Why is my speedometer not accurate?

I have the hypertech speedometer recalibrater installed in my truck. Seems to work. But I have a bigger issue with mine where it will be off by as much as 10 mph sometimes and other times be dead on according to the gps. That seems to mostly happen when I'm putting my foot into it. If I'm spinning my tires or doing donuts in the snow sometimes it will say I'm doing 15 mph when I'm stopped and will continue to be off by 15 mph till I shut it off and let it rest. Any ideas on that?

 

If you are spinning your tires or doing donuts then your wheels and drivetrain are turning even if you're not moving and the speedo is sensing the drivetraing turning.

 

I just checked mine this weekend with a gps, and at 60 indicated I was actually going 58 per the gps. I can live with the 2 mph difference, especially since the speedo is reading fast. This is in a 2010 TRD sport, so it has the 17 in wheels. If it was off by 5 or 6, that would be a little too much.

 

I have a 2009 Taco,access cab an when I was going down the road the police have a portable radar that tells you your speed. I held my truck at 65 kms and the radar read 62 kms.

later tig1

 

Yes I realize that. But it even happens when im not spinning out.

 

2011 Tacoma dbl cab gps 62 speedo 65. Dealer says this is normal on all Toyota's. Frist car-truck i had that did not match gps. Stock 16" Tires

 

That's what I plan to do next time I need tires so it good to know that the 215 to 235 will correct the error since I am also 5% low on my odometer/speedometer.

 

I'm bringing this back. After getting my Duratracs put on a few months ago I'm consistently getting shitty mileage now. I don't have a lift so mine are the largest you can put on the stock ride height, nothing extreme. My speedometer reads DEAD ON now that I have the larger tires. I'm also getting roughly 300miles to a tank now and about 3-4mpg less than before. My mileage readings have been quite consistent regardless of how much I focus on driving conservatively.

This confirms that larger tires on your truck will in fact slow down the rate at which your odometer reads miles.

 

My speedo on my '08 Access Cab reads 70 when my GPS says 65. I know the variation, and just cruise along at "70".

 

My mileage went to shit when i installed my duratracs also. I went from 18-18.5 MPG to 16 MPG now. I never checked my speedometer before i installed them. Will try it today.

 

I just checked my odometer against a gps and my odometer is dead on now with my 265/75-16 duratracs installed. I guess my MPG's now are acccurate compared to before.

 

I have been getting these exact results with the same size Duratracs. I thought the tread pattern was killing mileage. I didn't think that it's actually more accurate now.

PS some of you guys are thinking tire width will fix the speedo (the higher number: 2XX). Its the combination of the wheel size, width, and ratio that give you diameter.

 

Here is where it gets contradictory & bizarre.......265/70/16s, GPS says 70 and speedo says 67-68 BUT........the odo/trip matches the mile markers exactly on different frwys for 10-15 mile spans around town. I have done this on 4-5 interstate & state hwys around my area with same results. I mean they are never more than a tenth +/- off from mile to mile and avg out dead on for total distance. WTF do you adjust? I'm OK with the speedo being in my favor a couple or few mph for not getting a ticket & as long as the odo/trip are correct then my mpg calculations are correct. Just weird shit that speedo can be off a few but odo isn't. Seems both would be affected like in my old truck that had mechanical vs. electronic speedo/odo.