Speedometers and large tires
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Elder User
Joined: Oct 2004
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From: Fairbanks, Alaska
Speedometers and large tires
Does anyone know a reliable method of determining true speed or speedometer error when using larger than stock tires? Is there a chart or something someplace that could convert tire size to speedometer error (ie if you put on 33" tires you can expect your speedometer to be slow/fast by x miles and hour)?
I would like to know what different sizes would do to my speedometer to determine if it's worth the extra hassle in changing rear end gears etc, etc, etc.
I would like to know what different sizes would do to my speedometer to determine if it's worth the extra hassle in changing rear end gears etc, etc, etc.
Elder User
Joined: Apr 2004
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From: Sweet Home Alabama
Measured new tire size over old by tape measure, a known distance or time. Stock, the speedometer is a little fast. 285s make the speedo real close. I have 35s and it takes me 55 seconds to travel one mile at a stated 60mph which equates to about 65 mph real land speed. The longer you measure this, the more accurate. I always check this on the intersate at the beginning of all trips. You can also track actual odometer miles traveled over mile marker distance to figure it out. It is less than 10% off with 35s.
Cargo Master
Joined: Jun 2003
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From: Carlsbad, California
My tires stock are 31.65". Moving to a 33" tire would be a change of 100 * 31.65/33 = 95.9. This means that if the spedo were accurate stock, it would now read 4.1% slow. If you though that you were traveling 60 MPH, your actual speed would be 60 / 0.959 = 62.559 MPH.
The timed method of the mile markers works well. Another quick and easy was is to use a GPS if you have access to one.
The timed method of the mile markers works well. Another quick and easy was is to use a GPS if you have access to one.
Senior User
Joined: Jul 2004
Posts: 150
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From: Nevada,
Hey Everyone, follow the thread to a whole bunch of calculators and information:
http://www.andthensometoo.com/calc/tiregear.php
Good Luck, R.
http://www.andthensometoo.com/calc/tiregear.php
Good Luck, R.
Cargo Master
Joined: Jun 2003
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From: Carlsbad, California
I agree with their tire calculations but their HP calculation is off a bit 
My F-250 turned 16.0 s in the 1/4 stock. The calculator at the site said:
"Horsepower Results - 1/4 Mile Method
Your F-250 weighs about 7650 pounds and can complete a 1/4 mile in about 16.0 seconds. That means that you've got about 369.14 HP at the wheels, and about 479.88 HP at the flywheel."
With the SCMT Hi Performance tune installed I turned a 14.332 s 1/4 mile. Now the site calculator said:
"Horsepower Results - 1/4 Mile Method
Your F-250 weighs about 7650 pounds and can complete a 1/4 mile in about 14.332 seconds. That means that you've got about 513.60 HP at the wheels, and about 667.69 HP at the flywheel."
My F-250 turned 16.0 s in the 1/4 stock. The calculator at the site said:
"Horsepower Results - 1/4 Mile Method
Your F-250 weighs about 7650 pounds and can complete a 1/4 mile in about 16.0 seconds. That means that you've got about 369.14 HP at the wheels, and about 479.88 HP at the flywheel."
With the SCMT Hi Performance tune installed I turned a 14.332 s 1/4 mile. Now the site calculator said:
"Horsepower Results - 1/4 Mile Method
Your F-250 weighs about 7650 pounds and can complete a 1/4 mile in about 14.332 seconds. That means that you've got about 513.60 HP at the wheels, and about 667.69 HP at the flywheel."
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Elder User
Joined: Oct 2004
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From: Fairbanks, Alaska
Thanks guys! Darylhunter I think I will be staying around 33's or 34's...I don't want a monster truck, just a little more height! LOL!
a12play12, the link you sent is very helpful, thanks!
It's great to belong to a forum like this with so many knowledgeable people!
a12play12, the link you sent is very helpful, thanks!
It's great to belong to a forum like this with so many knowledgeable people!
Freshman User
Joined: Jul 2003
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Do all of the math above, then borrow someone's GPS and be sure. My truck is about 1 mph over at 65 (I set the cruise on the 62.5 line and GPS says about 64, which won't get you stopped in a 55 zone around here). This is running 285 tires. Stock, I was almost dead on the GPS.
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Freshman User
Joined: Jul 2004
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A queston about that horse power calculator, would'nt things like rear end gear ratio, tire size be factors.
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Cargo Master
Joined: Jun 2003
Posts: 2,203
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From: Carlsbad, California
I think that would be helpful, as would rolling resistance and Cd and frontal area, max RPM, and trap speed.
My guess is that they were trying to do a relatively simple F = m*a and find HP that way. The diesel doesn't rev as high as a car and make more of its torque down lower in the power band. Torque is the motive force and HP is (torque * RPM) / 5252. So if an engine is getting the same work done at less RPM, it will have more torque and less HP than estimated. If the engine revs are higher for the same work, then more HP is being produced and less torque.
My guess is that they were trying to do a relatively simple F = m*a and find HP that way. The diesel doesn't rev as high as a car and make more of its torque down lower in the power band. Torque is the motive force and HP is (torque * RPM) / 5252. So if an engine is getting the same work done at less RPM, it will have more torque and less HP than estimated. If the engine revs are higher for the same work, then more HP is being produced and less torque.