Not really a Tuner question
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From: Clifton Forge, Va.
Not really a Tuner question
This is not really a tuner question but I have an Xcal 2 and can use it to solve a problem if it really exists.
My son and I were on the open road - I was in my F150 and he was following in a Dodge Intrepid. My speedo read 60 and his read 55.
Without a GPS or other help I think the easiest solution to see if my speedo is off is to see what speed other trucks make at 2000 rpm.
I have a 2005 F150 super crew - 3:73 LS rear end and P255/70R17 tires.
Would someone with a similar setup please make note of their speed @ 2000 rpm?
Thanks for any help
Kenny
My son and I were on the open road - I was in my F150 and he was following in a Dodge Intrepid. My speedo read 60 and his read 55.
Without a GPS or other help I think the easiest solution to see if my speedo is off is to see what speed other trucks make at 2000 rpm.
I have a 2005 F150 super crew - 3:73 LS rear end and P255/70R17 tires.
Would someone with a similar setup please make note of their speed @ 2000 rpm?
Thanks for any help
Kenny
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Joined: Feb 2004
Posts: 10,390
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Hi Ken.
Actually, measuring your own tires true circumference & calculating rev/mile, then plugging that into the tune will be more accurate. Use the "chaulk-mark-roll-3-revs-divide-by-three" method on the rear tire.
Even with the same setup other factors like tire wear, different brands, etc their speedo error will render the results of your method questionable due to the risk of stacked errors.
Tire growth at speed is also a factor. Also you are stuck with increments-of-five granularity on the correction.
-OR- Buy a GPS, use it, then *cough* return it
Cheers
Hyphen-happy Bubba
Actually, measuring your own tires true circumference & calculating rev/mile, then plugging that into the tune will be more accurate. Use the "chaulk-mark-roll-3-revs-divide-by-three" method on the rear tire.
Even with the same setup other factors like tire wear, different brands, etc their speedo error will render the results of your method questionable due to the risk of stacked errors.
Tire growth at speed is also a factor. Also you are stuck with increments-of-five granularity on the correction.
-OR- Buy a GPS, use it, then *cough* return it
Cheers
Hyphen-happy Bubba
Last edited by MGDfan; Mar 21, 2007 at 02:43 PM.
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Joined: Mar 1998
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From: Virginia
Hi Kenny,
Kenny, you need to completely ignore what any other vehicle, including other F-150's, have their speedos say at 2000 rpm. There are so many different tire sizes & gear ratios that it makes this impossible - to say nothing of the fact that with all the different tire manufacturers & models, even of the same size, you have a significant amount of variance in their revs per mile spec and in the amount of "growth" in the tire size as speed increases, so this is about the worst way to try to make any accuracy test.
Actually, the *correct* way to do this is to contact the manufacturer of your tire, and give them the exact size and the model name of that tire, and ask them for their "revs per mile" specification. This is the number of revolutions that the tire must turn to travel exactly 1 mile - that is how *all* speedometers are calibrated in terms of the tire size.
Now one very important thing to remember is that with truck tires, most of them are rated for their revs per mile spec at a road speed of 45 mph, and the faster you go, the more the tire grows, so the more the speedometer is off - there is no getting around that, so you will always have *some* speedometer error. For example, at 70-75 mph, it's common to have 2 mph of error or so, and there really isn't' anything you can do about that - well, not unless you want to use a GPS unit and play around with the revs per mile setting in your XCal 2 and dial it in for your normal cruising speed, for example. Keep in mind that the resolution of the revs per mile in those devices is 5, whereas when we do the tuning, we can dial it in for the exact revs per mile, down a a resolution of 1 rev per mile.
The bottom line is, don't' worry about what other vehicles' speedo's say when going down the road, that is not the way to check it. Your truck is going to be as accurate as is possible within the accuracy control of the tire manufacturer, as we did the tuning - and that includes any compensation for any change to tire size or gear ratio.
So I would not change anything unless you have made a change to tire size or gear ratio AFTER we did the tuning, as it's just not going to be any more accurate - you will always have some speedo error as speeds get significantly above 45 mph, especially in trucks compared to passenger cars, and that is due to tire growth - it's just a fact of life. This happens more in light truck tires than in passenger car tires, and in fact, Car & Driver actually did a big article about this about 5 years ago or so to document this fact.
I can tell you that as long as you gave us correct info when you submitted your custom program worksheets for us to do your tuning, there is no way you are going to be 5 mph off at 60 or 65 mph - as long as we got accurate info, then in order for you to have a 5 mph error, you would have to be doing 120 mph or better.
Kenny, you need to completely ignore what any other vehicle, including other F-150's, have their speedos say at 2000 rpm. There are so many different tire sizes & gear ratios that it makes this impossible - to say nothing of the fact that with all the different tire manufacturers & models, even of the same size, you have a significant amount of variance in their revs per mile spec and in the amount of "growth" in the tire size as speed increases, so this is about the worst way to try to make any accuracy test.
Actually, the *correct* way to do this is to contact the manufacturer of your tire, and give them the exact size and the model name of that tire, and ask them for their "revs per mile" specification. This is the number of revolutions that the tire must turn to travel exactly 1 mile - that is how *all* speedometers are calibrated in terms of the tire size.
Now one very important thing to remember is that with truck tires, most of them are rated for their revs per mile spec at a road speed of 45 mph, and the faster you go, the more the tire grows, so the more the speedometer is off - there is no getting around that, so you will always have *some* speedometer error. For example, at 70-75 mph, it's common to have 2 mph of error or so, and there really isn't' anything you can do about that - well, not unless you want to use a GPS unit and play around with the revs per mile setting in your XCal 2 and dial it in for your normal cruising speed, for example. Keep in mind that the resolution of the revs per mile in those devices is 5, whereas when we do the tuning, we can dial it in for the exact revs per mile, down a a resolution of 1 rev per mile.
The bottom line is, don't' worry about what other vehicles' speedo's say when going down the road, that is not the way to check it. Your truck is going to be as accurate as is possible within the accuracy control of the tire manufacturer, as we did the tuning - and that includes any compensation for any change to tire size or gear ratio.
So I would not change anything unless you have made a change to tire size or gear ratio AFTER we did the tuning, as it's just not going to be any more accurate - you will always have some speedo error as speeds get significantly above 45 mph, especially in trucks compared to passenger cars, and that is due to tire growth - it's just a fact of life. This happens more in light truck tires than in passenger car tires, and in fact, Car & Driver actually did a big article about this about 5 years ago or so to document this fact.
I can tell you that as long as you gave us correct info when you submitted your custom program worksheets for us to do your tuning, there is no way you are going to be 5 mph off at 60 or 65 mph - as long as we got accurate info, then in order for you to have a 5 mph error, you would have to be doing 120 mph or better.
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From: Clifton Forge, Va.
Thanks Mike
I pulled my file and I made sure that I indeed sent Crystal the correct information(and I did). I also checked my odometer against the mile post markers on the way home from work and I appear to be "right on". I will call my son later this evening and warn him that when his speedo says 55 he is really running 60.
Thanks again Mike, not everyone has the level of customer service that you have. As you and I have discussed, you know that I am "tickled to death" with my tunes and the way my truck now performs. My truck, my tunes, my way.
Kenny
I pulled my file and I made sure that I indeed sent Crystal the correct information(and I did). I also checked my odometer against the mile post markers on the way home from work and I appear to be "right on". I will call my son later this evening and warn him that when his speedo says 55 he is really running 60.
Thanks again Mike, not everyone has the level of customer service that you have. As you and I have discussed, you know that I am "tickled to death" with my tunes and the way my truck now performs. My truck, my tunes, my way.
Kenny
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Joined: Dec 2006
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From: Phoenix, AZ
This might be helpfull as well...sounds like you got it worked out though!
http://www.discounttiredirect.com/di...foTireMath.jsp
http://www.discounttiredirect.com/di...foTireMath.jsp
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From: Virginia
You're very welcome Ken - and by the way, I didn't mean to step on anyone's toes, or rain on anyone's parade who are suggesting or using other methods - I guess that as a tuner, because I am used to how it's really done inside the PCM, that is how I look at it and prefer to have it done, as that is the most accurate - meaning, going by th tire manufacturer's revs per mile specification.
In light truck tires, they rate it at 45 mph, so the faster you go, the more error you will get just naturally from "tire growth" - now *some* people will use a GPS device and see what their error is at their favorite highway cruising speed (say, 90 mph just for example), and they will make a very small adjustment to tweak it in a little bit so that it's most accurate at their highway cruising speed - now this is usually only done by people that tend to cruise at higher speeds of course, as at lower speeds there isn't enough error to really speak of. This is just some additional general FYI stuff, not meaning to further complicate matters.
The bottom line is, the correct way to calibrate the speedo and how we do is is to use the tire manufacturer's revs per mile specification, and plug that into the tune, and we're all set other than normal tire growth at higher speeds that we basically just have to accept.
In light truck tires, they rate it at 45 mph, so the faster you go, the more error you will get just naturally from "tire growth" - now *some* people will use a GPS device and see what their error is at their favorite highway cruising speed (say, 90 mph just for example), and they will make a very small adjustment to tweak it in a little bit so that it's most accurate at their highway cruising speed - now this is usually only done by people that tend to cruise at higher speeds of course, as at lower speeds there isn't enough error to really speak of. This is just some additional general FYI stuff, not meaning to further complicate matters.
The bottom line is, the correct way to calibrate the speedo and how we do is is to use the tire manufacturer's revs per mile specification, and plug that into the tune, and we're all set other than normal tire growth at higher speeds that we basically just have to accept.