Tire Change = Lost Power & Speedo???
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Senior User
Joined: Jan 2004
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From: Central, KY
Tire Change = Lost Power & Speedo???
I have a 98 3.0 4x4 with 3.73 gears and stock 235/75R15 tires. I bought a set of 16x7 wheels. I am considering 245/75R16 or 255/70R16. I was wondering a few things.
Will speedo be off much?
Will ABS be "off"?
Loose any Power???
Thanks All!
Will speedo be off much?
Will ABS be "off"?
Loose any Power???
Thanks All!
Fleet Mechanic
Joined: Dec 2003
Posts: 1,436
Likes: 117
From: Virginia
I can't tell you how much the speedo will be off, but it will be off by 5+mph. If I had to guess I would say 8-10. Your speedo will read slower than you are actually going. (if I am thinking straight right now)
I don't know about your ABS. I wouldn't think it would be affected.
Your gas mileage will drop, and you will be slower on the take-off and have less throttle response through the whole RPM range. With more rolling resistance, and more wheel mass (all else equal) you have to work the engine harder to achieve the same acceleration as the OEM tires.
I don't know about your ABS. I wouldn't think it would be affected.
Your gas mileage will drop, and you will be slower on the take-off and have less throttle response through the whole RPM range. With more rolling resistance, and more wheel mass (all else equal) you have to work the engine harder to achieve the same acceleration as the OEM tires.
Fleet Mechanic
Joined: Dec 2003
Posts: 1,436
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From: Virginia
That is good info to have. I did say I was guessing. And it is also assuming the speedo is correct now. (before the tire change)
The mileage, and power I am sure about. Although exact numbers are harder to calculate since it kind of depends on the condition of the truck right now.
Good luck.
The mileage, and power I am sure about. Although exact numbers are harder to calculate since it kind of depends on the condition of the truck right now.
Good luck.
Post Fiend
Joined: Feb 2003
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From: Eastern WA
If you determine the circumference of the old and new tires you can quickly calculate the percentage change in speed shown on the speedometer; it's simply a ratio x 100. Visit the websites for the tire manufactures and you might be able to find tire circumference. As was correctly noted, larger diameter tires will cause a lower than actual speedometer speed. The added weight, especially at distances farthest away from the center of wheel rotation will decrease overall acceleration and increase stopping distances. However, your ABS should work just fine. Depending on how you drive, you may not notice the loss of acceleration much.
Last edited by CowboyBilly9Mile; Jun 22, 2004 at 11:27 AM.
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Senior User
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From: Central, KY
I am still unsure on which tire to choose from. The 255/70R16 is smaller diameter than the 245/75R16 but it is a wider tire. Some say that the wider tire will decrease mpg because the larger resistance of wind the tire has compared to the 235/75R15. Which makes sense that there will be more area of rubber for the wind to hit and hence slow down the vehicle some. Which in this case leads to more toward the 245/75R16. Many new 4x4 Ranger come with the 245 and 4x4 Explorers come with the 255.
Fleet Mechanic
Joined: Dec 2003
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From: Virginia
Originally Posted by KYFord
I am still unsure on which tire to choose from. The 255/70R16 is smaller diameter than the 245/75R16 but it is a wider tire. Some say that the wider tire will decrease mpg because the larger resistance of wind the tire has compared to the 235/75R15. Which makes sense that there will be more area of rubber for the wind to hit and hence slow down the vehicle some. Which in this case leads to more toward the 245/75R16. Many new 4x4 Ranger come with the 245 and 4x4 Explorers come with the 255.
You will have to make the choice on the tires you use, but you can minimize the effect weight has on your new wheels/tires. If you are going to 16" rims you have to get new ones.... find some that are as close to, or lower weight than your OEM 15". Stay away from crome 30lbs wheels, they will only make MPG, and power worse. You can also find a few lbs difference in tire mfgs too. Sounds dumb, but a few lbs on each wheel will make a difference.
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From: Central, KY
.... find some that are as close to, or lower weight than your OEM 15".
My wheels are the new 16x7 5 spoke 2004 aluminums. My factory wheels are 15x7 steel. The aluminums are much much lighter wheels. What size do you all think would be ok, 245/75 or 255/70?
My wheels are the new 16x7 5 spoke 2004 aluminums. My factory wheels are 15x7 steel. The aluminums are much much lighter wheels. What size do you all think would be ok, 245/75 or 255/70?
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From: Central, KY
[QUOTE=CowboyBilly9Mile]If you determine the circumference of the old and new tires you can quickly calculate the percentage change in speed shown on the speedometer; it's simply a ratio x 100. Visit the websites for the tire manufactures and you might be able to find tire circumference. As was correctly noted, larger diameter tires will cause a lower than actual speedometer speed. QUOTE]
Tire`````````Inches```65 MPH
-----------------------------
235/75R15````28.9`````Stock
255/70R16````30.1`````+2.7
245/75R16````30.5`````+3.6
For example: My stock tire is 28.9 inches. Say I get a 255/70R16 which is a 30.1 inch tire. You divide stock tire by the desired tire and multiply by 100. 28.9/30.1 * 100 = 96%. This means that my setup will produce a speed that will be 4% faster than what my speedo reads. Say my speed is 65 MPH. 4% of 65 is 2.6, I would add 2.6 to 65 which would give me 67.6 MPH. With this setup, slower speeds will be closer. The faster you go, the more the speedo will be off.
Tire`````````Inches```65 MPH
-----------------------------
235/75R15````28.9`````Stock
255/70R16````30.1`````+2.7
245/75R16````30.5`````+3.6
For example: My stock tire is 28.9 inches. Say I get a 255/70R16 which is a 30.1 inch tire. You divide stock tire by the desired tire and multiply by 100. 28.9/30.1 * 100 = 96%. This means that my setup will produce a speed that will be 4% faster than what my speedo reads. Say my speed is 65 MPH. 4% of 65 is 2.6, I would add 2.6 to 65 which would give me 67.6 MPH. With this setup, slower speeds will be closer. The faster you go, the more the speedo will be off.
Elder User
Joined: Mar 2004
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From: Houston
One thing you need to understand about the speedometer reading being off, as your speed increases you milage is off more. In other words you might be 3 mph off at 35mph, 4 mph off at 45mph, 5 mph off at 55, ect. I don't know why it works like this, but that is how it works, at slower speeds you are not off as much as you are at higher speeds. Anyway just thought I would throw my 2 cents worth in on the subject -
Waves
Waves
Elder User
Joined: Mar 2004
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From: Houston
I have to disagree with you it's not a fixed value Ken, I don't know why it works that way, but that is how it works, call any speedometer shop and they will verify what I am telling you. I have had four trucks all with over sized tires and that is how every one of them has worked. The faster you go the more off your speedometer is, that is just the way it is -