Speedo acceptable loss/gains

[Deviate]

Back in my days of building mini trucks and dragging frame, the goal was always as big a wheel/ tire you could tuck. This of course came with complete disegard for how close the speedo stayed to correct.
Now that being said, here is my question. What do you feel i an acceptable gain or loss on speedometer accuracy? There is always a percentage of change when you up size or down size your wheels. What do you think is acceptable for you? 3mph different than what the speedo says, slower or faster, for example? Or no way it needs to 99% accurate? Cause i dont believe that they're 100% accurate.
We've all tried to get the high score on the randomly placed radar game trailers, admit it.

 

[MerlintheMad]

I was okay with one to three MPH variance with reality, until this car: now I want 99%. Still, after over three years, I already feel like I'm moving c. 20 MPH slower on the freeway than I am actually going. I don't need any further inaccuracy coupled to that.

 

[Deviate]

Not feeling the speed of the car does prove to be an issue at times. "Oh look im doing 100...again" tends to be my almost daily issue.

 

[Ozstung]

From what ive been told down here the speedometers are calibrated 2-3km ph below true speed (@ 100kmph.) Its to err on the side of safety.

Thats why are GPS speed never seems to match the speedos.

 

[ShannonC]

Latest nav software in Aus displays GPS speed. When compared to speedo/hud display it's about 3-4% less than indicated. I tend to rely on gps rather than speedo/hud.

 

[Aarvix]

My tires are around 4% larger circumference than stock and I'm ok with that much speedo error.

 

[Deviate]

My tires are around 4% larger circumference than stock and I'm ok with that much speedo error.

I know for me 2/3 mph error isn't the end of the world by any means. At one point i was 10+ faster on the speedo in one of my trucks back in the day.

 

[vipeboy2000]

bmw's are the worst for having optimistic speedo's.. I remember reading a technical bulletin for bmw stating that up to 10% faster indicated speed is acceptable but only in the direction of reading faster than actual. I can for sure verify, our 525iT at 80mph indicated speedometer was reading 72-73 on gps, our 540iT 80mph indicated would say 75 on gps. Our old legacy outback 80 mph indicated 77 gps. Our hummer however at 80mph is 79 on gps. The suburban I used to have was just about dead on until 85 where it read 84.. however below that, it was accurate enough where speedo 80 was gps 80..
Our stinger is pretty accurate at 80mph indicated 78 mph on gps.

 

[JSolo]

According to my garmin (driveluxe 50 with most current firmware, says accurate to 10ft), the car's speedo, cluster, and gps nav display shows speed 1.x mph too fast. That is, at 60 on those 3, the garmin reads about 59ish. At 70, it's closer to a 2mph discrepancy (with car reading higher than actual gps speed).

Now mind you, im running 245/40/18 vs 225/45/18 stock, but in terms of rev's per mile, they're practically identical. OE was 804, current is 803 revs/mile.

Did another trip out into the country do put some more miles on the motor.

Car's trip computer
248.3 miles

GPS
246.7 miles

Difference
1.6 miles or .65% of the gps distance

Is this anything to loose sleep over? Not for me. It just means at 50K miles, the car will actually have ~325 miles less. Error is marginal IMO

 

[RM7]

I'm going to be 3.5% off when I switch to my winter tires and wheels. I'll probably just make a chart in 10mph increments and tape it somewhere for winter.

 

[vipeboy2000]

No, as long as the speedo reads faster than actual.. bmws acceptable threshold is up to ~~7% off or so erring in the side of displaying faster than actual. It's all over the place, our 525 reads 80mph when gps says 73mph, 540 says 80mph gps says 76, friends 340i reads 80 mph gps says 75mph... just like the article states , our suburban was almost perfectly accurate with gps at 75mph.. maybe .6mph faster than gps at those speeds .
Our Subaru outback reads almost perfect with gps now with 1 inch oversized tires .. before , it also was almost 7mph off at 80mph.
All factory tire sizes .
Speedometer Scandal!

 

[JSolo]

Thanks for the feedback and article. Rings true with the honda for sure. The accord was spot on compared to the gps. Same for the g37. Maybe I should of went with 245/45/18 to compensate.

 

[AusStinger]

I'm running 275 35 19 tires on the back which brings me into exact alignment with actual speed.

 

[RogueIV]

Mine should be reading a couple mph faster with my tire size but instead it seems to be pretty consistent with GPS. As long as I know how far it's off by it doesn't really bother me though but within 10mph is ideal.