Ok, guys I need a hand figuring this out. I have look at several conversion calculators and have contradicting results. I’m not sure what the stock tire size is on my truck (I believe 255/65/17 and I have on 265/70/18). I know that there isn’t a large difference but enough to throw off the speedometer. Now this is going to add more miles to my odometer and say it reads originally 65 (with stock tires) mph it would be something like 69mph correct (with new tires) (not so much exact numbers)? I want to know because I want to calculate my mpg correctly.

Here is an example of one of the sites: http://www.csgnetwork.com/speedocalibcalc.html

When I put in my new tire and old tire sizes it states that the speedometer will read less than what it is really going, but I had my gps in my truck and it showed that the truck is about 4 mph over what it states. What am I missing?

Thanks guys.

 

First, to find a base for any calculations, look at the rating plate inside the driver door sill - with the door open look to the right where the door would latch. That rating plate should have the size tires that were put on by the factory.

Once you have that, use one of the online calculators to find your speedometer offset - one to use is here:

http://www.discounttire.com/dtcs/infoTireMath.dos

As a general rule however, a larger than stock tire will give you a lower reading on your speedometer - example being if your speedometer is reading 55mph, you are actually going 60mph.

 

Ok, that confirms what I thought and the GPS showed. Is there a way to calculate the actual miles that you put on?

Say it reads 200 miles (with new tires)

Being off 4 mph average (roughly 7%: 55 to 59). 200 * 0.07 = 14. So I am really going 186 miles instead of 200 miles.

Is this a correct assumption? The problem is that at different speeds I believe the speedometer is off at different percentages.

Right or Wrong thinking?

Thanks

 

You are wrong in thinking that it will be off different percentages at different speeds. If its off 10% at 20mph, its off 10% at 80mph too. Your tires don't change size as you go faster or slower, so you aren't going to have varying results

If your tires are 7% bigger, then you just add 7% to your total mileage and then divide by the gallons you put in.

Mine were 9% bigger before I had the speedo fixed. So if the trip meter said 300 miles, I actually went 327 and then divided the gallons from there.

Hope that helps.

 

just go to your ford dealer and have them recalibrate.

my best friends dad did it for me saturday night, he's a mechanic at ford, so now i don't need to worry about that! lol

 

That only works if that tire size option was available for the year of your truck. I've got 20" wheels on my '04 and they couldn't calibrate the PCM because 20's weren't an option in '04.

 

Something to confuse things even more, the speedo is almost alwats 2-3 mph fast. Ford builds them that way. So, if you get the recalibration from the dealer, it will still be off from your GPS.

 

yeah your exactly right, but when you type in your tire size, its shows you how many revs per mile, so you go and find a similair size with similair revs per mile and you chose that one.

i had to do that with mine, closest i could get was 647- my tires make 643.

all they have to do is take it to the mian computer and calculate similair sized tires within ford certified limits and they'll get you as close as possible.

i passed one of those speed warnings on the side of the road, it was flashing 64 at me my speedo was reading 63-64, so im right on target.

 

Wheel size has nothing to do with recalibrating the PCM. I'm not sure why they wouldn't do it for you. It doesn't matter what wheel size option was available. They only need to know the overall diameter of the new tire or how much bigger or smaller, percentage wise, the new tires are than the stock ones.

If you go from a stock 32 to a 35 and your factory wheel was a 17, and you went to a 20, it won't matter. The 35 is still a 35 regardless of the wheel diameter.

 

and p.s.- it's all about the tires, not the rims.

only thing the rims affect is plugging in the tires size, if you know the diamter and width (in inches) of your tire, then you can figure out a similiar size within certified limits and get very close on the recalibration at the dealership.

sounds like you may have gotten a lazt mechanic lol

 

It's a good excuse to get a tuner so you can get it accurate yourself. The money you pay the dealer is like a downpayment on the tuner.

 

hahaha my thoughts exactly

 

I noticed that....I set my Edge so it would match the GPS, took FOREVER to get it close. It's not perfect, but at 80 on the GPS, it blinks between 79 and 80 on the Edge.

I don't even look at the cluster anymore, except for fuel level.

 

Same here. I keep the engine temp, tranny temp, volts and mph displayed. I forget to check my fuel level.

 

I failed to mention my 20 wheels have 35's on them and they wouldn't/couldn't calibrate it. Obviously, much bigger than stock. I really didn't care, the truck was not at the dealership for that, but the tech thought he's be nice and calibrate the PCM, but found out he couldn't. And, with the Edge and GPS, I know how fast I'm going.