Speed vs Tachometer
 

I could really use your guys input. When you are running down the highway at 70 mph, what does your tachometer read for the rpms? I've got a 98 GMC Sierra with the 5.7L and a rear end ratio of 3.73. Running down the interstate my tach reads 2500 rpm when my speedo is at 70 mph. I just wondered how that compared to other people because it seems to be a little high on the rpm side. If people could post what thier trucks speedo vs tach says that would be great. Thanks.

That does seem a bit high. Are you sure the truck is in overdrive? I can't say that I really pay attention to where mine is, but I don't even think my 4.10 rear end equipped truck tachs that high at 70. I am not sure how the 4L80E which I have compares to your 4l60E in the RPM department. I will check tomorrow and see exactly though.

i have a 94 and when im goin at abotu or more then 70 on the interstate it normally runs at 2000 in drive my overdrive craped out so ive been driving in drive until i can get it fixed
but 2000 is what mine normally runs at with my foot on the pedal if i set the cruise its around 1500 without cruse and i just coast at 70 without any foot on the gas it drops low as it should
mine also is a 5.7l
with the 4l60e trans

The truck is in overdrive at that speed and rpm. Could a bad fan clutch be causing my problem?

im not sure of that but i got to thinking the other day that the throttle kickdown cable can cause some wacky stuff if its not working properly

My 5.3 tachs 2100 at 70. I also got the 4l60 tranny and the 373 rear end, just the different motor. Are you sure your tach is not faulty?

Mine reads 2500 too, (4L80) but I may very well have had 4:10's installed.

Is there any way to tell if I have 4:10's or not? The previous owner didn't know and the glove box says GT4 (3:73)... but the rear has been opened and resealed, I get ****ty mileage, I have tons of power, and 80mph is close to 2800rpms.

Sorry to hijack your thread but I was actually thinking about this earlier today

i would say the only way to really truely check to see what you have is hate to say this but open the rear diff cover and see as you say its been opened before
thats the only way i know of how to check maybe there is another way

I think you can check it by jacking up your truck, put it on jack stands, put the truck in nuetral with the parking brake off, mark a spot on 1 of your tires, have some one turn your driveshaft by hand until the spot on your tires goes completely around until it returns to the same spot. Now I can not remember if you multiply or dividee the turns your driveshaft makes by a certain number. Mabe a tranny/ rearend shop can tell you how to do it with out opening it up. I do not remember it has been a long time ago and an old tranny old timer told me how to do it.

I could be wrong, but could i draw a line on the tire and one on the driveshaft and see how many times the tire goes around per drive shaft turn... I'd imagine it'd be easy to tell the difference between 4.1 turns and 3.73 turns...????