Speed over-reporting speed with stock tires?

[dieck]

Speedo over-reporting speed with stock tires?

Hey Guys,

I'm upgrading to larger tires next week and was kind of stressing about not being able to change the speedo/odometer so it read correctly.

However, I've tested my reported speed vs actuals with a couple different GPS and both show that the speedo is over-reporting speed by 1-2 MPH at 60-70MPH.

By my calculations this means when I go to a tire a little over 33 inches, I'll actually be spot on from a speedo and odometer perspective. Has anyone else noticed this?

This also means the MPG computer and manual calculations are slightly overstated on stock tires because the mileage is overstated slightly.

 

[STX4Runner]

Hey Guys,

I'm upgrading to larger tires next week and was kind of stressing about not being able to change the speedo/odometer so it read correctly.

However, I've tested my reported speed vs actuals with a couple different GPS and both show that the speedo is over-reporting speed by 1-2 MPH at 60-70MPH.

By my calculations this means when I go to a tire a little over 33 inches, I'll actually be spot on from a speedo and odometer perspective. Has anyone else noticed this?

This also means the MPG computer and manual calculations are slightly overstated on stock tires because the mileage is overstated slightly.

Just went to 275 MT/Rs (32" approximately) and my speedo is now spot on according to those "Your Speed" signs at school zones. If you go to 285s, which are just slightly bigger than 275s it will still be pretty close.

 

[Apex Overland]

ive 285/70's and mine is dead on to GPS and speeding signs.

 

[BlackWorksInc]

In my experience I have found the speed signs to be varying with accuracy.

Most of the automotive world's little secret is that most of the time, speedometers will read 1~2mph faster than what the vehicle is actually doing. There's probably a legitimate reason behind it (help you not get speeding tickets?), but for the most part Toyota says 1~2mph below is acceptable.

 

[STX4Runner]

In my experience I have found the speed signs to be varying with accuracy.

Most of the automotive world's little secret is that most of the time, speedometers will read 1~2mph faster than what the vehicle is actually doing. There's probably a legitimate reason behind it (help you not get speeding tickets?), but for the most part Toyota says 1~2mph below is acceptable.

I know those signs aren't that accurate. But it was still nice to see my speedo and the sign agreeing. Not that it'd be that far off just going to 275s.

 

[dieck]

I'm curious now if the odometer is accurate despite the speedometer being conservative.

In my experience I have found the speed signs to be varying with accuracy.

Most of the automotive world's little secret is that most of the time, speedometers will read 1~2mph faster than what the vehicle is actually doing. There's probably a legitimate reason behind it (help you not get speeding tickets?), but for the most part Toyota says 1~2mph below is acceptable.

 

[High_Country]

OP, I believe you are right on track here. I switched to 255/80/17's and have checked my speedo with two different and accurate hand held GPS units and my speedo is now spot on at 75mph. This leads me to believe the stock configuration was reading fast.
It starts to make me mad when I think about - I was racking up miles quicker (shorter warranty and increased depreciation) and I was getting inflated mpg calculations. I never checked BEFORE switching tires.
Now, in the big scheme of things, let's say there's 6% difference in actual vs shown. That only equates to a 4runner showing 106,000 miles vs 100,000 so it's pretty insignificant in all practicality. But still....

 

[adam2562]

yea it over reports the speed because now that i have bigger tires it's on point! lol

 

[03_4x4Runner]

My 2014 Limited speedometer reads exactly what 2 portable Garmin gps reads with the stock tires on the digital speedometer.

 

[flyinhawaiian01]

I'm curious now if the odometer is accurate despite the speedometer being conservative.

Don't you have a smart phone with a gps on it? You can download free mph apps for both android as well as Iphone, that should tell you if you're within 1-2mph accuracy.

I'd check it at 35 and 75mph, just to make sure its all good!

 

[dieck]

I do. And using that and a second GPS I've confirmed my speedo reads 1-2 mph faster than I'm actually going. My question is is the Odometer also conservative?

Don't you have a smart phone with a gps on it? You can download free mph apps for both android as well as Iphone, that should tell you if you're within 1-2mph accuracy.

I'd check it at 35 and 75mph, just to make sure its all good!

 

[1engineer]

Lol how accurate do you want your speedometer to be? Do you want that accuracy in the morning at 50 degrees or in the afternoon at 80 degrees? How about at 5000 feet or sea level? These things matter guys. The SAE spec calls for a plus/minus of around 2% if I remember correctly. Sure GPS is more accurate but that is not the issue. Truth is depending on tolerances some 4Runners will be more accurate than others in exactly the same conditions. Oh, this is the case with all vehicles and not just the 4Runner. Last is the angle you are looking at your speedometer dial. Forget the digital for a minute. What you see is not exact.

 

[NoDak]

if I remember correctly ... fed regs require the speedometer to read exact or slightly more than actual, but it CANNOT read under actual speed.

also oem's don't want to be held responsible for your tickets for under reporting your speed.

most oem's will game the speedometer to read about 1.6 mph (1 kph) over actual to make sure they are within fed regs and not get dinged by variations from vehicle to vehicle, tire wear variations, etc.

 

[1engineer]

if I remember correctly ... fed regs require the speedometer to read exact or slightly more than actual, but it CANNOT read under actual speed.

also oem's don't want to be held responsible for your tickets for under reporting your speed.

most oem's will game the speedometer to read about 1.6 mph (1 kph) over actual to make sure they are within fed regs and not get dinged by variations from vehicle to vehicle, tire wear variations, etc.

I don't want to pay the $72.00 to get the correct document from SAE and go through it.
Speedometer Scandal! - Feature - Car and Driver

 

[NoDak]

I stand corrected, I must have been thinking about the euro spec that states it must read accurate or higher but never lower.

hence why I always seem to see Toyota's read about 1.6 mph higher.

I know mine reads 74 (digital) but road side indicator and garmin gps show 73.


edited to put in digital ref

 

[3wbdriver]

most oem's will game the speedometer to read about 1.6 mph (1 kph) over actual to make sure they are within fed regs and not get dinged by variations from vehicle to vehicle, tire wear variations, etc.

Wouldn't 1.6 mph be closer to 3 kph? Or 2.57495? :hiding:

 

[NoDak]

lol i put the numbers in backwards. it should have been 1.6 kph and 1mph.

im old, i have an excuse

 

[dieck]

That was an unusually informative article from C&D. Thanks for sharing!

I don't want to pay the $72.00 to get the correct document from SAE and go through it.
Speedometer Scandal! - Feature - Car and Driver

 

[Chris4r]

Where over-reporting mileage makes a difference is when the warranty comes into play, or leased vehicle mileage when turning the vehicle in. I had a leased vehicle some years ago that over stated mileage by about 5% until I had it adjusted. It worked out, because I was halfway through the lease, and the revised mileage under stated by 5%....

 

[wfo9]

Just another data point.

When I have 33s (actual 32.5...) on, my spedo and GPS match perfectly.