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...????

@gmcsierra fan thats what i was thinking also after some one suggested make a mark on the tire in a post above
maybe that would work maybe not

I would tend to agree with you, which is what I was basically saying, but when we tried it with my dad's 90 model chevy; it was hard to figure out. We were trying to determin if it had 373s or 342s. It was so close it took us numerous tries. Ended up using the vin number since the rear end had never been gone into. Tried it and see.

lol i know according to my glove box numbers i have the 3.42 gearing in the rear end but i havent looked to see if its been opened up before or not
as i want to try to figure it out so i know because its driving me crazy not knowing

anyway how did you come across finding out what it was

I got mine from a web site that was able to tell me everything about my truck by the vin number. It had all the manufacturer's codes and was able to tell me every option the truck left the factory with. It also determines how many trucks of a certain year rolled at of the same factory that were identical to the one I have. It was pretty cool. I will try to find it and let you guys know.

i worded it wrong i ment to say if anyone knows the ratios and math to figure it out cause i really dont wanna open my trucks rear diff up
also i used a website that told me everything
but it said i had or have 3.42 gearing in the rear diff
but i am thinking it maybe different as when i am in a slippery spot both my rear tires grab and start to spin
if it was the stock 3.42 it would be a one legger

and for the website there is an old old old forum on here some where that has the link thats where i found the one i went to

It's still possible that you have 3.42 gears. Most of the trucks rolled of the line with a limited slip diff. Especially the ones with V8s and towing packages. That's why your truck isn't a one-legger

your probably right although i dont have the tow pakage i just have teh basic 94 4x4 pakage and i took a look at my rear diff and it doesnt apaer to have been removed at all
so someday i will be getting 4.10 gearing in there and also in the front as well

Since we have thoroughly discussed gear ratios now, are there any other ideas about what could be causing my problem with higher than normal rpms? I also noticed that at 50 mph in overdrive my tach reads 1900 rpm. Also, I get better fuel mileage in stop and go city driving than I do out on the interstate. Any ideas would be greatly appreciated. Thanks

i cant really think of anything else other than maybe a trans issue or throttle problem