25 mph @ 2500 rpm in first with 3.73 and white gear?

TheUser · Oct 10, 2004

I supposedly have 3.73's in my GT and it feels like it and i've rotated the tires and counted driveshaft rotations and it was 3.73, but my speedo says 25 mph at 2500 rpm in first and I just changed the speedo gear to the white one. I used an rpm calculator and it says I should be at 15 mph in first w/ 3.73's...what's up?

Ray@VSK · Oct 10, 2004

rpm calculator, where can I use this ?

TheUser · Oct 10, 2004

http://mr5oh.tripod.com/gears.html and he's also got a spreadsheet you can download and edit yourself

5spd GT · Oct 10, 2004

If your calculations are correct you don't have 373's...my 373's are somewhere around 4-4500rpm @ 25mph...maybe a bit more...

My 2000 GT had 327's (stock) and were at 3k at 20mph...

I would say you have 308's? or 273's...

I use the 23tooth white speedo gear (fror 355's) for my 373's...

This is all assuming you have the right speedo gear to accompany 373's...if so you don't have 373's...

TheUser · Oct 10, 2004

ok here's what happened...I bought the car and was told it had 3.73's and, as I mentioned above, I have counted and the driveshaft spun 3 3/4 times for one tire rotation. I just put in the white gear (from Jeep, but I've heard it works and is stronger) and pulled out a yellow gear. Before, it said I was doing 100 when I was actually doing like 60-70, so I figured it was normal w/o the correct gear, replace gear and now this happens: 1st gear 2500 rpm=25 mph, 2nd gear 2500 rpm=35mph, 3rd I think was 50 mph at 2500 rpm...this is nuts! Maybe someone put a really high first gear in the tranny ha ha ha. Are the tach and speedo adjusted from the same gear or do they get their readings from seperate places?

Michael Yount · Oct 10, 2004

The tach gets it's reading from the engine electronically - and if it's a factory tach, they're known to be quite inaccurate - sometimes registering over 7000 rpm before the 6250 ecu rev limiter cuts in.

The speedo gets it's input mechanically from a cable attached to the gear you've been changing out.

Don't bother trying to calibrate to your tach, or to low speeds in lower gears. Get out on the Interstate and compare your odometer to the mile markers. Also, run along at an indicated 60 mph and have a friend time between mile markers. They should come up every 60 seconds at 60 mph. However high or low you are (time-wise) is directly proportional to how far the speedo is off. On some cars, it's necessary to change the 'drive' gear inside the tranny to get a truly accurate reading. I've done it twice on mine - requires pulling the transmission to do it on my car.

5spd GT · Oct 10, 2004

TheUser said:
ok here's what happened...I bought the car and was told it had 3.73's and, as I mentioned above, I have counted and the driveshaft spun 3 3/4 times for one tire rotation. I just put in the white gear (from Jeep, but I've heard it works and is stronger) and pulled out a yellow gear. Before, it said I was doing 100 when I was actually doing like 60-70, so I figured it was normal w/o the correct gear, replace gear and now this happens: 1st gear 2500 rpm=25 mph, 2nd gear 2500 rpm=35mph, 3rd I think was 50 mph at 2500 rpm...this is nuts! Maybe someone put a really high first gear in the tranny ha ha ha. Are the tach and speedo adjusted from the same gear or do they get their readings from seperate places?

I'm guessing you have the wrong speedo gear if you know you have the 373's?

At 50mph my 327 geared 2000 was at 3k...in third gear

TheUser · Oct 10, 2004

the gear was the white one. I wasn't thinking clearly enough to realize it's probably just the stock tach off as Michael pointed out. I'll take it on the interstate and do it w/ mile markers and see if the speedo is accurate....I don't mind shifting by ear, I do mind not knowing how fast i'm going...at least within a few mph

5spd GT · Oct 10, 2004

TheUser said:
the gear was the white one. I wasn't thinking clearly enough to realize it's probably just the stock tach off as Michael pointed out. I'll take it on the interstate and do it w/ mile markers and see if the speedo is accurate....I don't mind shifting by ear, I do mind not knowing how fast i'm going...at least within a few mph

Has your tachometer ever been messed with? Guages?

I really doubt (if they haven't been messed with)...that they are off that much....

Does the 50mph your seeing on the speedo seem like 50mph...same with the 25mph?

TheUser · Oct 10, 2004

to be honest, I can't tell speeds very well...which is a main reason why I want the speedo to be semi-accurate. The sound also distorts the speed perception, so I have no clue and I calculated those results yesterday, which makes it even harder to remember what it felt like.

I would try now, but it's wet out and water+bald tires+3.73's=

ZelenVR · Oct 15, 2004

I just went up to the local Ford dealership today and picked up the gear I needed for my 3.55's and he looked it up on a chart that matched the gear with the size tires I was running and came up with it from there.

25 mph @ 2500 rpm in first with 3.73 and white gear?

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