Technical Help! C10 won't go 65 mph!

I need help, I've got a 66 c10 with a 250 straight 6, ****** is the stock 4 speed, granny low, and it's got a 12 bolt rear, 3.07 gears, tires are 215/70/r15's I can only go about 45-50 mph in this thing and the rpm's are up there pretty high, I picked up a T-5 ****** out of an 89 S10 but after doing some research I found that the 5th gear ratio is .86, that doesn't seem like it's gonna be much of an improvement, not worth the time and money that it would take to put it in just to get an extra couple of mph. Is there another transmission that will bolt up to my straight six that will get me up to highway speeds?

 

Something else is wrong.

You might have a slipping clutch, or you don't have 3.07's.

You should be at 2500 rpm at 65.

 

I just put the rearend in 3 weeks ago, I took the cover off and checked the gears so I'm sure they're 3.07's, if it is the clutch wouldn't I feel it slipping, in all the gears I mean?

 

what are the RPMs at speed? do you have a gauge or just going by sound?

as stated, there are only so many things that it can be, certainly sounds like a slipping clutch but I would think you would smell it after driving it a while.

 

If you have a GPS,it should have a speed indicator on it,maybe you speedo is reading wrong with the new rear end?. I had a couple f those old trucks,they werent speed demons,but should do 65 easily. Harvey

 

Most transmissions the final gear ratio in high gear is one to one. Most chevy trucks has 410 rear gears from the factory. The three speeds you could wind them thirty in low and 60 in second. Change the rear gears and not recalibrate the speedometer and it would still show 30 in low and sixty in second. My daily is a 66 GMC with a 250 six SM420 four speed and 336 gears. And its plenty long legged. It will run faster than I care to drive it. I can pull my stock trailer 70 MPH no problem.

 

Take the emergency brake off.

 

66 half ton Chevy trucks usually had 3.73 gears from the factory.

Tell us about how you checked the rear gear ratio. And also tell us what you've done to confirm the ratio is what you think it is (you can do this relatively easily, by turning things and counting turns)

How is the engine running? does it feel peppy in lower gears?

 

Take it out on a 4 lane hi way with a friend in a modern car running along side you, and check your speedometer with him going 60 mph.

 

I think its the Rear End.!
What your saying that the Rear end Gear's are is Wrong for a Standard Shift
Car.!
A Automatic Mabey but Not a Stick
I have a Automatic in my Merc. which had a Standard & I wish I had gears like a 3:23
Ratio in stead I have 4:11 which is Good for a Standard.!
Like Squirrel said Check the Rear End Again.!

just my 3.5 cents

Live Learn & Die a Fool

 

Did you change the speedometer driven gear in the trans after you installed the rear end?

 

How tall are the rear tires? We used to run the 17.5" or 17" stock wheels and tires to help with it winding out too much.

 

I checked the gears by taking the cover off and looked at the numbers stamped on the ring gear 14:43, The rear I took out was 3.73 according to the numbers off the top of the axle tube, and it was doing the same thing, it had 31 inch tires on it when the 3.73's were in, and it would go about the same speed- 45ish revved up, then I bought some whitewalls (the 215/70's) and when I was driving home from the tire shop with the smaller tires I was barely movin- I mean I was probably going around 35-40 topped out, that's when I got the "new" rear and put it in, that put my speed back up around 45. My speedo and tach don't work so I'm just going by sound, this truck has been my daily driver since last September, it has good throttle response and I don't smell anything getting hot, when I took the old rearend out I pulled the cover and it seemed ok, no metal shavings or busted teeth or anything, maybe something wrong with my ******? I know that when I start from a stop (in second of course) if I don't throttle it just right it shakes and hops, but I just chalked that up to it being an old truck, I haven't messed with the clutch at all, it's probably due for a new clutch regardless. So with the rearend and tire changes the speed gap has been consistent - taller tires/ short gears 45 mph, smaller tires/taller gear 45 mph, if it were just that I need a new clutch that would be nice......

 

You can down load a speedometer app for your phone for free then buy or borrow a tach so you can accurately see what's going on. Five minutes on google will tell you how to RE-check you gear ratio.

 

No, I haven't messed with the transmission at all.

 

Simple answer is your speedo is lying to you.

 

he said his speedometer does not work, so I doubt he's paying attention to it.

Have you checked the ignition timing on the engine?

 

I downloaded one of the speedo apps last week and it verified that I was going around 45-50 (***uming the app is accurate) but app and speedometers aside, I know I ain't going anywhere near 60 mph, I drive a 9 mile stretch of highway to/from home and I have people right on my @$$ the whole way, and I wouldn't even think about taking this thing on I-5.

 

I've put a timing light on it a few times, I put it at 6 or 7 degrees over if I remember correctly.

 

You seem to be way off on your rpm guesses. Your new tires are smaller than I would run but that rear axle ratio ( if correct ) will let your truck go LOTS faster if it is running good and brakes and clutch are working properly. Gary

 

Missed a post along the way. Must be a mechanical problem and there are smarter guys than this speedo tech to figure that out.

 

I'm just going by sound with the rpm's, it's hummin pretty good at 45, I don't want to push it any harder for fear of burning up the motor.

 

I've got a 68 Chevy pickup with the inline six and stock rearend, 3 speed on the column, it will fly up to it, but top speed is about 60.
Getting ready to change that with a 5 speed overdrive and 396.

 

Jack up the rear, put a mark on the driveshaft near the U joint where you can see it and a mark on the top of the wheel or tire and turn the tire until the drive shaft makes 1 revolution while counting the revolutions of the wheel and any part of a revolution, that will be your ratio 1: ?

 

You need a tach to find out what your motor is really turning. Sometimes some motors just sound like they are winding up, when in reality there is plenty of rpm's left in it. Have you looked to see if the carb is opening fully when you have it floored?

 

I guess the days are gone when you can buy a cheap tachometer at any auto parts or hardware store. You need something to check actual rpm. That 250 should turn 4000 rpm easily. With 29 inch diameter tires and 3.07 gears would get you to 112 mph. 65 would be under 2500 rpm.

 

you can still buy a cheap tach, but getting modern cheap tachs to work with point ignition can be challenging.

 

After reading this entire thread closely, I have to agree with BJR in post #25. I had a '62 Chev. 3/4 ton , and a 4.56:1 floater axle rear that would run all day at 80-85mph if you didn't minb listening to that old wore out 283 spinning up!

 

These 6 cylinder trucks came with low gears for a reason....drivability.
Simply put a engine has to have the *** (power) to pull a high gear, very tall tires and or a overdrive efficiently. That's why it's common for these trucks to have 3.73 and lower gears. The sixes need the lower gears to have good all around performance to say 70MPH.
You put a 3.07 gear plus low profile tires....This action has likely negated any gain at the top end ratio wise. The low profile tires may have canceled out the gear change. With tall stock size tires on the new gear, the engine may not have the *** to pull it.
Step one
Check the clutch...
I suspect something like a worn, tired engine and a well oiled clutch(not a good thing)....slipping..all the time. Look for a leak at the rear main.....oil on the back of the block, dripping from the clutch cover.
Step 2 is to give the engine a tune up. A good one. Compression check, points plugs, carburetor adjustment...the whole bit. Get the engine at it's best or everything else is for naught.
With the engine tuned and it's best, then you can go further.

 

strange, my kid's off topic 1970 model Chevy has a stock 250 six, PG and 2.73 gears and it goes 75 mph just fine.