look at that image.

my speedo reads about 78 (due to the angle it doesnt look it, but it is). the gps tells me im going 75.2.

i was testing it out using cruise control on solid numbers. when my speedo read 80 on cruise, my gps was reading 77... it was a consistent difference of about 3mph.

which is accurate? why is there a discrepancy?

im about to throw on 265/75 mtrs (next week!) and i want to have some way to guage the difference.

thanks.

 

Trust your GPS. Mine's the reverse: when my speedo registers 70 I am actually doing around 73. Are your tires stock?

 

stock size, yes... 265/70...

 

the speedo's on 3rd gens are about 3-4% fast (speedo reads faster than actual speed). going to 265/75's will increase the tire diameter by 3-4%, and ta-da, your speedo is fixed!

 

I recently switched to the 75's from the 70's and I now concur with the state troopers!!!

I'm not sure what the acceptable accuracy of an analog speedometer should be, but 2.8 mph with respect to a 110 mph full scale is 2.5% which sounds reasonable as far as meter accuarcy goes. Considering all the variables between the road and the meter, accuracy is tough to manage.

 

what gps model is that?

 

Trust your GPS. It's a much more sophisticated system for measuring speed than most speedometers. I found a discrepancy in my GPS and speedometer as well-something like 5 mph off at 60 mph. I was going 55 when it read 60. I fixed that with a part from Tacoma Speedometer & Instrument. It might not seem like a big deal, but I am picky about some things.

 

If my speedometer says I'm going 78, My GPS will say about 90... due to the increased tire size

Of course, my 4runner can only go that fast on flat ground with the wind

 

i know with my truck, its reads like this:

reading (actual)

17 (20)
20 (25)
25 (30)
29 (35)
34 (40)
38 (45)
dont know 50
46 (55)
51 (60)
dont know 65
59 (70)

thats without the gps, just used a chart from interco.com

 

sweet, thanks man.

thats a garmin ique 3200... its a pda/gps.

seriously one of the best things ive ever gotten off ebay. and certainly the best deal too!

 

yeah, like mentioned above, the GPS is way more accurate. your speedo being off could be alomost anything, varying tire sizes, gear ratios, etc.

 

also, as your tires wear (street tires), you'll lose nearly a half-inch in diameter. on a 30" tire, that's nearly 2% difference right there. on a tire w/ deeper treads, you can have a much higher variance since they usually start out at a slightly larger overall diameter and then wear down to an overall smaller diameter when completely worn out.

 

Ditto, the same thing happened to me. I went to 265/75's and my speedo went dead-on, but... my odometer started showing 3% low.

So, check the odo with the GPS as well once you get the new tires on.

 

Look at the original specs for the 3rd gen 4 runner you will see that they spec'ed the bottom of the line with 245/70/16s. I did the math on one of the online tire calculators and wow it is exactly what my speedo is off. I have the 265/70/16s also. So for some reason Toyota upgraded the tires from 245s to 265s and left the speedo alone (cheaper).

SLG

 

bump! andy was right, adding the 265/75s has my speedometer 100% right now

 

Check your odometer as well. I'd bet that it reads 3% short.

 

good call, i will check it out. thanks.

 

Minor hi-jack; does the GPS care if you are going in a straight line, or does it take readings often enough that is can judge speed on say, a curvy road?

 

it updates every other second or so on mine.