1971 Chevy Pickup revs too high

[chazdo2]

Before I start thanks for all of your patience for us new guys!!! I posted before my truck wasn't shifting into 3rd gear. I was wrong its working fine. Its just that it runs about 4 grand when going around 55 mph. I am assuming my rear end is geared too low. How do I confirm this and what can I do to resolve this? Truck runs and idles fine.

[Grumpy old man]

Need more details ,what engine ,what model truck ,what trans, what rear differential ,what size tires ???

[TwoFiftyShifter]

Your transmission is slipping? Even 5.13 gears wouldn't be 4000@ 55.

[joeydurango]

Check the transmission fluid, Step #1.

[chazdo2]

350 chevy 3 speed automatic. 30 inch tires. The transmission doesn't feel like its slipping.

[chazdo2]

checked the fluid when it was in gear per instruction. Fluid looks good and has good level

[chazdo2]

how do I find out what differential gearing I have?

[TwoFiftyShifter]

You need to search that one man!

[rcazwillis]

Are you confident you are shifting into 3rd? With a 30" tall tire and 4.10 gear, 2nd gear at 4,000 rpm would 56 mph.
My favorite speed calculator:
http://www.catherineandken.co.uk/sti/tyres.html

[chazdo2]

I can down shift going slow from drive into second and first and can feel it shift twice the same as starting in drive from a start and feeling it shift twice. Sorry for the caps too lazy to change now.

[Ironangel]

Read this, http://www.crankshaftcoalition.com/w...ar_ratio_check and this, http://www.how-to-build-hotrods.com/gear-ratio.html The best method is to pop the cover, (have a new cover gasket ready to install as well as two quarts of new 80w-90 gear oil with a GL-5 API rating) and read the number of teeth stamped on the edge of the ring gear. Such as 14 43, divide the the large number by the smaller number to get the ratio. In this case, this ratio is a 3:07...By popping the cover you take the guess work out of the equation and you determine whether or not you have a positrack as well as changing the gear lube that God only knows the last time it was changed.~Ghostrider~

[rcazwillis]

To get 4000 rpm you would need a gear ratio in the 6.5:1 range. Chances are slim that you have a gear of that nature. Are you confident the tach is accurate? Does it have adjustments for 6 cylinder vs 8 cylinder?

[joeydurango]

You can also jack up the rear and rotate a wheel, count number of turns at wheel vs. driveshaft, and do the math to figure your ratio...

[GASoline71]

You said "about 4 grand". Do you have a tach in the truck?

I'm betting it's cruising at around 2,500 to 2,800 RPM's and you're just used to modern vehicles with Fuel Injection and Overdrive transmissions That cruise at 70 mph at 1,800 RPM's.

These old heaps can cruise at 3,000 RPM's all day long and not even give a rip.

Gary

[chazdo2]

Its the stock dash tac. The engine is from the 90's. Found that out when I replaced the starter! Its a 350. Ill put a strobe on the motor and see if the rpms are accurate. Thanks you guys.

[chazdo2]

Thanks great info looks like I have 410 gears in the rear end. Two full tire rotations were a little over 4 revolutions of the driveshaft.

[demian5]

My impala has a flex fan and no fan clutch and it sounds like its screaming 5k at 2500...

[n33k0]

My truck with 4.10 gearing and a TH350 screams at around 3k rpm at 60. At first I was afraid to go over 60 because I thought it would hurt the motor, now I'm afraid to go over 60 because it will hurt my wallet.

[chazdo2]

just strobed it tacs pretty accurate. Found a pretty good tranny leak. Fixed it and will try it soon.

[demian5]

Originally Posted by n33k0

My truck with 4.10 gearing and a TH350 screams at around 3k rpm at 60. At first I was afraid to go over 60 because I thought it would hurt the motor, now I'm afraid to go over 60 because it will hurt my wallet.

My son's first car is an 86 Scottsdale with a 350, TH400 and 4:10. Its hurting MY wallet...

[Ken B]

I have 410's in my 70 Longhorn with a 350 and a 4 speed. I usually keep it at about 2600 while on the highway for extended period's. I'll bump it up to 3 grand for a short burst every now and then but that is when the fuel gauge really begins too drop like a stone. I know for a fact the original owner of my truck was going to work it and he wanted the 410's. It was real easy to do the speed limit with a full load with 410's.. Back in the day 55 mph was the rule of the day and I think either most people didn't push the envelope as much or they enforced the speed limit more???? Its happens almost every day now that a freight train of cars blow by me doing at least 85... If you want to spend a considerable amount of time in the left lane you are going to need a gear change... Or... just do what I do, sit back and enjoy the slow ride....

[davepl]

Silly question perhaps, but it's not a Pertronix III ignition is it? They can fire multiple times per piston event but don't have a tach wire so if you're hooked up to the coil for your tach, it can read double actual RPMs at times.

[webfoot]

Originally Posted by demian5

My impala has a flex fan and no fan clutch and it sounds like its screaming 5k at 2500...

THIS.. my truck has a flex fan and I thought the same thing and I've been driving it around in 3rd going 25.. hooked my tach up and it sounds like it is screaming at you at 3K.

Clutch fan is on its way!

[Frank_Zappa]

I thought that my 1971 K20 was reving to high also. Then I bought a aftermarket tachometer and found out that my stock in dash tach from 1971 was just wrong by about 600 RPMs.

Also I found this website very helpful, just put in your tire size, tranny type, and rear end ratio and it gives you exact numbers for what your RPMs should be. http://www.grimmjeeper.com/gears.html

[chazdo2]

Thank you!!!!