Online mph/gear calculators accurate?

[justinschmidt1]

Whenever I calculate my Rpms vs my gears/tires and mph it always seems like my car is going slower at that rpm than it should be.

for example, with a 27 inch tire and 3.73s in third gear with a 1.00 ratio I should be able to do 129 mph @ 6k, yet it doesnt seem accurate.

Any reason why this might be?>

 

[auto_x5.0]

Does the calculator take into consideration the weight, wind drag, etc? All the othe vairables

 

[lutter94]

how fast are you going at 6k in your top gear? It should be pretty close to your top speed which would be up there somewhere....you still need to realize that drag will use up some power and have a higher rpm/slower speed as a result

 

[blown98gt]

yeah i'm getting you at 129mph but the key to remember with this is that it's suppose to be in 4th gear. if you are using the formula
(tire height X rpm) / (336 X gear ratio X trans ratio) = mph

and considering that your trans ratio is actually 1 because if it's not 1 that could throw the equation also.. anyways keep in mind it's not a 3rd gear formula from what i understand. someone can correct me if i'm wrong though

 

[CC'S95GT]

I thought 4th gear was 1:1.
I don't think it matters what gear your in. only that you get the ratio right.

 

[justinschmidt1]

3rd gear with the auto is 1.00

 

[CC'S95GT]

K i was thinking you had a 5 speed.

 

[justinschmidt1]

nope, stalled auto...

Its weird....maybe the RPM gauge is off, I dont know.
just seems like my 3rd gear mph is lower than it should be.

it says i should be doin 108 mph at 5k in 3rd gear so Ill check but im pretty sure its not that fast...maybe more like 100

 

[CC'S95GT]

Stock tachs are notoriously inaccurate.
you could plug in a good code reader to get the actual # to the puter