3K RPM's Get's Me 70 MPH What Do U Get?

[Green Monster]

That's a great tool. Thanks for sharing.

I just realized the speedometer displays ~10% slower than actual speed. I'd say it's moving closer to 65 MPH at @3,500 rpm. I get 5.29 with those figures. Looks like it's splitting the difference and could be either a 5.13 and 5.38.

I think this may have come with a 3.73 originally. If that's correct, the guy in the youtube has a calculation that compensates for changing tire size yet maintaining similar transmission shift point. (New Tire Size in inches x Original Gear Ratio) / (Old Tire Size in inches)

The old tire size was L78-15 that translates to P235/75/15 which is 28.88" tall according to this tire dimension converter..

If all these numbers are correct, the calculation yields 4.26. A 4.11 or 4.56 gear ratio would probably provide better drivability (lower RPM) on the highway.

[DeadheadNM]

On a related note, here's what I did to correct my speedo (350, th350, 3.73s, 33" tires): http://67-72chevytrucks.com/vboard/s...d.php?t=804681

[Green Monster]

Thanks for sharing, Pete. Looks like the adapter does a nice job correcting the speedo. Now I just have to pick one up.

What are your thoughts on the 3.73 combined with 33" tires and TH350? My Blazer ratio us the low 5s but I was thinking 4.11 would be ideal. Curious what you thought and if you're happy with the 3.73.

[DeadheadNM]

That adapter worked great and it resembles an OEM piece. The business was really easy to deal with. I need to add a speedometer drive shaft seal this spring as I’m getting some seepage there at the tcase. I’d recommend doing that too if you get an adapter.

Low 5s? What about lifting the rear to confirm by rotating the shaft once and counting tire rotations?

I’m really pleased with the 3.73/33” combo and would recommend it. For the driving I do it’s a good balance. Not terribly high rpm on the highway but I can’t quote you an rpm off hand.

[jocko]

Originally Posted by Green Monster

I just realized the speedometer displays ~10% slower than actual speed. I'd say it's moving closer to 65 MPH at @3,500 rpm. I get 5.29 with those figures.

What makes you say your speedo is ~10% slower than actual speed? Just curious.
Have you compared your speedo reading with a GPS at a few various speeds?
If you note your actual speed on a GPS vs RPM, we can tell you exactly what your gear ratio is.

[Green Monster]

Good suggestion. That is how I got that 10% figure. GPS and the speed calming devices police set up on the side of the road (that read YOUR SPEED IS...) clocking your actual speed have consistantly shown the speedo is reading 10% slower that actual no matter what speed it clocks me at.

[Big Kev-O]

700R4, 3.73 gears, 265/60/16 = 28.5"diameter tire gets me 97mph @ 3000RPM, 65mph @ 2000RPM. Just switched fro 4:10 to 3.73 and think they are a good match for the 700R4, much better acceleration but can still tow my boat.

Truck came with the 700R4, if money was no issue I would get a 6L80 or 8L90.

The free Tremec Tool Box app has some great tools for figuring out mph/rpm and drive line angles.

[Tomchev]

4L60e 3.08 rear gear 28.5 inch tires 2000 at 80MPH

[Sheepdip]

SM465 with GV O.D.- 4.10 rear gears/O.D.=3.20 final drive ratio- tires are 30.4" dia.- 55 mph = 1944 RPM's
Speedo checked with GPS is 2 MPH slow

[RustyBucket]

Here's a good selection of automotive calculators: http://www.wallaceracing.com/Calculators.htm

My 2wd. Blazer is running 80mph at 2,000rpm. TKO 500 trans. with a .67 OD ratio. 3.08 rear gear/30" rear tire. Mild 454, 750 Holley. It'll give 14++ mpg. I rarely cruise it at interstate speed though, wind buffeting is terrible. Can't hear or talk. Sometimes I use earbuds & wear shooting ear protection over 'em .... like I do on the mower.

[Accelo]

Originally Posted by Desert Rat

Next truck i build i have a 468 going into a 67 SWB project and id like to put in a 700R4,love to loaf along the freeway at 125 mph at 2000 rpm and get 20mpg,awaking from my dream,now.hey i can wish.I think a fuel injected,468 ci motor, over 500hp and 20 mpg would be sweet,7-10 mpg sucks even in a cruiser when you got $10 in your pocket and you still need to buy some beer,and tanks on E.its called 2 gallons of gas,and a bunch of cheapy malt liqour 40oz ers.or a nasty $7 bottom of the shelf 5th of whiskey.

Dam you had me rolling on the floor laughing. : Thanks
Cheers.

[custom10nut]

Originally Posted by Richie

I get 2500 at 50,about 3000 around 60 and right at 3500 to 3750 at 70.I am running a
350/350 with 275/60-15's on the rear.That 700r sure looks good sitting in the garage, Can' wait till it is between the frame rails!

------------------
'72 longbed,350\350,4\6 drop,p.s,p.b,tach.
http://hometown.aol.com/rp2bp/myhomepage/auto.html

This is the same that I have. Factory 3.73 rear gear.

[Accelo]

[QUOTE=lukecp;10699]3000 rpm's gets me roughly 75 mph. 3.07 rear gears, and 255x60-15 tires. I hope to upgrade to a 700R4, and i may keep the high gears. Then i will be praticly idling at 75 . I heard somewhere that the 700r4 won't cruise at high speeds in OD because the tranny will kick down at 2/3 throttle. Is this true, and if so is there a way to get it to stay in OD at full throttle? I dont wanna drive too fast, but a run up to 130+ sure would be fun .

--
It's simple to figure out the new cruise RPM if you upgrade to a 700/4L60
Multiply the current cruise rpm x the transmission's overdrive ratio (.7 in this case) it would be 2100 rpm. (3000*.7)

Some 700/4L60 will do full throttle in OD. They can be modified by changing the valve body to the Corvette configuration. Anyone that rebuilds transmissions can do this modification for you or they sell kits online.

If you are dealing with the newer stuff shift speeds are controlled by the ECM and can be configured to your liking if you modify the software.

I converted a C10, with a 396with 4:11 gears, to a 700R4. It would do 100mph in OD. Any faster and it wanted to kick down into third gear. I also lost my review mirror doing this trick. Bolt retaining the mirror was rusted. Mileage went from 10mph to 13 mph on the highway. Also much quieter on the road. I was amazed how much noise comes from a motor at 3000rpm.
Without the mirror mileage and top speed went up again! LOL.

I never connected the Lock-up converter. If you do connect it the rpm will drop another 200 rpm.
Cheers

[truckster]

My 72 Blazer runs right at 80 MPH at 3000 RPM. Turbo 350, 3.73 gears, and 33 inch tires.

And, yes, there is a lot of wind noise at that speed.

[D.B]

My 72 C20 Cheyenne Super has a 511 big block (approx 600 hp), Turbo 400 with a Gear Vendors, Dana 60 with 4.10 gears (final ratio 3.20) and LT305R70/16 (33") BFG tires. I know it runs around 2200 to 2300 rpm at 65 mph. The calculator says it should be 2150 rpm. At 70 mph should be around 2300 rpm and at 3000 rpm it says 90 mph.
I will say I was at Texas Motor Speedway a few years back when they were doing the Friday Night Drags and they allow vehicles to run two laps before the races start. I was coming out of turn two at over 100 mph. I stopped looking at my dash and kept my eyes on the track so I am not sure exactly what the rpm at that speed was. The race sponsor got on to me for doing that because the tires on the truck were not designed for speeds over 100 mph. Didn't want a crash before the race and ruin it for everyone.

[Steeveedee]

Originally Posted by D.B

My 72 C20 Cheyenne Super has a 511 big block (approx 600 hp), Turbo 400 with a Gear Vendors, Dana 60 with 4.10 gears (final ratio 3.20) and LT305R70/16 (33") BFG tires. I know it runs around 2200 to 2300 rpm at 65 mph. The calculator says it should be 2150 rpm. At 70 mph should be around 2300 rpm and at 3000 rpm it says 90 mph.
I will say I was at Texas Motor Speedway a few years back when they were doing the Friday Night Drags and they allow vehicles to run two laps before the races start. I was coming out of turn two at over 100 mph. I stopped looking at my dash and kept my eyes on the track so I am not sure exactly what the rpm at that speed was. The race sponsor got on to me for doing that because the tires on the truck were not designed for speeds over 100 mph. Didn't want a crash before the race and ruin it for everyone.


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God's dentures that a nice looking truck!