Speedometer Accuracy

[gawettla]

I just got a gps and have discovered that when running my stock 235/70/16 tires, my speedometer reads about 3% high. When it says I am going 50, I am really only going 48.5.

This makes my gas mileage even worse. My EVIC already reads about 7% higher MPG than I really get. So taking the speedometer error into account my EVIC is off by 10%.

Has anyone else noticed this.

When I run my MT tires 247/75/16 the speedometer reads about 1 or 2% lower than actual speed. I guess that is a good reason to run the MT's all the time.

 

[4Factor]

I just got a gps and have discovered that when running my stock 235/70/16 tires, my speedometer reads about 3% high. When it says I am going 50, I am really only going 48.5.

This makes my gas mileage even worse. My EVIC already reads about 7% higher MPG than I really get. So taking the speedometer error into account my EVIC is off by 10%.

Has anyone else noticed this.

When I run my MT tires 247/75/16 the speedometer reads about 1 or 2% lower than actual speed. I guess that is a good reason to run the MT's all the time.

You're assuming that your GPS is 100% perfect? It's likely that neither are absolutely perfect. I'd be more tempted to go with your speedometer, the GPS tracking (and therefore speed measurement) can be affected too easily if you don't have a great view of LOTS of satellites. Notice the degree of error displayed right there on the screen. That alone tells you that it's not 100% perfect.

 

[gawettla]

You could be right about the GPS. I am not an expert in GPS or speedometers for that matter, but I don't think bad satelite coverage would show as consistent lower than actual speeds. Would it not show up as inconsistent speed readings.

There must be some others with GPS's that might provide some input.

Maybe my speedometer really is reading high.

 

[farwellbooth]

This link is from a gps forum and someone (for what it's worth) says it's accurate within .1 mph. Maybe not that accurate but definitely more accurate than auto speedo. I'm usually communication with 5-8 satellites. http://www.pocketgpsworld.co.uk/modules.php?name=Forums&file=viewtopic&t=13472 I use this same program and also use gps while flying. You really have to watch the lag ie you change your heading and/or speed and the receiver takes a little while to update.

 

[Bennett]

I'm gonna have to argue that GPS by far is more accurate than a speedometer. Actually most cars these days are off by +- 3 mph, weird, but it's true. I read it in a magazine a while back. Actually, my friend has an escape and he says his speedometer is slow by 3 mph with stock tires.

Bennett

 

[CoasterG]

Next time you go into your Jeep dealer for service have them adjust the computer to compensate for the tires - I had 235s put on and it took the tech about 3 minutes to do, no charge since I was having other service done.

He also ran thru the other basic computer adjustment options but nothing else fun to play with from what he read off.

George

 

[grogiefrog]

Mine is also about 3 high I have noticed. I would rather have it 3 high then 3 low!

 

[Paul M]

O.P. is correct...our Liberty with stock 225's ran at least 3 mph low. I checked this with mile markers on the flat over distance, speed cameras, and speed signs, both around town and at highway speeds. Despite what some have said, GPS is extremely accurate...if I use my iPhone it'll be more accurate than a stopwatch using mile markers. When I installed 245/75's (about 1" larger in diameter over stock tires) I checked all these same indicators and found I was dead on with speed versus speedometer. Turns out for years I've been running slow and didn't know it. My wife's company vehicle, a Ford Escape, was the same, about 3 mph under. Considering why this might be happening on more than one vehicle I merely left it up to the "I don't trust the Government" axiom. That part of me says the inaccurate speedo is on purpose to make folks think they are driving faster than they really are...sort of a subversive plot to keep us all driving slower or to artificially elevate MPG stats. :disgust:

 

[rkkj]

Speedos may be +-, but some people usually do 20%+ over posted limit anyway! I've got an aftermarket EVIC type unit and there's about a 3-5% difference in the speed reading (and I assume in all other readings). Unless you're doing 200mph I wouldn't worry about the slight variation.

 

[Paul M]

Speedos may be +-, but some people usually do 20%+ over posted limit anyway! I've got an aftermarket EVIC type unit and there's about a 3-5% difference in the speed reading (and I assume in all other readings). Unless you're doing 200mph I wouldn't worry about the slight variation.

My point exactly...in our modern tech era having a vehicle drive at the rate indicated on the speedo is childs play, even when the tire diameter changes due to tread wear (reduces) or when a new set is put on (increases). The ability to adjust the speedo "on the fly" with sensor inputs is cake because there is more computing power on board than in the first Lunar Lander. Granted, most folks drive at 5mph over the posted limit, but with all these speedos off by 3 or more mph they are actually driving closer to the posted limt than they think.

 

[LaneKJ]

When you run bigger tires, it slows your truck down. And since the speedometer is programmed to your stock wheels, it reads differently and throws it off. This is the case with my dad's F150.

 

[Ry' N Jen]

O.P. is correct...our Liberty with stock 225's ran at least 3 mph low. I checked this with mile markers on the flat over distance, speed cameras, and speed signs, both around town and at highway speeds. Despite what some have said, GPS is extremely accurate...if I use my iPhone it'll be more accurate than a stopwatch using mile markers. When I installed 245/75's (about 1" larger in diameter over stock tires) I checked all these same indicators and found I was dead on with speed versus speedometer. Turns out for years I've been running slow and didn't know it. My wife's company vehicle, a Ford Escape, was the same, about 3 mph under. Considering why this might be happening on more than one vehicle I merely left it up to the "I don't trust the Government" axiom. That part of me says the inaccurate speedo is on purpose to make folks think they are driving faster than they really are...sort of a subversive plot to keep us all driving slower or to artificially elevate MPG stats. :disgust:

They are not that extremely accurate.
Just recently I have litterally had four different makes of GPS in my vehicle and each one has read out a different KMH and MPH!
Tom Tom
Magellan
Garmin
iPhone App.

 

[sota]

i've verified mine is off by what looks like 10% fast.
I've used those radar sign boards, had various people pace me, even convinced a cop to radar me! each time I'm just about 10% fast. I seriously wonder if the prior owner was running a different than stock size tire and had the system recalibrated to match. 225/75R16 is the current size.

 

[sota]

I even have math to back up the theory it's been changed...

assume I'm exactly 10% off for the moment.
current size is 225/75R16.
manufacturer says that tire has a 29.3" diameter.
since the 10% error is related to the circumference of the tire, to find the error in diameter we'll divide the error by pi. giving us a rough change in diameter of 3.183%
if we multiply the current diameter by 1.03183 we get 30.232", which is pretty darn close to two common sizes...
235/75R16 tires are about 30" (goodyear says 29.8" for the silent armors)
245/75R16 tires are also about 30" (goodyear says 30.5" also for the silent armors)

now given the slight error I introduced by using only the known current tire diameter instead of the unknown (but determinable with a lot more algebra!) diameter, I'd say there's a pretty good chance my jeep was once wearing one of those 2 sizes of tire. since I plan on changing to the 235 size later on anyways (not to mention new springs) I'll live with the fact I need to do an indicated 72 or so just to be doing a 65mph limit.

 

[Paul M]

They are not that extremely accurate.
Just recently I have litterally had four different makes of GPS in my vehicle and each one has read out a different KMH and MPH!
Tom Tom
Magellan
Garmin
iPhone App.

Might depend on where you are or distance traveled. Further distance adds accuracy (versus going just one mile, say.) Suffice it to say location services is pretty accurate. Regardless, I use a five mile marker run on a highway at a constant indicated speed, then time it using my iPhone stopwatch. Works to within +/-1/2mph accuracy each time I've tried it. Our Jeep ran slow 5mph from the factory, now it runs dead on. Not sure what this does to the mileage ticker. Of course, none of this is important on backroads...who cares, right?

 

[Cardhu]

Might depend on where you are or distance traveled. Further distance adds accuracy (versus going just one mile, say.) Suffice it to say location services is pretty accurate. Regardless, I use a five mile marker run on a highway at a constant indicated speed, then time it using my iPhone stopwatch. Works to within +/-1/2mph accuracy each time I've tried it. Our Jeep ran slow 5mph from the factory, now it runs dead on. Not sure what this does to the mileage ticker. Of course, none of this is important on backroads...who cares, right?

Mine was also near 8 kph off from factory. 245 75 16 fixed it pretty close ie less then 2kph according to my garmin. i got more faith in my garmin then any app. considering it was optomistic for last 5years its great. hard to ge used to it working correctly.

 

[Ry' N Jen]

When you run bigger tires, it slows your truck down. And since the speedometer is programmed to your stock wheels, it reads differently and throws it off. This is the case with my dad's F150.

That's why it's important to get the pinion reset done at a dealership.
Takes a Chrysler mechanic all of five minutes to do with their Snap on
computer.
Re-calibrates the speedometer, the transmission doesn't hunt for gears
anymore, and the transmission doesn't have to work as hard.
Makes a huge difference.

 

[Paul M]

That's why it's important to get the pinion reset done at a dealership.
Takes a Chrysler mechanic all of five minutes to do with their Snap on
computer.
Re-calibrates the speedometer, the transmission doesn't hunt for gears
anymore, and the transmission doesn't have to work as hard.
Makes a huge difference.

I read somewhere in the Forum that disconnecting the battery to reset the computer, after reconnecting then going for a short drive to allow the ****** to "find" the proper RPM's for better shift patterns. I've done that after switching to the 245's and it seemed to work. But if that's bogus then I will take your advice and make the hour ride to the dealer for a proper calibration. Thanks for the heads up. :waytogo:

 

[rockymountain]

I read somewhere in the Forum that disconnecting the battery to reset the computer, after reconnecting then going for a short drive to allow the ****** to "find" the proper RPM's for better shift patterns. I've done that after switching to the 245's and it seemed to work. But if that's bogus then I will take your advice and make the hour ride to the dealer for a proper calibration. Thanks for the heads up. :waytogo:

It still won't be right if your speedo is off. If the computer thinks you're going 68 and you're actually going 75 what do you think the transmission will do when you step on it a little bit. If you don't have the right tire size programmed in, the transmission will learn false readings and hence have erroneous shift points.

 

[Paul M]

It still won't be right if your speedo is off. If the computer thinks you're going 75 and you're only going 68 what do you think the transmission will do when you step on it a little bit. If you don't have the right tire size programmed in, the transmission will learn false readings and hence have erroneous shift points.

You are correct...tried to see if short-cutting around the Dealer would work, even with only a 1" tire diameter change. Moral is: There's never a quick fix over proper calibration/programming, even if sensor inputs try to work it out.:badidea: