Taller tires to Smaller on TV

[Western Traveler]

Our new acquisition tow vehicle has me scratching my head.
The prior owner changed out the standard 18" King Ranch rims for 17" King Ranch rims. He did not however make up the difference with tire height.
I know this is a common practice for winter driving but I thought it is matched to a taller tire.

He had upgraded the truck and addressed most of the 6.0L issues but due to a change in Job status from travel to permanent location he was selling the truck.

I checked the spare tire when I was inspecting the vehicle for purchase and noticed what appears to be the original spare tire and rim. LT 275/70R18
DOT P515 1VB3

I asked him about the difference and he said he made the change to reduce tire costs for present and future.

He Changed to TOYO LT 285/70R17 E rating (a respected tire manufacturer I think).

Now I know my gauges are off but not sure to what extent. I ran the numbers through a tire size calculator and came up with 1.4" circumference difference and a speedometer difference of 1.1 mph at 70 mph.

I know the speedo reads fast but are the odometer miles inflated or deflated? Are my miles to empty gauge reading more miles to empty than I have?
Did my 3.73 axle go up or down? Up I think.

I got this from from about.com;
"Here’s the basic issue: Your speedometer, odometer, traction control, torque and gearing settings are all based on the distance that your tire travels over one complete revolution, which is determined by the outside diameter of the tire-and-wheel assembly. A tire with a different outside diameter will travel a different distance over that one revolution with a different amount of torque."
Duh...

I plan on matching the spare...

I know I can have a Ford dealer change the computer and probably will but how important is it at about $200 and how does that address the traction control issue?

[Ken / Claudia]

Check the white sticker near the driver door, it's named Federal Standard Sticker. It and or a yellow one nearby will show the tire and rim size that the truck came with along with abunch of other useful information. Even with the right size tires the speedos can be off. The police cars have cerified speedos and we had to log them checked every 6 months and every tire change, They were allowed +/- 2 mph from the true sped vs speedo and many were that much off, both fords and chevs. We would check them at 55/65/75 miles per hour. They were checked with a radar in moving mode that was also checked with its tuning forks. My opinion is if that was my truck, I would not worry about it the error. Besides the dealer there maybe a shop that can fix the speedo to the true speed. You should have a hour meter on the engine that is as good or better regarding engine life.

[hankpage]

If you don't have a GPS, borrow one .... it will show you the speedometer error at all speeds and you can judge from there. But with <1mph error at 60 mph I would not be concerned. This is just my opinion but as long as all 4 tires are the same size I don't see it affecting traction or stability control. JM2¢, Hank

[Western Traveler]

If you don't have a GPS, borrow one .... it will show you the speedometer error at all speeds and you can judge from there. But with <1mph error at 60 mph I would not be concerned. This is just my opinion but as long as all 4 tires are the same size I don't see it affecting traction or stability control. JM2¢, Hank

Even with the right size tires the speedos can be off. They were allowed +/- 2 mph from the true sped vs speedo and many were that much off, both fords and chevs. My opinion is if that was my truck, I would not worry about it the error. Besides the dealer there maybe a shop that can fix the speedo to the true speed. You should have a hour meter on the engine that is as good or better regarding engine life.

Dawn breaks over Marble Head!

@Ken,

We used to calibrate the Highway cars on a treadmill. 80-100 mph standing still (always seemed crazy at some level) and kept the error at the different speeds on a calibration card.

As I now recall it was presumed that The Big Three used to purposely build error into the vehicles to make it look like they got better gas mileage. I never did see an accurate speedometer back then.

@Hank
I paced myself against another vehicle and I was 5 mph off, but now I realize two un-calibrated vehicles would give an even bigger error.
GPS, duh! I have a couple...that will tell me if the error is wide enough to bother with it.

[Ken / Claudia]

I forgot about GPS, we had hand held with dash mounts in pickups and all boats. I found that they worked good for paceing speeders after they were checked against the radar and speedo. I would say to take 3 speed readings like we did. Hold speed at 35 with GPS than read speedo, do that at 45 and 55 if allowed do at 65. It should be off the same at each speed. I found it rare that it would change, say 45/43, 55/52, 65/60. Those vehicles were taken to dealer to fix what ever the problem was.

[LittleJoe]

I would not worry about the tires. 1.? circumference diff is next to nothing.

I would however get same size spare as damage can be done to your LS rear differential if mismatched sizes are used.

the 18" tires are very $ compared to 17's. if you do not need the few extra lb rating for your application.