The 32.5" BFG M/T KM's & Truxxx Fit Great!!!

[Chris03HD]

I'm from Toms River over by the Home Depot on Rt9.

My buddy is from Freehold.

Thanks.

[ericrwalker]

Your Speedometer is not off by 4.8 MPH, it's off by a percentage.

The faster you go the more it will be off. Test it with a GPS then you will know what I am talking about.

Originally Posted by Chris03HD

Thanks.

Yes, as posted above already, it was 4.8mph slower then the actual speedometer.

[Chris03HD]

Thanks Eric, will do.

I should have said, "at 60mph on the speedo, I'm actually going 55.2mph."

I'll have to tell him to keep the ever changing percentage in his head while he drives.

I know with my Harley truck, I can use the hand held programer to change the tire size, and it makes the nec. changes in the PCM.

[ericrwalker]

Originally Posted by Chris03HD

Thanks Eric, will do.

Example, at 60mph on the speedo, I'm actually going 55.2mph.

I'll have to tell him to keep the ever changing percentage in his head while he drives.

Yeah so at 60MPH you are off my 4.8 MPH.

So when your speedo says you are going 5 MPH I am sure you are not going 0.2...You are probably going 4.6 MPH.

And when the Speedo says 80 MPH you are probably going 73.6 MPH

By the way, the tires look cool. Really gives it an aggressive look. What's it cost for tires like that?

[Chris03HD]

Thanks Eric.

I can do the tires for pretty cheap. Pm me if interested in a set.

[Chris03HD]

Originally Posted by arteegee

Thanks for forgoing the white letters.

It's actually a black wall tire.

[zuhl]

How is that possible that you are going slower than your speedometer with tires that are considerable larger than stock? I mean, it just does not compute. I believe you, but it does not make sense how that could happen.

For every one turn that the wheel sees, it is registering the distance travelled as the circumference (3.14 * 29.5 = 92.67 in.) of your factory tires. Your circumference (3.14 * 32) is larger than that by 8.4%, so at 60mph on you speedo you should be doing roughly 65 mph in reality. My error with 31.4" tires is about +7.5% as measured by two different GPS systems. You should have to get smaller than factory tires to accomplish what you have reported here.


Was your speedo dead on before the tire swap?

[Chris03HD]

Here's an example off a tire sizing calculator comparing the factory rubber to the BFG size rubber now on the truck.

245/65/17 @ 60mph
255/75/17 will now be 55.20mph

Speedometer Difference: 8.537% too slow

Diameter Difference: 7.87% (larger then factory diameter)


This means you are actually going FASTER then your speedo is showing.

[roguemoog]

Originally Posted by zuhl

How is that possible that you are going slower than your speedometer with tires that are considerable larger than stock? I mean, it just does not compute. I believe you, but it does not make sense how that could happen.

For every one turn that the wheel sees, it is registering the distance travelled as the circumference (3.14 * 29.5 = 92.67 in.) of your factory tires. Your circumference (3.14 * 32) is larger than that by 8.4%, so at 60mph on you speedo you should be doing roughly 65 mph in reality. My error with 31.4" tires is about +7.5% as measured by two different GPS systems. You should have to get smaller than factory tires to accomplish what you have reported here.


Was your speedo dead on before the tire swap?

Exactly. Think about two cars equal in every way from the factory. Swap out the tires on one for larger tires, with a larger circumference. Take those two cars and have them do the EXACT same speed - the one with the larger tires will register LESS revolutions per any distance X than the one with original tires. A larger circumference means to cover X distance, you have to turn less revolutions. If LESS revolutions are measured in the driveline for any distance X, what do you think the speedometer in the car with larger tires is going to do if it never got told the tires changed size?

[roguemoog]

Originally Posted by Chris03HD

Here's an example off a tire sizing calculator comparing the factory rubber to the BFG size rubber now on the truck.

245/65/17 @ 60mph
255/75/17 will now be 55.20mph

Speedometer Difference: 8.537% too slow

Diameter Difference: 7.87% (larger then factory diameter)


This means you are actually going SLOWER then your speedo is showing.

No, your speedometer is showing a slower speed than you really are doing, so you're going FASTER than it's showing! Think about it!