission.

As many have noted, the speedometer reads accurately with the 265/75 tires. When it indicates I'm going 62, the GPS reads 62. That means that the stock tires caused the speedometer to read a bit fast. At 60 mph, with the stock tires, I was reading about 3 miles per hour faster than the GPS indicated.

My main interest was the odometer. I used the trip computer on the GPS and the 4Runner with both set to zero. Then I drove several hundred miles to compare readings. The GPS trip computer read about 3.5 percent higher than the 4Runner trip computer. Assuming the trip computer and odometer are the same, that means that if the 4Runner indicates that I drove 1,000 miles, I really drove 1,035 miles.

For those that track gas mileage at fillups, you will be reading a worse mpg than you are really getting. For example, if my 4Runner says I drove 200 miles and I bought 10 gallons of gas, I theoretically got 20 mpg. In fact, I would have really driven 207 miles (3.5 percent more). The resulting gas mileage is really 20.7 mpg. Not a huge difference, but with our low gas mileage, every tenth matters!

I hope this is useful for someone. The results are inline with tire diameter calculations, so the odometer is pretty darn accurate with stock tires.

Have a good 2009 everyone!

MadCityRich

 

thanks for putting the time in to get the numbers. i'm running 265/75, and i was wondering if my mileage was low- i think you've confirmed my suspicions..

 

How do you like the Firestones? I just put that same tire on my GMC Sierra 2500 and like them on that truck. Smooth and very good in rain and snow. Thinking about putting them on my 4runner.

 

I've got 21k on the the Firestone Distination A/T's in 265/70/17 and am also very pleased with them on my 2004 T4R. The taller profile has a rolling diameter 8% more than stock. 70mph on the speedo is actually 73mph on the GPS. They improved my (corrected MPG), fill the tire wells nicely and bumped my clearance at the skid pan by 1 1/2 inches over the stock 265/65/17's.

 

I've got about 15k miles so far. I've driven cross-country, in rain, in snow, and in mud. I like them. Very good hydroplane resistance in heavy water on the highway. They've done pretty well in snow this winter, though I added 100 lbs in the rear, which really improved my snow driving.

The biggest weakness was in mud. Not terrible, and these tires are definitely not MTs. No surprise there. I still didn't get stuck and the treads cleared out pretty well. So far I'm satisfied.

MadCityRich

 

How does one go about correcting your speedo/odo when you put bigger tires on?

 

One of the specifications for my Firestone Destinations was revolutions per mile. I compared that to the revolutions per mile on the stock 65 profile Dunlops and came up with a percentage difference. The 70 profile was has less revolutions per mile due to the larger rolling diameter, in my case it worked out to 8% less so I multiply my miles per gallon figure by 1.08 to correct for the lesser odometer reading. Comparing the GPS distance reading vs the odometer reading is another way to get to the same factor.