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  #1  
Old 10-01-2019, 07:26 AM
FishinFreaks FishinFreaks is offline
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Default Bigger Tires - Adjust Speedometer?

On my 09 Ranger, I'm going from stock P235/75R15 to 30x9.5R15. How will this change my speedometer reading?

This truck is driven by my 16 year old, so I don't want to have a speedometer that's off. Can I tune the speedometer myself? I've read a couple posts here that talk about a guage you can buy, but what do I need? And how is it done?
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  #2  
Old 10-01-2019, 01:42 PM
dvrich dvrich is offline
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Default Re: Bigger Tires - Adjust Speedometer?

The speedometer will only be 3.9% off with the bigger tires, not enough to be a problem.
If you go 65 mph with the bigger tires, the actual speed will be 67.5 mph.
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Last edited by dvrich; 10-01-2019 at 02:43 PM.
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Old 10-06-2019, 03:58 AM
hat_man hat_man is offline
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Default Re: Bigger Tires - Adjust Speedometer?

Like dvrich said, 4% is nothing to worry about.

I wonder if an '09 has a way to re flash the computer to adjust for the new tire size?

Definitely not necessary though.
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Old 10-06-2019, 02:59 PM
sgtsandman sgtsandman is offline
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Default Re: Bigger Tires - Adjust Speedometer?

The speedometer will actually ready pretty spot on going from 235/75R15 to 30X9.5R15. Rangers speedometers tend to read about 2 mph faster than you are actually traveling. The size change will correct that.
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  #5  
Old 10-07-2019, 06:35 AM
dvrich dvrich is offline
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Default Re: Bigger Tires - Adjust Speedometer?

Quote:
Originally Posted by sgtsandman View Post
The speedometer will actually ready pretty spot on going from 235/75R15 to 30X9.5R15. Rangers speedometers tend to read about 2 mph faster than you are actually traveling. The size change will correct that.

If you use the stock tires listed on the door jamb, the speedo will be dead on. Thanks for the laugh.
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Old 10-07-2019, 07:38 AM
sgtsandman sgtsandman is offline
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Default Re: Bigger Tires - Adjust Speedometer?

Quote:
Originally Posted by dvrich View Post
If you use the stock tires listed on the door jamb, the speedo will be dead on. Thanks for the laugh.


Vehicle manufacturers are allowed a certain amount of inaccuracy in what the speedometer reads and tend to err on the reading fast side as a result. If you punch the numbers in on the 16” tires the 2010 and 2011 Sport models use, they work out to 30” tires. There has also been and article a time or two over the years on the subject. It has been found that none of the vehicle manufacturers have accurate speedometers. I imagine for scenarios like the Ranger where tire sizes range from 225/70R15 27”) to 255/70R16 (30”) and they don’t want to be held liable for the speeding tickets a person who buys their product might get. It’s easier to explain away a speedometer reading too fast than one reading too slow.

From personal examples, I find this to be true when comparing indicated speed to what a GPS tells me. My 1999 and Honda CR-Vs both read 3 mph fast. My 1998 and 2011 Ford Rangers read 2 mph fast. And my 2019 Ford Ranger reads 1 mph fast.

Now, whether you choose to believe what I’m telling you or not is up to you. It won’t hurt my feelings.

You could also choose to test it for yourself to prove or debunk what I am saying as well. Again, it won’t hurt my feelings. I’m confident you will find similar results.

But as also stated above, the variance from one tire size to another that the OP was asking is small enough that even if the original size was spot on, with the given buffer the police usually give, the difference ins’t enough to cause a speeding ticket in itself.


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  #7  
Old 10-07-2019, 10:13 AM
dvrich dvrich is offline
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Default Re: Bigger Tires - Adjust Speedometer?

The 2009 4x4 XLT Ranger 4 door ex cab should have 255/70/16 tires on it.
I looked it up.
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Old 10-07-2019, 11:08 AM
sgtsandman sgtsandman is offline
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Default Bigger Tires - Adjust Speedometer?

In 2011, that was an option. Mine came with 235/75R15 off the dealer lot. The Sport model came with 255/70R15 stock. I can’t speak for 2009.

In any case, the speed settings seem to be set for the largest tire option and not adjusted for the smaller sizes. One setting means less time in building the truck and less money spent by Ford in manufacturing an truck they wanted to be done with.


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  #9  
Old 10-08-2019, 03:27 AM
dvrich dvrich is offline
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Default Re: Bigger Tires - Adjust Speedometer?

It seems the OP doesn't care so why should anyone else.
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Last edited by dvrich; 10-08-2019 at 03:57 AM.
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  #10  
Old 10-19-2019, 08:52 AM
hat_man hat_man is offline
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Default Re: Bigger Tires - Adjust Speedometer?

Or you could just say "I still don't think you're right but I'll have to check it out for myself."

If you want to debate the point then great. It makes for more information for everyone to use. But to say someone may be incorrect and then, when called out on it, lay it off on the OP is a pretty weak exit strategy.

The OP seems slightly concerned. It's fairly obvious he doesn't want his 16 year old new driver to start his driving career with a ticket. He may have been thinking the speedometer was going to be off by 5+ mph. You gave him a great answer and it probably made him feel better about the situation. It's too bad that when someone else tried to add information to the conversation it had to devolve.

Either check it out with a GPS for yourself or agree to disagree. But take the high road. Even saying "Your Results May Vary" would be better than throwing the OP under the bus.
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Little Black Ranger: R.I.P Born-1997 Died-2018. 297k miles of good times. You will be missed.

Oh Deer: 2003 XL 2.3L 5-speed stick w/OD. Current mods: Body work after hitting a deer 12 hours after I bought it. Averaging 32 MPG on the highway.
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