Speedometer accuracy?

Why does the speedometer read 2 MPH slower than the actual speed I'm traveling? My Tacoma is a 2010, SR5, double cab, long bed, 4x4 with 16" wheels.

 

Mine was off by a little but since I switched to 265/75 it is right on now.

 

Thanks Chief. Just got my Taco in Oct '14. Wanting to increase ground clearance, put larger diameter wheels, add some lift, etc, and someone said I might want to watch out for speedometer accuracy issues.

 

2 MPH is within specs. You have a buddy with a calibrated speed gun or are you measuring the mile markers placed by road workers? Many markers were placed WAYYY before they had GPS.

Might want to check for the .005 mm offset of the door handle. It's a TSB ya know.

 

When you save up to put on lift and tires, budget a couple hundred bucks to get your speedo re calibrated.

 

Or just realize it's a tiny bit off and deal with it and save your money.

 

Ive had 3 GPS's running at the same time in my truck, and they all lined up pretty well with my speedo. I changed to bigger tires and now it seems even more accurate... How are you testing your speed?

 

Thanks to all. I've used GPS units and all read the same while speedo tracks 2 MPH slower than each GPS. Don't know about the TSB.

 

I have stock-sized tires and was noticing the neighborhood "Your speed is" radar guns were reading 3-5mph slower than my speedo. Guess I'll have to grab a GPS app on my phone and test it.

 

No speedometer is perfect because ratios change due to tire wear. Also no two brands of tires are exactly the same size even new, and there are other factors especially if you have an automatic. A plus or minus of 2.5 percent is considered acceptable. If they are accurate with new tires at a certain speed then the same speedometer is not accurate as the tires wear.

So your speedometer is not off 2mph. It may be off in ratio to wheel spin and at a certain speed it may be off 2mph but not at all speeds. So if it is off by 2.5% then at 35mph it is off by .875 mph, and at 65mph it is off by 1.625mph and at 83.98mph it would be off by about 2mph.

If you are a perfectionist and seeking a fix there is none to fix it perfectly. Even a GPS does not give perfect speed readings although it may be closer to reality and the cheapest way to go. Your best bet if you want the best accounting is buy a laser speed gun and mount it on the vehicle.

Personally I would just forget about it as the cost of perfection is VERY expensive, but to each their own.

 

I would bet that every new (non-performance) car and truck sold nowadays has the speedometer purposely set to read a couple MPH fast. The average driver will never notice. But they would sure notice if the error went the other way and they ended up eating a ticky tack ticket when they thought they were only going four or five over the limit.

 

Well within allowable specifications.

 

X2

Every Toyota that I've checked reads higher than actual speed. But they were all "dead on" at 0mph

 

In general, if you put on larger tires, the speedometer will then read lower.

Example: at a given real world speed (actual speed) of 65 mph- Your speedometer reads 67 mph. But put on larger tires... The speedometer will now likely read 65 mph, matching the actual speed.

Example: your new truck speedometer says 67mph with stock 245-75 tires. but you're actual speed is 65. Change to the taller 265-75 tire, and your speedometer speed of 65mph will match the actual speed of 65mph (speedometer is reading lower than before).

Someone please confirm this, as wording on these is touchy. Lots of threads on this topic. Hope this helps.

 

You got it right.

Initially I thought Toyota set all the speedos the same and only the models with the largest tires were accurate. Turns out they all suffer from the same thing regardless. Go +1 on new tires and it will be pretty close.

 

Ok thanks, I can live with actual speed 2 MPH slower than speedometer reading. End of Thread

 

I'm just curious... What size tires are you running? really tall ones?

 

If you do this, be aware your odometer will be off a small amount. Virtually every manufacturer reads 2-3 mph high, and odometer is close to exact. Having said that, ALL vehicles will change accuracy as the tires wear. My OEM tires lost a full inch in diameter before they were down to the wear bars.

 

I wondered about how off the 33s would make speedometer

 

Just use any if the online tire size calculators or comparison tools. The percentage change in circumference (rotations per mile) is your answer.