How hard can I push it?

[dbenamati]

I've posted about my 67 250 4 spd farm truck I fixed up. The way that its geared, going 55 was pushing the rpms abit. I've considered changing out the rear end gears and havn't had much luck finding something with out getting really crappy gas mileage. Someone posted last week about swapping rear ends and from the topic pages, you guys use all ratios and have no problems with speed so I decided to push it today and open it up on a long stretch of road to see how fast I could get it going before it exploded. The engine is in decent shape with no rebuilds and just cleaned up looks pretty good. I got it up to 65 and the rpms didn't sound that bad. Am I pushing my luck to get it going faster or should I go back to the rear end change idea. I don't know the ratio in my truck, I am going to change the diff cover gasket this spring to get the ratio but it is geared awful slow (3/4 ton 4 spd). Any suggestions about pushing it? Thanks, dave

[Stroker]

I would guess you have a 3.73 unless it's been changed out or an option is listed on the glovebox. My 72 had a 250 and 3.73 with 28inch tires I drove it 70-75 MPH daily to work and back. Never had any problems.

[71402BB]

since you have the I-6 in it you would have 4.57 coggs and no I don't think you would really want to push that truck much over 65 mph.... But if you really want to know what your motors spinning at then I would go out and invest in a cheap tach so that you know for sure how hard your motor really is working!

[Stroker]

I didn't notice it was a C20 was the 4.57 ratio standard on the I6 3/4s?

[crazy longhorn]

im not sure what the book says , but the ones i tore down (20 series ,with 6 ) were a 4.57 gear. Best (cheapest bet) on that is a set of 4.10s, just swap the carrier & that will help some. Good luck, crazyL

[mikep]

Ive got 4.57's and I dont have any problem running up to 100+ mph. I'm running 32 inch tires but thats a pretty common size with 3/4 tons. Remember that 3000 rpm in high gear is 65 mph with those gears and 32 inch tires. I know the 6 cylinders will do better than 3000 rpm under load.

That is of course assuming you have 4.57's. Unless you are optioned at 5.14's or 4.11's then you most likely do have the 4.57's. If it were a one ton you'd probably be looking at 5.14's . Ilike the 4.57's. With the tall tire its just about the same as having 3.73's or 4.11's in a passenger car. The number sounds real big and scary but theres other factors than just the physical gear ratio to consider.

[theo goebel]

dbenamati: I share your concerns. I have a '67 250 I-6, C-10 that also seems to top out at 55 to 60 mph. My truck is also a granny 4 spd and according to the build sheet in the glove box a posi 4.11. I have been thinking I should change the rear end to a lower number like 3.73 or in the 3.50's so I could get an easier cruise on the freeway as most often traffic around Seattle is 60 plus and I can hardly keep up besides spinning the engine at what seems to be the top available RPM. Also, last summer I pulled the head and had the seats redone so I believe my I-6 to be in top running condition and I only get 10 MPG. Also, I have the standard steel wheels (6 bolt) with a mud and snow radial tire. Not sure how tall they are in comparison to Mikep stats.

[mikep]

Dude, you have other problems besides your gear ratio. The last '69 C10 LWB I had had 4.10 gears with 31 inch tall tires and a 250 CID 6 cylinder with a 3 speed manual tranny . On a trip from Seattle to Sacramento once I averaged 18 mpg . If your'e getting 10 mpg with that combo and your'e topping out at 65 mph theres something drastically wrong. I get 10 mpg with my combo and I'm pushing 400 HP with a 4.57 gear ratio and a LEAD foot.

I work industrial maintenance and if theres one thing I know its that you have to find the source of the problem and solve it to get things to run right. Dont treat the symptoms . Treat the disease .

[oldsub86]

My old 67 stepside had 4:11 gears and a 3 speed standard transmission. I never had any trouble getting 70 mph out of it and the motor was not in wonderful shape. I tore it down and put rings and bearings in but didn't bore it out and the cylinders were a bit worn especially at the tops. It never gave up. I sold it after I wrecked the front end - slid into a big Buick on an icy road.

[dbenamati]

I think someone told me it's probably a 4:57s for that truck. I'll find out once I pull the cover next month. As for my mileage, I don't drive long distances but the gas mileage is poor (Guessing about 12-14 mpg) Thats not as big concern as the speed. You can get run over by a big rig in these parts with windy country roads. I put smaller wheels on it last year because I couldn't find tall ones. I got a set of 16.5's from a dodge full size van. I like the lower profile. It originally had 17's anyway. Thanks for the input, maybe I should leave well enough alone. My dad says that my truck was built to haul weights that other trucks couldn't (Farmers special) and it was a trade off for speed. I'm not a farmer, I am a mortician, what do I know about farm stuff, haha.

Thanks, Dave

[mikep]

Your'e Dead On !