Tiresize change-microtuner recommendations
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Junior Member
Joined: Sep 2003
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From: San Francisco
Tiresize change-microtuner recommendations
Just changed from 275/60 R17 to 260/70 R17 Revos (great tire) It runs rough around 45 - 50 mph since tires installed. O.K. around city riding. Think I need to change shift points to match increase in tire height? Went from 689 revs/mile to 657 revs/mile. Any feed back on easy mircotuner to fix this problem?
Thanks Joe
Thanks Joe
Last edited by jaamina; Jan 18, 2004 at 06:10 PM.
Senior Member
Joined: Mar 2001
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From: Western Massachusetts
I would like to add a bit to this question, if you don't mind.
I will have to have my chip set up for a new tire size also (finally, better tires!
). My question is, if I put the new wheels/tires on before having the chip flashed, how far will it be off. I'm going from a 265-70T-17 to a 285-60V-18.
Mike, I will be calling you soon about this. I'm pretty excited about how my truck will be soon!
I will have to have my chip set up for a new tire size also (finally, better tires!
). My question is, if I put the new wheels/tires on before having the chip flashed, how far will it be off. I'm going from a 265-70T-17 to a 285-60V-18.Mike, I will be calling you soon about this. I'm pretty excited about how my truck will be soon!
Former Sponsor
Joined: Mar 1998
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From: Virginia
Hi Joe,
Thanks for your post, good question!
As you correctly pointed out, in 1999 or newer F-150's, the way the powertrain program actually deals with tire size with regard to the speedo & odo calibration (and shift points in the automatics) is via the tire manufacturer's "revs per mile" specification - which is the number of revolutions that tire must turn in order to travel exactly 1 mile.
Well, in the Micro Tuner you can easily correct for tire size and/or gear ratio changes, but you don't enter the revs per mile spec - instead, what you do is to physically *measure* the actual tire height - then *that* is what you enter into the Micro Tuner, and it's "resolution" is 1/4" - meaning you round off to the nearest 1/4" in height. Then the Micro Tuner takes care of actually changing that revs per mile spec in the program, and the speedometer & odometer are automatically corrected as a result.
If you have a 1997 or 1998 F-150, you must do this via changing the speedo gear in the tailshaft of the transmission - actually, you can do it that way for virtually all pre-1999 FoMoCo's. If memory serves, I think it's the 1992-1996 models that also have a programmable speedo/instrument cluster, which can be reprogrammed up to 6 times.
The bottom line is, for all 1999 & newer F-150's you measure the tire height and enter *that* figure in the Micro Tuner, to the nearest 1/4", and you're all set!
Thanks for your post, good question!
As you correctly pointed out, in 1999 or newer F-150's, the way the powertrain program actually deals with tire size with regard to the speedo & odo calibration (and shift points in the automatics) is via the tire manufacturer's "revs per mile" specification - which is the number of revolutions that tire must turn in order to travel exactly 1 mile.
Well, in the Micro Tuner you can easily correct for tire size and/or gear ratio changes, but you don't enter the revs per mile spec - instead, what you do is to physically *measure* the actual tire height - then *that* is what you enter into the Micro Tuner, and it's "resolution" is 1/4" - meaning you round off to the nearest 1/4" in height. Then the Micro Tuner takes care of actually changing that revs per mile spec in the program, and the speedometer & odometer are automatically corrected as a result.
If you have a 1997 or 1998 F-150, you must do this via changing the speedo gear in the tailshaft of the transmission - actually, you can do it that way for virtually all pre-1999 FoMoCo's. If memory serves, I think it's the 1992-1996 models that also have a programmable speedo/instrument cluster, which can be reprogrammed up to 6 times.
The bottom line is, for all 1999 & newer F-150's you measure the tire height and enter *that* figure in the Micro Tuner, to the nearest 1/4", and you're all set!
Senior Member
Joined: Jan 2002
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From: Maine
question for mike troyer
i know you said you are supposed to physically measure the tire, and this may be a dumb question, but do we measure it off the truck or while its on the truck and on the ground? wondering cause with my 305/70r17 there is almost an inch difference between one way on the other....
thanks
marcr
thanks
marcr
Former Sponsor
Joined: Mar 1998
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From: Virginia
Hi marcscrew,
Ahh, sorry 'bout that!
We advise measuring it on the vehicle, and on one of the rear tires, too. You're measuring height, not circumference, so just from the pavement to the top of one of the rear tires.
Now the reality is you *can* measure tire height *uninstalled,* meaning on the rim and inflated to normal pressure but not installed on the vehicle - and you'll eliminate the deflection due to the weight of the vehicle, and depending on the parameters, that can sometimes give you a tad more accurate reading, but that's pretty rare - we prefer you measure it on the vehicle, that's how we do it, as that is the condition of the tire when the vehicle is being driven, and thus the "baseline" for the speedo calibration to begin with.
Good luck!
Ahh, sorry 'bout that!
We advise measuring it on the vehicle, and on one of the rear tires, too. You're measuring height, not circumference, so just from the pavement to the top of one of the rear tires.
Now the reality is you *can* measure tire height *uninstalled,* meaning on the rim and inflated to normal pressure but not installed on the vehicle - and you'll eliminate the deflection due to the weight of the vehicle, and depending on the parameters, that can sometimes give you a tad more accurate reading, but that's pretty rare - we prefer you measure it on the vehicle, that's how we do it, as that is the condition of the tire when the vehicle is being driven, and thus the "baseline" for the speedo calibration to begin with.
Good luck!
Former Sponsor
Joined: Mar 1998
Posts: 13,385
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From: Virginia
Hi Tom (wittom),
Call me for details on that, I can 'splain it to you better over the phone.
There's plenty of posts here in this section over the years on exactly how to do that, via the search feature - just search on speedo gear.
Basically, all the pre-1999 F-150's (and most other older vehicles for decades & decades) have a gear in the tailshaft of the transmission, a "speedometer gear," and that is what controls the speedometer calibration - when you make a tire size or gear ratio change, you replace that gear with one that has the correct number of teeth and that corrects the speedometer, odometer, and the shift points if it's an automatic tranny model, too - the gears cost under $20, and it's a 10 minute installation, pretty simple - it's under a little spring-loaded door on the tailshaft of the tranny.
Call me when you can if you like, and I'll go over it all in detail, show you the math on how to determine the number of teeth on the new speed gear, etc., etc. if that would be helpful.
Call me for details on that, I can 'splain it to you better over the phone.
There's plenty of posts here in this section over the years on exactly how to do that, via the search feature - just search on speedo gear.
Basically, all the pre-1999 F-150's (and most other older vehicles for decades & decades) have a gear in the tailshaft of the transmission, a "speedometer gear," and that is what controls the speedometer calibration - when you make a tire size or gear ratio change, you replace that gear with one that has the correct number of teeth and that corrects the speedometer, odometer, and the shift points if it's an automatic tranny model, too - the gears cost under $20, and it's a 10 minute installation, pretty simple - it's under a little spring-loaded door on the tailshaft of the tranny.
Call me when you can if you like, and I'll go over it all in detail, show you the math on how to determine the number of teeth on the new speed gear, etc., etc. if that would be helpful.