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Tire size affect speedo?

Mikes72

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I’ve been reading through a bunch of posts about changing the tire size on our cars to wider tires but keeping the same aspect ratio ( like from 215/60 R15s to 255/60 R15s). Has anyone seen a change in their speedometer reading due to these types of tire changes? Seems like if the new tire results in a larger diameter the speedometer would show a slower speed than actual. If so, what do you do to solve this?
 

hofilbert

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Uh....yes. Us oldfarts have known of this since way back when these cars were new. They make speedo transmission gear ratio adapters but I've never had much luck with that. They will get you close if you upsize your tires. You're best bet is to keep the same O.D. (Outside Diameter) on the tires. You can change to huge 21" wheels (please don't) and if you keep the OD on the new tire the same as factory OD tires, you'll have perfect speedo measurements - charts are available online. Or, put on some behemoth rear tires, and find a digital radar speed testing site and check your speed and remember that when you're speedo shows 50, you are really going 10 mph faster.
 

Rapidfire

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Yes tire size diameter will change the speedo reading! You will need to change your transmission speedometer driven gear accordingly!
 

Mikes72

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Thanks for the confirmations guys. It is as I suspected. I’ll pay attention to the outside diameter when I get my new wheels and tires.
 

Dodgeboy

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Another easy way to figure out what size speedo gear you need is to calculate the percent error from your indicated speed by your actual speed. That is the percent change required in tooth count. Add that to your current speedo gear. I am in the middle of doing this myself.
 

Rons340

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Here is something else you might use.
 

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fastmark

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I change gears for a customer and going by those charts did not work well for me. I used on of those phone apps on my phone to get the right gear. It was no where close with the charts using the diameter of the tire.
 

Cuda Hunter

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So, I'm a big speeder. I get my max level of speeding tickets yearly.
One year I got a speeding ticket that topped out my points at 18. I had to get a lawyer to keep the license that year.
Anyway, before I got the lawyer I wanted to prove that the speeding was due to having larger tires on my jeep.
I took the jeep, with 205/75/15's to a dyno shop and got the readings. I then installed 31" tires on the jeep (won't turn as they rub) but they work great straight on a dyno machine.
After changing to those tires I could not come up with a difference in speed that would help me in court. 1 m.p.h. basically.
The lawyer got me out of everything for a small fee.


I have a suburban that I upped the tires size and it is 7 m.p.h. higher than the stock tires. 225 to 275. Funny it did it to the burb but not the jeep.

Just to say it, speeding is the only way I have ever found to "buy" time. Some years I drive 30K miles and if I save 15 minutes per day that adds up.
 

Dodgeboy

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So, I'm a big speeder. I get my max level of speeding tickets yearly.
One year I got a speeding ticket that topped out my points at 18. I had to get a lawyer to keep the license that year.
Anyway, before I got the lawyer I wanted to prove that the speeding was due to having larger tires on my jeep.
I took the jeep, with 205/75/15's to a dyno shop and got the readings. I then installed 31" tires on the jeep (won't turn as they rub) but they work great straight on a dyno machine.
After changing to those tires I could not come up with a difference in speed that would help me in court. 1 m.p.h. basically.
The lawyer got me out of everything for a small fee.


I have a suburban that I upped the tires size and it is 7 m.p.h. higher than the stock tires. 225 to 275. Funny it did it to the burb but not the jeep.

Just to say it, speeding is the only way I have ever found to "buy" time. Some years I drive 30K miles and if I save 15 minutes per day that adds up.
I was told a long time ago that going 20 over for 20 minutes would save you 5 minutes
 

Cuda Hunter

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If I go 60 miles per hour at the speed limit then 60 miles it is.
If I go 80 Mph in a 60 zone i get to the location 20 miles earlier. Meaning I just saved about 20 minutes. Something like that.
I have to drive 4 hours today for work, I'll make up about 45 minutes (if not more) of that time by speeding.
Sure, getting pulled over takes about 20 minutes and that does add up. But after thousands and thousands of mile between pull over it's well worth the 20 minutes to bull **** with a cop anyway. Surprisingly enough I tend to get a warning every other pull over. At least there are some nice things about getting old.
 
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