help with calculating milage with bigger tires

[BoatsRcool]

ok i can live with the messed up speedo, cuz i dont think i can change the tooth with 32's and 3:27 gears. anyway, how can i figure out my correct milage i know my odometer has to be off.. thanks

 

[KEbert]

It is really hard to do unless you change the gear. I think Ford can help you. I would at least ask.

 

[BoatsRcool]

well i usually divide my trip odometer with how many gallons i fill up with... which would be pretty correct.. but there must be a # i could multiply that trip # by isnt there...

 

[Byrd91]

Your actual mileage is going to be off by exactly the same PERCENTAGE that your speedometer is off, which is real close to 10% with 32's instead of 235's. So just add 10%, or 10 miles for every 100.

 

[Bill Kemp]

Find an expressway with mile markers and have a friend tell you when you are taking exactly sixty seconds to go one mile. Use a three mile average. Whatever speed you are at will be 60 mph. Whatever percentage that speed is of 60 is the percent your odometer is off also.

 

[beaston1]

hey how do the 3.27's and 32's work out. Is it a bit sluggish or what???

 

[Robert]

Take your new tire diameter and divide it by your old diameter. Take your result and multiply it to your odometer to get the actual miles you have driven. Divide the number of gallons into it to get your MPG.

 

[Fusion213]

Well, what's the size of your stock tires? then i can figure it out.

 

[WILL]

You are all making it way too difficult, here is the easiest
Needed:
1. 2 people (safety)
2. Open hwy (60mph)
3. Set cruise to 60mph
4. Zero trip meter
5. With stopwatch measure 60sec
6. Record mileage on trip
7. Repeat 3 separate times (for an average)

The idea at 60 mph you are doing 60 miles in one hour(60 min), in one minute you cover one mile. If the cruise is set at 60 and you complete less than one mile in 60 sec that is your difference. For example if your 60 sec time gives you .8 on your trip meter. Then at each fillup multiply your trip meter times .2 to get the actual mileage traveled. This works for all vehicles and is pretty easy to do.

 

[WILL]

Originally posted by Robert
Take your new tire diameter and divide it by your old diameter. Take your result and multiply it to your odometer to get the actual miles you have driven. Divide the number of gallons into it to get your MPG.

Rob I agree with your math but in my experience when you purchase a car, the speedo isnt always right. Just try the 60 sec 1 mile and you may or may not notice that on some cars. Me I am not too good at measuring using those small lines on a measuring tape.

 

[Fusion213]

Here, go to www.4lo.com
click on the speedometer check on the calculators bar.
All you need to do is some math, that's all.

 

[Robert]

Originally posted by WILL


Rob I agree with your math but in my experience when you purchase a car, the speedo isnt always right. Just try the 60 sec 1 mile and you may or may not notice that on some cars. Me I am not too good at measuring using those small lines on a measuring tape.

The speedometer isn't always right, but it is a lot closer than the error you would introduce by using your odometer for one mile. Your chance for error there is +/- 10%. The speedometer shouldn't be anymore than 5% off. Mine was dead on compared to my GPS. What you are attempting to do with your theory is basically the same thing as I did, only the way you are doing it is incorrect:

"The idea at 60 mph you are doing 60 miles in one hour(60 min), in one minute you cover one mile. If the cruise is set at 60 and you complete less than one mile in 60 sec that is your difference. For example if your 60 sec time gives you .8 on your trip meter. Then at each fillup multiply your trip meter times .2 to get the actual mileage traveled. This works for all vehicles and is pretty easy to do."

Let's say he went 180 indicated miles on his tank. If he multiplied it by the .2 you are saying, he only went 36 miles. He won't divide it by .2 either since that would indicate 900 miles to his tank (he wishes). The correct thing to do is be to divide 1 mile by .8 or whatever his odometer said. He would use whatever number he came up with to multiply by his indicated odometer distance at fill-up. For instance using your numbers, if his odometer indicated .8 miles over 1 actual mile, he would get 1.25. Then if at fill-up his odometer indicated 180 miles, he actually went 225 miles. Whatever he comes up by measuring over the course of a mile will be the same as if he just divided his new tire diameter by his old diameter (only not nearly as accurate). Using his old tire diameter and new diameter will at least get him as accurate as his odometer/speedometer was when it came from Ford. The problem with using the odometer is you only have 1 decimal place resolution which can make him off by +/- .1 miles over 1 mile traveled (+/- 10%). Now if he did it over 100 miles then he would be within 1 %) but that would take a little longer than just dividing the two diameters.

 

[X24]

Rob is right.

A question for everyone. I know I have asked this before but I just want to get some more opinions.

I have a set of 16" EB/Sport OEM wheels that I will be putting on my 98 XLT in place of the 15" OEM wheels. A guy at my ford dealer said that doing that could mess up my ABS sensors and my 4X4 sensors (auto 4X4). The tires that I plan on getting on these rims are 255/65/16's which are .6% bigger than my stock 235/75/15's. That is as close as I can get it. I think that this guy thinks that I am going to be putting on much bigger tires. I can see how that might cause some problems, but if my outside diameter is that close there shouldn't be any problems... right? After all, it is the outside diameter that matters not the rim size, correct? Thanks.

 

[Fusion213]

I really don't see what's wrong with that because my explorer came stock with 16" rims and 255/70/16 which are about 1" taller than 255/65/16.
At 4lo.com there's also metric tire conversions.

 

[01EB5.0]

For the problem with the speedo being off, it is very easy to replace the speedo gear and I would just get that fixed. it's like $10 and saves a heck of a lot of hassle!

As for the larger tires on a 15" model. that may be true, but as long as you get the speedo recalibrated, it will all be fine. I have had my P265/70R16 on my 95 XLT for 15,000 miles and the ABS and 4X4 still work flawlessly.

 

[Bill Kemp]

98 FordX24, I changed to exactly the same tire/rim combo a year ago and noticed no difference except maby better handeling.
Timeing your car on a measured mile to see what speed takes 60 seconds to complete the mile is the easiest way to determine any speedometer/odometer error.
It's also nice to know if you push the speed limit.
Unless you have a computer (milage, distance to empty ect.) it's almost imposible to get a real accurate MPG figure. The way the gas tank filling system is set up it is very difficult if not impossible to know that you have filled it to the same level each time. If a pump runs real slow you can get an extra gallon or more in the tank.

 

[WILL]

Well Rob thanks...guess I am not a math teacher, I have been doing just that. I am not too good at explaining it though...I left out a step. Yes the accuracy is pretty shty huh. Well I have to replace the gear just havent had the time

 

[X24]

hey bill, do you have any pics of your truck that you could send me or post? my email is JG24CV@aol.com thanks. we have very similar mods. i just want to take a look at it and see what you have done.

 

[WILL]

no hopefully in the next few days I can borrow my uncles digital cam and get some pics finally. I have no idea what to do with them once I take the pic...give me a few days hopefully this week

 

[WILL]

oops bill not will...common mistake