Can someone explain this to me?

[RangerRocker]

Ok, so I had the stock tires for my truck for a while. 235/75/15. I recently lifted my truck and put on 33x 12.50 tires. I have 4.10 gears. And for some reason, my speedometer is dead on, and my truck still revs at 2500 doing 65 down the highway. According to the tire and geat chart, it should be lower than that. Any ideas?

 

[Big Jim M]

Dude... Ya have to get another vehicle and tell it to set the cruiz to 65mph.. Then you follow closely to see what your actual speed is.
Ya see... the engine and speedometer are using the tranny to tell the mph...not the tire size. So you are NOT going as fast as the speedometer says.. You are going FASTER.
Big JIm

 

[Viper Command]

Dead on big jim. You can ask a cop to use a radar gun.

 

[Andres629]

What I did was found one of those little things on the side of the highway that tells your speed as you go past it, and matched that up to the speed on my speedo. Voila! Problem solved. At least, in my case.

 

[RangerRocker]

Alright, thanks guys. Thats what i figured. I did what Andres629 said, but I did it at 35 mph. Haha. It was dead on then.

 

[Big Jim M]

Most likely

Alright, thanks guys. Thats what i figured. I did what Andres629 said, but I did it at 35 mph. Haha. It was dead on then.

NOT! You were prolly about 3 mph faster than the 35 the sign said. But it was hard to make out on the speedometer.. The gain in speed increase is incremental.

You can use the tire circumference to know exactly how much faster you will be at any speed.
Of course the circumference is in inches and the end result needs to be in MPH but there is a formula somewhere to use. I can do it but don't how to write the formula.
Big JIm

 

[Scotty J.]

GPS is your friend

 

[Bob Ayers]

GPS is your friend

+1

Check my speed out here:

 

[master hec0]

i use a garmin nuvi to tell my correct speed my bronco is the only vehicle with correct speed all my other vehicles are off by at least 5kms

 

[dennis461]

tranny type?

Auto or manual,
what gear were you in.
Based on the numbers you posted, I calculate 60 mph road speed.
It will be lower if automatic, assumed 1:1 ratio., higher with overdrive and manual.

Remember, the speedometer and tachometer ratios were NOT changed, so they will always match. (until you change the gears in something, like axle.)


For example.
Take your original setup, put the rig on jackstands and you will find 2500 RPMs match 65 MPH on your rig.
Change the tires, and repeat.
They will still say 2500 RPMs at 65 MPH.

And your road speed is ZERO (because you're on jackstands.)

 

[babytruck]

GPS is your friend

X2