Speedometer is off but I wonder why?

About a month or so ago, I got pulled over by a local State Trooper for speeding. The speed limit was 65 mph and I had my cruise set at around 68 or 69 mph. He clocked me doing I think it was 73 mph. Since I was going up and down hills, when he clocked me, I thought maybe my truck had just sped up briefly while on cruise. Well, the other day, I was on the freeway and again had the cruise set. The speed limit there was 75 mph and I had my cruise set at 78 mph. However, one of those little road radar sign hit me doing 82 mph. There was road construction ahead and right before the speed limit dropped they had one of those portable machines.

Now for the life of me, I can't figure out how my speedometer got 4 mph off from what it should be? I know about a year ago I got new rims and tires put on the truck. I however kept the stock tire size. However, the one thing I did change was the rim size. I have a TRD Off Road package 4x4 that came stock with 16" wheels but hear on the aftermarket ones I had to go to a 17" wheel so I did that. I know if you go to a bigger tire size, your speedometer will be off but is it the same thing with rims as well?

Also, does anyone have a clue how much it will cost me to get my speedometer reset?
Thanks all.

 

Bigger is bigger, wheel or tire. What size tire did you wrap around the 17s?

 

Ditto. What size tire?

265/70-16 compared to 265/70-17, the 17 is larger by a full inch, and will introduce a speedometer error of 3.3%.

That might be enough to give you a 78-82 discrepancy... would think it would be closer to 78-80... but those radar signs are not calibrated.
Should not be enough for a ticket (unless you have out of state plates ) if you are sticking to the posted limit.

 

The original tires on my truck were 265/70-16. What is on there now is 265/70 -17. Basically, same size tires as stock but 1" bigger rims.

 

What they said. If you did indeed go from a 265/70r16 to a 265/70r17 that is a 4% difference in revs/mile. You are reporting about a 5% diference with 4 mph at about 80 mph. What air pressure do you run? Overinflation for the load on the tire will also change the rev/mile numbers, as well as wear down the center of the tread.

To the best of my knowledge, the speedometer cannot be recalibrated with a simple parameter setting, but there are aftermarket options. For 4 or 5% I would just drive accordingly, but thats me.

Edit, you posted the tire sizes while I was posting, thats your main factor.

 

There ya go.

265/70 reflects the tread width and aspect ratio.
You have the same tread width and aspect ratio, but those tires are on rims that are 1" larger, therefore, the overall diameter of the tire is 1" larger than stock.

http://www.tacomaworld.com/forum/tirecalc.php

Your stock tires were roughly 30.6", your new tires are roughly 31.6"
To match stock diameter, you would need to go with a 265/65-17.

But as I said... the 3% error introduced by the larger tires will have a measurable impact on calculated MPG (20.0mpg calculated is actually 20.6mpg), but as for speed, if you hold your indicated speed to the posted limit, it's not going to result in a ticket...

You're only looking at a 2.12mph error at 65.

Indicated..............................Actual
20 mph (32.19 km/h) = 20.65 mph (33.24 km/h)
25 mph (40.23 km/h) = 25.82 mph (41.55 km/h)
30 mph (48.28 km/h) = 30.98 mph (49.86 km/h)
35 mph (56.33 km/h) = 36.14 mph (58.17 km/h)
40 mph (64.37 km/h) = 41.31 mph (66.48 km/h)
45 mph (72.42 km/h) = 46.47 mph (74.79 km/h)
50 mph (80.47 km/h) = 51.63 mph (83.1 km/h)
55 mph (88.51 km/h) = 56.8 mph (91.41 km/h)
60 mph (96.56 km/h) = 61.96 mph (99.72 km/h)
65 mph (104.61 km/h) = 67.12 mph (108.03 km/h)

 

654 revs/mile on the LT265/70r17 BFG ATs, 680 revs/mile on the P265/70r16 rugged fails, assuming thats what was there, which is a 3.9% change. Diferent brands and tire models vary a bit even for the same size.

 

Cool, thank all for your help. I figured it was due to the larger rims but was unsure. I am now wondering how far off it will be when I put 285/70 r17's on it next? I'll just have to be extra cautious I guess. The State Trooper was also pretty cool when I got pulled over and only gave me a warning so that was a blessing as well.

 

Assuming you pick BFG ATs again, they are 635 rev/mile, so a 7% change from stock. Any other brand will be close.

 

FWIW - If it bugs you, and as mentioned above, there are electronic modules that can be programmed to correct your speedometer calibration. They generally plug inline into the circuit behind the dash.

 

Isn't there a page on his forum that has a tire size calculator?

 

Yes, there is....

vvvvvvvvvvvvvvvvvvv