What rear gear ratio is good for highway???

[Oldesole7472]

My 72 c10's top speed is about 55...i wanna swap the rear with one with a higher ratio...any suggestions???

[SWEET7T]

Mine is tall, 3.07 ratio. I run 75mph at 3k (255/70R15 tire).
That is about as fast as I hold it for long periods of time (traveled this way for 2 years on the interstate from northern to southern Utah during college years).
It would do over 100 if I let it but the motor was singing pretty good.

I just found out last week that my motor isn't a 327, it is a 307ci according to the casting #
Avg of 75mph was getting me 13-14mpg.

[69GMCLonghorn]

Well, first of all, get all your information first so you know what you're dealing with:

1) Verify that your speedo is correct.
2) If you don't have a tach, install one and see what your actual RPM is at 55.
3) Figure out what rear end ratio you have now, what tire size, etc. Then do the math and decide what to go with. If you feel it's only comfortable at 55, you'll probably want to go with a 3.08 or 3.42 rear end as long as you don't have an OD tranny.

[Oldesole7472]

very good ideas...thank you...the engine sounds like its racing @ 60mph but it would he a good idea to get a factual RPM figure before searching for a replacement rear.

What would I look for on the rear that would tell me what i have?

I have 307 CI, 3 on the tree...no OD.

Thanks

[Stocker]

Surest method is count the teeth on the R & P gears and do the math... but you can jack up the rear, turn a tire one rotation, and count the times the driveshaft rotates.

[Oldesole7472]

thank you very much.

[big_al_71]

3:08 gears would be vote for best highway gears....

[Oldesole7472]

yeah thats what im gonna keep my eye out for...any estimate on how much that swap would cost?

[leddzepp]

3.07 would probably be your best bet for highway gears.

[67chevemall]

I have 3:07's in a box ready to go in....be interested in how much people are paying for the install??

I can't see myself doing it.....

[GREASEMONKEY72]

mine has the 3.07 rear gear with a turbo 350 tranny and 31in tall tires and it cruises down the freeway under 3grand keeping up with traffic

[Sublvr72]

with the 350 auto 3.73's were a pretty good gear.

I have a 307 and 3 on the tree in the panel. Haven't taken it on the highway yet, so not sure.

[Ackattack]

I've found it difficult to acuratly count the teeth on the pinion gear with the differential installed.

Get a tach and do some calculations.

[70cst]

I have 3:73's in conjunction with a 402BB and 400 auto, I am still going to be getting 10-12 mpg.
(I Hope)

[1972]

To check the ratio, I jack up only 1 rear tire and have a friend spin it 20 times. I get under the truck and use a paint marker to mark the driveshaft, then count the number of times it turns. Divide that number by 10 for your gear ratio.

This method is accurate enough to tell the difference between a 3.50 and 3.55 ratio.


Edit: I know our trucks don't have 3.50 or 3.55 ratios... I was just using those as examples to show the accuracy of this method.


.

[69GMCLonghorn]

Gear ratio can be figured out by doing this:

(RPM x Tire Diameter) divided by (MPH x 336)

[jholley]

Originally Posted by Oldesole7472

yeah thats what im gonna keep my eye out for...any estimate on how much that swap would cost?

I just got my truck back from the shop yesterday. It had a 3.73 open rear. They put in a brand new Eaton Posi, New 3.08 gears, all new bearings inner and outer and did some other odds and ends. It came to around $1500. You could save a lot obviously finding a used one but you never know what youre getting unless you can drive the vehicle it's coming out of. Oh, and I can still turn the tires from a dead stop, only now it's both of them instead of just one. The marks on the pavement are a lot more impressive

[67chevemall]

Makes me remember how BAD I am at math. So simple but it fries my brain

[Longhorn Man]

your 307 with a 3 speed manual should have a 3.73.
If you can only do 55, you have bigger problems. You should be able to do 100+ MPH.

[Oldesole7472]

for real? i dont have a tach hooked up but id think that 3rd gear is 1:1 but im far from an expert...actually i dont know much at all about these trucks but thats what my Early Bronco's original 3 spd 3rd gear is...

first gear is high on my c10 but the engine sounds like im racing it anything over 55mph to me...i dunno...im gonna hook up a tach and get some real stats.

[Longhorn Man]

its one to one.

[GREASEMONKEY72]

these old trucks make alot noise usually, do hook up a tach to make sure youre not over revving it but i wouldnt be surprised to see that youre only doin about 2500-3000k rpms

[chris91]

As far as RPM, will a 327 rev higher than a 350? I am wanting to drive my truck on the highway, but it sounds like the engine is revving really high. Maybe I need a tach.

[ERASER5]

Originally Posted by 1972

Edit: I know our trucks don't have 3.50 or 3.55 ratios... I was just using those as examples to show the accuracy of this method.

But they can have a 3.54. Some of the Dana 60s came with them.

[Longhorn Man]

Originally Posted by chris91

As far as RPM, will a 327 rev higher than a 350? I am wanting to drive my truck on the highway, but it sounds like the engine is revving really high. Maybe I need a tach.

You'll be amazed when you put a tach in there. Like mentioned earlier, you're probably just shy of 3000 RPMs on the freeway.