NeedMoreCowbell
New member
Hey, just curious how all of you who have moved to larger-than-stock diameter tires have recalibrated your odometers and spedometers to account for the difference between tire sizes
OD adjustments
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martini
New member
A bunch of people have used the speedo healer with good results: Speedometer correction - Toyota 120 Platforms Forum
mikey's2010SR5
New member
After I got the tires installed I used the GPS to track how fast I was going compared to the speedo and it was spot on.
Has anyone done this with a 5th gen and documented it? It would help those of us (well, me) who are wiring diagram challenged and don't know exactly which wires to cut/splice/etc.
Mikey, notwithstanding your GPS, I think it is impossible for your OD to be "right on" with 285 tires, unless your OD was "off" in the first place. Different diameter/circumference etc. Here is a link where you can plug in the OEM tire size vs your current tires and get the speedo calculation.
Tire Size Calculator - Compare Tire Sizes
Good luck and be safe.
GrantK
New member
Unless you have a calibrated speedo, it's off by a few mph. All mfgs design their speedos to read a little faster than the car is actually going so they can't be sued for speeding tickets etc, etc.
1engineer
Moderator
Manufacturers design their speedometers to give the operator the exact speed they are travelling with everything stock and new tires. Where did you get your information? Think for a second - what would their liability be if they knowingly manufactured a faulty odometer to misrepresent speed and milage? I am sure it would be a lot more than speeding tickets.
You can only use civilian GPS to get an approximation of your speed.
A stopwatch timed mile run at 60 mph will give you a close speed.
A light activated timer over a mile run will be the best.
Or, you can trust your current speedometer and calculate (or use a web based calculator) the difference and make a small cheat sheet and put on visor. This is usually the best and cheapest solution.
The difference between a worn tire and a new tire the same size will equate to 1-2 mph at 70 mph. The police "give" you some leeway because of this and other factors.
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1engineer
Moderator
Sure, if you think it's worth it. Going from 265's to 285's will result in a 3.5% difference. So if your speedometer reads 60 mph you are actually going 62.1 mph. This is assuming a generic new tire. Everyone knows that all tires of different brands are not the exact same size. At 70 on the speedometer you will actually be going 72.45 mph, assuming again, the generic 285 size. For reference, a worn out 285 and a brand new 265 are very close to the same diameter. Again, you have to make that decision on if it's worth it. BTW the values above are calculated. This Tire size calculator is the calculator I have used for years, since 1997 or so and it's slick, simple with no ads. It assumes again a generic tire. In the past I have gotten tires in one size and after looking at actual dimensions it is much closer to another size, as listed on the calculator.
To the OP - "I've got a fever....and the only thing that can cure it is -"
And another thing, someone is using a picture of the "Great Cornholio" as an avatar and I laugh everytime I see it!
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offroadingatc
New member
FWIW mine was reading a bit fast with stock tires but is basically dead on with duratracs. This is using an app on my iPhone so not sure how accurate that is but its probably pretty close.
My buddies VW golf reads close to 5 kph fast on stock tires, that's the most extreme Ive seen a car from the factory.
My buddies VW golf reads close to 5 kph fast on stock tires, that's the most extreme Ive seen a car from the factory.
Thanks. I like your calculator link since it uses drop down boxes. Make it a bit easier. Did you try the one I posted above? It gives you that "cheat sheet" you spoke about above.
Tire Size Calculator - Compare Tire Sizes
Be safe.
Tire Size Calculator - Compare Tire Sizes
Be safe.
GrantK
New member
Saw in an magazine article back when. Also, I've NEVER been in ANY SUV, car, truck, VAN that didn't read faster than it was going. They are usually just a couple off though some have been as much as 4. You can also Google it to find more of same.
What would be there liability for a speedo that protects drivers from going faster than they really are - NONE.
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1engineer
Moderator
The liability comes from an OEM willingly building something that is used in legal documents (titles) not manufactured to be within spec. If this were so, everyone would use this as their main defense. (your honor, everyone knows speedometers are not right. Mine is just too slow.)Please find me one case where an OEM is quoted to building an inaccurate speedometer. Here is a Wikipedia article quote:
"Most speedometers have tolerances of some ±10%, mainly due to variations in tire diameter.[citation needed] Sources of error due to tire diameter variations are wear, temperature, pressure, vehicle load, and nominal tire size. Vehicle manufacturers usually calibrate speedometers to read high by an amount equal to the average error, to ensure that their speedometers never indicate a lower speed than the actual speed of the vehicle, to ensure they are not liable for drivers violating speed limits."
I buy this except the last part about speeding tickets. Notice the "citation needed" comment. That means it is here-say or an opinion not fact.
In the U.S., manufacturers voluntarily follow the standard set by the Society of Automotive Engineers, J1226.
They do have standards and it is + or - a percentage. Around 2%. There are exceptions for temperature variances and alternator current variations.
Saying things like "every manufacturer does this" without citations is just continuing a myth. To be fair I will say I have been in a number of vehicles myself that seemed high. How did you check your speedometer accuracy?
This is going to start a pizzin match...lets all agree to disagree.
You CANNOT judge you speed by your GPS...it will track land speed based on a single dimension, point A to point B.. the reason this is not accurate is because it can not account for hills and distance traveled on the apex of a turn. It tracks from one geological location to the next.. think of it as drive on a sheet of paper.
you speedometer is more accurate because it calculated revolutions of your actual wheel. there is allot more math involved in calculating accurate ground speed than you can do with a iPhone or a modern civilian GPS. you could calculate over a given section of road, the distance covered down to the inch and divide it by the time traveled and get a good idea, but who is going to do that.
I run 285s and according to the little "caught-ya" speed signs, I am off about 2 mph at 60...but wait there's more...I also have a second 4th gen on 285...and guess what...same thing.
Bottom line.. If you vehicle is sluggish when cold, let it warm up before you drive.......if it is stiff over bumps going fast, slow down for the bump....if your speedo runs a little off slow down to compensate.
:roadkill:
Victor_inox
New member
Not true,every vehicle i owned since 1993 was spot on,gps proven.you are not questioning gps accurasy,do you?none of themhas speedo calibrated,
my motorcycle was the only one with inaccurate speedo,corrected easily
with speedohealer.
NeedMoreCowbell
New member
Back when we used to prerun in the desert, people would get "closer" to their actual speed by regearing. Bigger tires were offset by higher final drive ratio, which also helped maintain acceleration off the line.
offroadingatc
New member
that's how you are proving that they are accurate? it seems like a pretty bad argument for a court case since speedometer accuracy would be an incredibly simple thing to verfiy. I know for a fact that a friends VW was out by like 5% right from the factory. Also I agree with the other poster I dont really think indicating a higher speed than you are traveling would open the companies up to additional liability, in fact I tend to agree that it might protect them from the odd poorly tuned radar gun...
re: GPS accuracy, it depends on what type of GPS you are using and where. They can be incredibly accurate and a lot of them account for altitude changes and turns etc. Regardless if you are using it to verify your speed and do so in a relatively straight line they calculate speed very accurately for the most part. If you think you cannot judge speed with a GPS i suggest never flying again.
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