How fast am I going at this rpm?

[1981silverz28]

The speedo in my car is bad wrong and I was wondering if there is a formula for figuring out speed by the rpm's turned? I have a 25.4" tall tire and at 55 mph I'm turning 2500 rpm with a 3.08 gear.I got the 55 mph figure from running beside my buddies bone stock truck for about a mile(couldn't get him to go any faster).I want to know ,how fast am I going at 5000 rpm? Oh yeah I have an auto if that makes a difference.Thanks

[astroracer]

Here is a free calculator...
http://locost7.info/gearcalc.php
I haven't tried it so let us know what you find out if you do...
Mark

[1981silverz28]

Thanks, I loaded that and it shows that I'm running about 135 at 5500 rpm.That's a little faster than I would have guessed by not by much.Again thanks for the help!!!!

[sinned]

You are actually not going that fast. The calculator assumes a whole bunch of things, not the least of which is that the torque converter is locked up with zero slippage which is most certainly not the case in a 1981 F body.

[1981silverz28]

Yeah ,I kinda figured I was going 105 / 110 or so.With all stock suspesion and such it feels like you are really flying.Not real fast,just real fun. Thanks for the help.Oh yeah ,I have a 200r4 w/lockup ready to go in as soon as I get time.That should help out a little.

[graycamaro]

go to fbody.org they have a calculator on there that will map out every gear and rpm possibility.

[protour_chevelle]

Yeah ,I kinda figured I was going 105 / 110 or so.With all stock suspesion and such it feels like you are really flying.Not real fast,just real fun. Thanks for the help.Oh yeah ,I have a 200r4 w/lockup ready to go in as soon as I get time.That should help out a little.

Hahah, tell me about it. 30 year old suspension doin 135km/h on the way home.... Its like your floating and taking up both lanes.

-Matt

[1981silverz28]

Yeah it feels great as long as you are the only one on the road

[sinned]

go to fbody.org they have a calculator on there that will map out every gear and rpm possibility.

But still doesn't take into account for torque converter slippage or drivetrain losses for worn out parts, so it isn't accurate.

[Fuelie Fan]

at 5500 rpm slippage should be less than 10%, meaning if the calc'd speed is 135, i would expect no less than 122.
Worn out drivetrain parts will consume more power but not lower your speed (unless of course your engine can't make up the lost power, but in that case you would never reach 5500 rpm). In other words, a set of 4.10's does not ever become 4.20's from wear. The exception i guess would be slipping clutches within the transmission, but if this is happening you better get a new one SOON.
A tire's rolling radius is different than its measured radius, due to deflection of the sidewall (which will vary with speed and load), but the effect is negligible for what we're talking about.

[sinned]

The exception i guess would be slipping clutches within the transmission, but if this is happening you better get a new one SOON.

Exactly, you may be suprised by how many folks run transmissions that slip quite a bit and never think twice about it. Torque converter slippage in a high mileage stocker is pretty high too, especially once you put some heat in them and get the RPM's spinning.