Does anyone knows if you are running a 3.76 RMM LSD, That you notice that car is going faster than it really is, or is the speedometer affected at all??

From my understanding- it shouldn't affect the speedometer because it raising engine speed faster..
But-I was having a conversation with a buddy with a mustang and he said when he change his rear end up to a 3.73 he was going like 7 to 8 Mph faster on the instrumentation, like for example-Speedometer says hes doing 80 but in reality he's doing 72....

Could this be true?


QWK

 

With the 3.76 ratio, you are actually lowering the gearing by approx. 15%. You can translate that to your speedometer registering 15% faster speed than actual.

Another way to explain it is by lowering the ratio, you effectively decreased the tire diameter. Now that you have "smaller" tires, your speedo will indicate 69mph if you're actually traveling 60 mph - that's assuming a 15% change in the ratio.

 

Thanks for the info.. That sounds like it makes sense.
I was wondering since i went from 17's to 19 inch rims.
Thought my rolling diameter is almost stock, I would figure the added size up in the rim would compensate some what in the speedometer?

But, I notice before and after on the HWY, when cruisung at 90mph it was around 3000 RPM Stock(3.27)
Now-81 or 82 is 3000 RPM w/(3.76)-So is this my real speed or am I going 73-74 MPH?

Kinda confusing - depends on which way u think..

Thanks,
QWK

 

It won't affect speedo, which comes off speed sensors on the wheels.

It WILL affect rpms at any given speed and gear (compared to stock).

 

Thats what I thought! But Hey, I 'm trying to get everyones opinions/knowledge and come up with the right facts..

Anyone Else? got their 2 cents...

QWK

 

I had 3.42 gears in my '97 Z28 and when I went to 3.76 my speedo was off by almost 10mph. It showed me going faster than I was...

 

Right, but that's a Z28 which I'm sure doesn't have electronic wheel speed sensors.

 

I think Bit is correct since the speed sensors, which I forgot, are at the wheels. Now, you only need to be concerned of the rubber size in your larger rims to make sure that it has the same diameter as the stock wheels (16s) to get a true speed reading.

 

Well absolutely, I did not know about the wheel sensors!
Our cars are so cool..Anyways I went to tire rack to get the information on how to caluculate tire diameter.. And The results are great for us who have upgrade wheel plus size.

According to tire rack the formula is :
For example-We will do the stock 16's first.

225/55-16 (Factory 16's) - take the outside # 225 (in mm) and Multiply the tire height : 55, so its 225 X .55 = 123.75 , remember the tire is made up of 2 sidewalls..
Then you take 123.75 X 2 =247.5
Now you have to calculate wheel size (25.4 X (wheel size))
so 25.4 X 16 = 406.4
Now you take 247.5 + 406.4 =653.9 divide that by 25.4, and you rolling diameter is 25.744094

225/55-16 Calualations : 225 X .55 = 123.75 X 2 = 247.5 + 406 which you got from (25.4 X wheel size) =653.9/25.4 =25.744094

245/35-19 : 245 X .35 = 85.75 X 2 = 171.5 + 482.6 = 654.1/25.4
Equals = 25.751969

Thats very close I mean by 0.007875 - Thats right on the money!!

And The 17's are aways off - So for those of us with 17's from factory optional wheels- We were off and didn't even know it..

Here are the numbers for the 17's with 235/45-17
235 X .45 = 105.75 X 2 = 211.5 + 431.8 = 643.3/25.4
Equals = 25.32677

I couldn't believe it when i saw it..But numbers don't lie, so when you upgrade you are actually going to exact factory specs! COOL huh?
Check out the difference between stock 16's to the 17's
=== 16's: 25.744094 - (minus) - 25.32677 (17's) = .417324 OFF!!! of original specs!!
Thats why our cars ran so damn good w/ those 17's..But the Speedometer was off using 17's...

Well check my numbers, and judge for yourself..
Thanks Everyone,

QWK

 

This page on my web site is a bit of a mess but might provide useful info to someone.

Tire/rim size chart