Lower rpms
 

I've got a 84 1/2 ton c10, 2wd swb with a mild 350, th350 trans, rear tires are 265/50/15's measuring 26 inches around with a 10 bolt rear end with 2.73 gears. My problem is I do a lot of highway driving and the truck revs really high at 70 its around 3000 rpm. I have been wanting to put in a 700r4 but money is tight right now, and cant have the truck down for long. is there a different rear end I could put in it. or maybe a different size back tire I could run and if so I need to know what would be the same width tire that I have now as my rims are 12 inches wide any help would be great, thanks

Re: Lower rpms
 

Save for the T700R4 or a T200R4. Anything else is not going to give you a big enough drop in rpm for the price.

A T200R4 is the cheapest since it is the same length as a T350, and saves the drive shaft shortening.

Re: Lower rpms
 

My trucks got the long 9 inch tail shaft so it shouldn't have to be shortened for the T700R4 correct? I have a T700R4 that's in my parts corvette but was told that it was different, not for sure tho. I have an 84 corvette with the same motor, intake, carb and cam as my truck and it pulls like 1500 rpm doing 70 mph. I bought truck to pull the corvette to shows thought it would be kinda cool since there the same year and gonna paint them to match each other. Also what all would I need to have to put in the T700R4 torque converter, dipstick, TV cable and bracket, what would I need to make it lock up in OD?

Re: Lower rpms
 

Save your money and your time and get your tach fixed.
At 65 mph you're turning 2400 rpm.
An overdrive tranny won't magicly give you more miles per gallon. Likely drop.
Your motor won't pull 1800 rpm without getting deeper into the pedal.
Here's a speed/tire/rpm calculator.
http://www.powerblocktv.com/resources/

Re: Lower rpms
 

I kinda agree. My dually with 4.10 axle ratio will run about 3000 rpm at 70mph. If your tires are 26" tall then your rpms should be around 2400-2500 at 70 mph. Those rpms arent bad in my opinion but an overdrive tranny would lower your cruising rpms but not sure how much mileage would be improved.

By the way.....Slick truck!

Re: Lower rpms
 

Are you sure your gears are 2.73 Like Chevy3+3 said your rpms should be 2500ish with those specs

Re: Lower rpms
 

I'm not running exactly 3000 RPM I said around, as in 2700 and up to 3000, I'm on my second tach and they both read the same. As far as the gears the 2.73 was figured by wheel to driveshaft rotation, 2 full rotations of the wheel = 2 and almost 3/4 rotations of the driveshaft, that was how I was told to figure it so if I'm wrong I apologize. I am not to worried about saving MPG's just would like the RPM's down a little when the truck goes down the interstate for an hour and a half a day 5 days a week. I'm not looking to drop a 1000 RPM just maybe a 100 or 2. I would like to have taller rear tires cause I know 26 inch tires are not stock and the trucks been repainted and all the stickers have been removed from inside the glove box and off the inside of the driver door so I have no clue what stock is. I know speedo is 6 mph fast checked by radar, speedo has to read 76 to actually be doing 70. So would taller tires help me or hurt me?

Thanks Chevy3+3

Re: Lower rpms
 

A taller tire might help a little. I think these trucks ran a 235/75/15 tire originally or close to that size. That would make the factory tire height around 28-29" tall. As far as your Speedo goes.....usually taller tires make your Speedo read faster than your actual speed. So running smaller tires should make the Speedo read slower than your actual speed.

If you have a smartphone download a free GPS speedometer and double check your dash speedometer against the GPS speedometer. I'm wondering if you might have a Speedo problem.

Re: Lower rpms
 

I went from 4.10's and no over drive to 3.73 with over drive on my pickup. With the 4.10 ratio my freeway speeds were 65-70 and I averaged about 12 mpg. After the trans and gear swap, driving 75, I get about 14.5 now the couple of times Ive checked it. This is in a 3/4 ton 4 wheel drive, was originally a 6.2 and someone swapped a 70's 400 motor in it before I bought it. The higher gears are nice but if there is a head wind or a few hills to go over, it just doesnt have the torque to pull it and it drops out of over drive. I have an 87 burb with a 6.5 turbo and the same gearing but slightly larger tires and its great unless you are driving into a 35 mph or higher head wind then the pyro starts reading a little too hot and I drop it out of overdrive. With the turbo it has enough extra torque to pull the gearing most all the time whereas the 400 motor doesnt. The 4.10's from the pickup replaced the 3.73's my 87 blazer with a stock 350 and over drive with the same tires as the burb and I think its the best combination for the gas motors. Kinda wish now I would have left the 4.10's in the pickup and just went with the overdrive especially since I tow the heaviest loads with the pickup. I think the tires on the burb and blazer are 30 inches tall and the pickup tires are the stock size, maybe 2 inches shorter. I dont have a tach in any of them but according to the gear calculator posted here , the burb should be running about 2200 and the blazer is 2400 rpm. This is what I have found with all my messing around with gears and over drives, hope this helps.

Mike

Re: Lower rpms
 

My 84 came with 205 75 15 stock. In theory 27 inches tall. Closer to 26.5 installed. Not much taller than yours.
You need a poor man's o/d. Taller tires! 235 75 is 28 inch, 255 75 is 30 inch.
The 235's will drop your rpm 10%.
Damn fine looking truck by the way!!

Re: Lower rpms
 

Dang! I didn't realize they came with 205/75/15 stock? I always thought they were 235/75/15. Maybe I just always thought that because I see alot of people running that size on their trucks.

I actually put 255/70/15 on a 78' Chevy I had. I think they were about 29" tall. It ran down the highway great but I still had enough low end I could roast them off from a dead stop with a mild 350. Taller tires would definitely be your easiest and cheapest route.

Re: Lower rpms
 

Alright went out today and downloaded speedometer app and went for a drive and at 70 MPH on the app, speedometer was reading 76 MPH

Re: Lower rpms
 

Well throw yourself some taller tires on it. That's the easiest and cheapest fix in my opinion. If your tires are 26" tall then I would be looking for at least a 28" tall tire. I would try and find a 29" tall tire so you can really feel the RPM difference. Take a look at 255/70/15 (29") or a 275/60/15 (28"). Your wheels may be a little to wide for that size though.

Re: Lower rpms
 

Went out and downloaded a speed app and went for a drive. At 70 MPH on the app my truck speedometer was reading 76 MPH.
Attachment 1231806
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RPM's were right at 2800 at 70 MPH
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Tires are 26 inch diameter
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And a little over 12 inches wide
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Pulled the paint off the sticker on the door and it says 237/75/R15 tire so what size tire can I get that's the right diameter as stock and as wide as I got on there now or wider?

Re: Lower rpms
 

a stock 700r4 shouldnt be costly anyhow i bought one for a 100 bucks last summer. i had to look twice your trucks twin just went through big lots parking lot. same wheels even.

Re: Lower rpms
 

I also think that the gauges in my truck might of been changed at some point in time cause the gear selector reads P,R,N,OD,D,2,1. I installed the white face gauges the day after I bought it and changes every bulb in the truck to l.e.d. and the original gear selector read the same doesn't work cause needle is broke. kinda make me curious tho.

Re: Lower rpms
 

That's funny!! Need to ask him if he's got an extra center cap. I'm missing the center Progressive sticker off one wheel and cant find a replacement.

Re: Lower rpms
 

i honestly cant tell the difference and yours and his.

Re: Lower rpms
 

but still for the cost of a rear swap id say you could buy a od trans.

Re: Lower rpms
 

sounds like it was a od truck orignally.

Re: Lower rpms
 

Is there any way to find out what was originally in it. The glove box has been changed so there's no codes. I also noticed the other day that the transmission cross member has only 1 bolt in each side for a total of 2 and the second holes from frame to cross member don't line up exactly like its been moved already.

Re: Lower rpms
 

Just put it in od. Its got the indicator...:lol: No, but really. Get some large tires.... Those tires will only slow you down. I had them in my blazer and. Everyone would pass. Me on the highway. I put some 235 75 15. And all was good. Felt like I was speeding all the time but was only doing the real speed limit.

Re: Lower rpms
 

The hole with the bolt in it is toward the rear of the truck, the other almost lines up to get a bolt thru. same on both sides of frame
Attachment 1231849
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Re: Lower rpms
 

Closest you can come to stock height and same width, would be a 265/70r15 about half an inch higher than the 235/75 Maybe that little extra drop in the rpm would be fine though

A 255/70r15 is the same as the stock 235/75 but is not wider like you want

Tire selection is hard around this sizing to get something that covers all factors including being able to get tires on the 12inch wide rims with out having to much trouble seating them and not looking funny with a lot of stretch

Re: Lower rpms
 

I've been thinking the past few weeks of trading my wheels for some stock rally wheels 6's in the front and 9's in the rear. sucks for me cause I just bought 2 brand new back tires and they weren't cheap either. Gonna start looking on good ole craigslist.