Speedometer seems off at higher RPMS/Speeds?

[justinschmidt1]

Ok, so when I calculate my speed with my 27 inch tall tire, 3.73 gear, and 3rd gear ratio of 1.00 it says I should be going about 129 MPH at 6,000 rpms, 125 mph at 5800 rpms.

Ive always notice that once I get above about 5000 rpms my speed always seems to be lower than it should actually be compared to rpm.

For example: I looked down today and was at 5900-6000 rpms and was only going 120 mph according to my gauges.

I dont know whether to think my rpm gauge is off or my speed is off?

I just dont understand how I can be going like 10 mph slower than I should be.

 

[mygreengt]

sounds like somebody has a heavy foot

 

[MustangChris]

theres a webpage that does this math for you... maybe you forgot to carry the 1 when you should have subtracted the tangent from the remainder??


at 3.73s, there is no accurate speedo gear for the transmission.
also, at the tall tires, it throws another monkey wrench in the situation by slowing your speedo.

i assure you, if you did the math properly, chances are your math is correct but your speedo is off.

has your car been on the dyno (i know it has, lol) did the tach line up with the computer? does it idle at the proper RPM?

im not 100% sure how the rpm gauge works, but if it runs off the crank sensor, thats a 4 bolt fix if you want to replace it.

 

[justinschmidt1]

theres a webpage that does this math for you... maybe you forgot to carry the 1 when you should have subtracted the tangent from the remainder??


at 3.73s, there is no accurate speedo gear for the transmission.
also, at the tall tires, it throws another monkey wrench in the situation by slowing your speedo.

i assure you, if you did the math properly, chances are your math is correct but your speedo is off.

has your car been on the dyno (i know it has, lol) did the tach line up with the computer? does it idle at the proper RPM?

im not 100% sure how the rpm gauge works, but if it runs off the crank sensor, thats a 4 bolt fix if you want to replace it.

I used a calculator online and it took into account that my tires were an inch taller and everything.

The math is right...even with a 26 inch tall tire I should still be going about 125 at 6k

The speedo gear I had usually is supposed to read high I thought...but the 27 inch tall tires evened that about out.

I dunno wether the speedo is off, the rpm gauge is off, or if Im really not going that fast. lol

sounds like somebody has a heavy foot

lol....I do

but it takes me all of a few seconds to get from 75 to 120 mph

 

[duff daddy]

have someone do a marked mile and then you do it per your odomiter see how far it is off ine feet then convert it.

 

[CC'S95GT]

gps's are pretty accurate. or at least close enough to tell if you have a prob with your speedo.
as for your tach, some of your better scanners can read rpm's.

 

[Win]

at 3.73s, there is no accurate speedo gear for the transmission.

there is you just have to look for it. in the 7.5" rear end a 3.73 gear was offered. You should be still able to go to ford and get the speedo gear.

 

[96_SVT_Cobra]

sounds like ur car is just too fast for ur speedo to keep up

how are u liking the vortech

 

[Paul]

It's very common for the speedo to be way off at high speed.

 

[justinschmidt1]

It's very common for the speedo to be way off at high speed.

Ive heard this before....but do you think the speed is off or the RPM? lol

sounds like ur car is just too fast for ur speedo to keep up

how are u liking the vortech

Thats probally it haha...umm Im liking it for the fact that my belt stays on track and it hasnt blown up yet. lol

My procharger did sound quite a bit cooler though.

As far as power I swear my procharger moved more air mid range than the vortech but I did make a bit more power on the dyno with the vortech.