Tires size and MPG
#17
In the Excursion rear end it is my understanding that the speedo runs off a tone ring that is part of the rear ring.
When I changed my gears from 3.73 to 4.30's I did not have anything reprogrammed and my speedo lined up right where it was before (BTW...before or after...my speedo says I'm going 2mph faster than my GPS states at 65mph)
A PCM reprogram to tell it which gears are in the back may be needed for improving transmission performance and when I got my tuner from 5-Star he programmed in that I had 4.30's to improve shift points/etc.
But a gear change should not affect speedo based on where the speed sensor is located...and I can confirm that I did not notice any shift in speedo performance post gear swap.
Joe.
#18
Join Date: Mar 2009
Location: Mt. Shasta California
Posts: 11,798
Likes: 0
Received 8 Likes
on
8 Posts
I believe this last part may be inaccurate.
In the Excursion rear end it is my understanding that the speedo runs off a tone ring that is part of the rear ring.
When I changed my gears from 3.73 to 4.30's I did not have anything reprogrammed and my speedo lined up right where it was before (BTW...before or after...my speedo says I'm going 2mph faster than my GPS states at 65mph)
A PCM reprogram to tell it which gears are in the back may be needed for improving transmission performance and when I got my tuner from 5-Star he programmed in that I had 4.30's to improve shift points/etc.
But a gear change should not affect speedo based on where the speed sensor is located...and I can confirm that I did not notice any shift in speedo performance post gear swap.
Joe.
In the Excursion rear end it is my understanding that the speedo runs off a tone ring that is part of the rear ring.
When I changed my gears from 3.73 to 4.30's I did not have anything reprogrammed and my speedo lined up right where it was before (BTW...before or after...my speedo says I'm going 2mph faster than my GPS states at 65mph)
A PCM reprogram to tell it which gears are in the back may be needed for improving transmission performance and when I got my tuner from 5-Star he programmed in that I had 4.30's to improve shift points/etc.
But a gear change should not affect speedo based on where the speed sensor is located...and I can confirm that I did not notice any shift in speedo performance post gear swap.
Joe.
#19
well when you're both going down the high way at the same speed, you're going down high way at the same speed, whether she's behind me, in front of me or next to me.
#20
#21
If your speedo is not calibrated properly it will be more off the faster you go. For example mine is not calibrated and it is off by 9.2% (theoretically), on account of the 35" tires, so it is pretty close at 25 MPH I am going 27 MPH... but @ 70 MPH I am going more like 77 MPH.
I say theoretically because it has come to my attention that the default calibration is not exactly correct. It seems that with stock 265/75R16 tires the speedo is actually over reporting your speed. I have noticed this on lots of vehicles.
Many Excursion owners have reported that their speedo is more accurate when they move from the stock 265/75R16 (~31.5" tire) up to a 285/75R16 (~33" tire)
If this is true then the speedo is under by about 3.5-4% as it comes calibrated from the factory.
I say theoretically because it has come to my attention that the default calibration is not exactly correct. It seems that with stock 265/75R16 tires the speedo is actually over reporting your speed. I have noticed this on lots of vehicles.
Many Excursion owners have reported that their speedo is more accurate when they move from the stock 265/75R16 (~31.5" tire) up to a 285/75R16 (~33" tire)
If this is true then the speedo is under by about 3.5-4% as it comes calibrated from the factory.
2-3 MPH @ 70MPH as you state.
But, what is commonly forgotten is (in my Ex) the ODOMETER (trip meter on dash board),
distance traveled on the trip computer (lie-o-meter),
and data from ODBII port (MPH, RPM, distance traveled) appear to be
completely accurate (and agree with GPS. I've driven 600 miles and
the GPS calculated within a mile or two - which when you factor in
the on-off ramps, rest stops, etc. is "exact" to me.
So keep in mind the accuracy of the speedometer is not necessarily the
same as the other variables which go into calculating MPG.
So, changing tiresize might make the speedometer correct,
but would likely mess up all the other variables (ie: distance traveled)
and would give inaccurate MPG results - even if there is no
difference in the actual MPG.
Sounds like we need Mythbusters and a test track and do our own
"Daytona 500" on 2 different sets of tires?
#23
Size is only one portion of the equation though. (The largest one.)
Tread type would be another and weight of tire would be also.
#24
With 265/75R16 tires my speedometer is accurate @ 25, but off by
2-3 MPH @ 70MPH as you state.
But, what is commonly forgotten is (in my Ex) the ODOMETER (trip meter on dash board),
distance traveled on the trip computer (lie-o-meter),
and data from ODBII port (MPH, RPM, distance traveled) appear to be
completely accurate (and agree with GPS. I've driven 600 miles and
the GPS calculated within a mile or two - which when you factor in
the on-off ramps, rest stops, etc. is "exact" to me.
So keep in mind the accuracy of the speedometer is not necessarily the
same as the other variables which go into calculating MPG.
So, changing tiresize might make the speedometer correct,
but would likely mess up all the other variables (ie: distance traveled)
and would give inaccurate MPG results - even if there is no
difference in the actual MPG.
Sounds like we need Mythbusters and a test track and do our own
"Daytona 500" on 2 different sets of tires?
2-3 MPH @ 70MPH as you state.
But, what is commonly forgotten is (in my Ex) the ODOMETER (trip meter on dash board),
distance traveled on the trip computer (lie-o-meter),
and data from ODBII port (MPH, RPM, distance traveled) appear to be
completely accurate (and agree with GPS. I've driven 600 miles and
the GPS calculated within a mile or two - which when you factor in
the on-off ramps, rest stops, etc. is "exact" to me.
So keep in mind the accuracy of the speedometer is not necessarily the
same as the other variables which go into calculating MPG.
So, changing tiresize might make the speedometer correct,
but would likely mess up all the other variables (ie: distance traveled)
and would give inaccurate MPG results - even if there is no
difference in the actual MPG.
Sounds like we need Mythbusters and a test track and do our own
"Daytona 500" on 2 different sets of tires?
#25
It should not affect the speedo and in my case and numerous others with Excursion gear swaps going from 3.73's to 4.30's...it has not appreciably affected the speedo...
But I guess it could affect the speedo as EXv10 points out...
#28
I went from 3.73 to 4.30 running my 315'w on 10in rims and did not change my highway MPG but did increase average and town and really made a big difference with towing. Plus I can stay on CC running down the road now even going up hills where before it would shift in and out all the time. Not to mention it just drives better, takes off quicker and has more throttle response at all speeds. Definitely a big improvement.
My speedo is off about 5mph at 70mph but I have a scan guage setup to monitor speed and use it.
My speedo is off about 5mph at 70mph but I have a scan guage setup to monitor speed and use it.