Gear swap or tire swap?

[themechanic]

I bought a built 489 Eaton Tru Trac rear diff from Doctordiff with a 3.55 ratio a few years ago for my 1969 Coronet. It is fine around town but at highway speed (60-70 mph) it hovers around 3000 with 26.6" 295/50 R15 BFG RWL tires.

What would a reputable local shop charge at $100/hour to swap the gears?

Since I need new tires (nothing wrong with mine they were just made over 5 years ago) what size taller tire would yield similar results on a 15" wheel?

FYI, I do not know what cam or stall converter is in the car when I bought it. I was told it has a purple RV cam. Smooth idle, no loping.

 

[dadsbee]

Glad there's no tach in my SuperBird... Atlanta Georgia to Alexander City Alabama with 3.55's and never gave it a thought while she ran smooth as a top! Goodyear Polyglass F60-15's

 

[themechanic]

I saw in another thread the 700r4 option and the Gearvendors option. Both very expensive. The car seemed to run fine at 3000. It was 185 on a 70 degree day. I won't know until I fill up again what the mileage was (stupid aftermarket senders). But, mileage is the last thing one cares about when driving a car with a big block.

 

[mrhemi]

I once drove a '68 R/T, 440 4spd., 3.54 Dana from Boston to San Diego in 42 hours. The planned 2.76 didn't work out at the last minute, so had to put the Dana back in. That 440 was rarely below 4 grand for the whole trip.

 

[67belvedere/225]

I saw in another thread the 700r4 option and the Gearvendors option. Both very expensive. The car seemed to run fine at 3000. It was 185 on a 70 degree day. I won't know until I fill up again what the mileage was (stupid aftermarket senders). But, mileage is the last thing one cares about when driving a car with a big block.

i have 355 in my Belevdere i did the 700r4 swap in my slant six and she purrs at 2100 at 60 mph

 

[33 IMP]

First, questions. What body style are we talking about, and what size tires do you have on it now?
I have 255-70x15 on my 62, on an eight inch rim. Bfg radial t/a, 29+" tall, rwl on the inside, 8"tread on the ground,10 1/2 or 11 "section, i dont remember for sure. If you have 25/26"tall now, they will make a difference, if you are 27 1/2, not so much. I would measure what you have now, and see if you can get a 29"+ tire on it .
285-70x15 are even taller. i wanted to use them, but i was pretty sure they would have been too wide on the rims i wanted to use.
Edit: SORRY! I completely missed the part in the op's post of his tire size. Still think a tall tire would help.

 

[themechanic]

First, questions. What body style are we talking about, and what size tires do you have on it now?
I have 255-70x15 on my 62, on an eight inch rim. Bfg radial t/a, 29+" tall, 8"tread on the ground,10 1/2 or 11 "section, i dont remember for sure. If you have 25/26"tall now, they will make a difference, if you are 27 1/2, not so much. I would measure what you have now, and see if you can get a 29"+ tire on it .

View attachment 858108

See first post.

 

[33 IMP]

Yep i missed it. Concentrated on the question, completely missed the info. My bad.

I still think a tall t/a or cooper would help.

 

[5.7 hemi]

Use a 28 inch tall tire, 275-60-15, you’ll be happy.

 

[khryslerkid]

Play around with this calculator and see what you get. It seems pretty close with my setup.

https://spicerparts.com/calculators/engine-rpm-calculator#

 

[Stroker68]

Use a 28 inch tall tire, 275-60-15, you’ll be happy.

Yeah...28 inch tire. Look cool too.

 

[themechanic]

I've looked at the calculator. The tires are the way to go for now. I don't have $2700 for a GV OD unit.

I'm looking at Mickey Thompson Sportsman S/T and Cooper Cobra Radial GT tires.

MT front 225/70-15 (not available in 235/70-15)
MT rear 275/60-15

Cooper front 235/70-15
Cooper rear 275/60-15

 

[themechanic]

For comparison my old BFGs 295/50-15 vs the new tires 275/60-15:

 

[themechanic]

Ordered MTs today. Seems odd none of the 3 major tire stores carry them.

 

[66Satellite47]

A 28" tire would sure help. 3000 RPM @ 65+/- sure seems like a lot with3.55's.

 

[themechanic]

Maybe I should take out a second mortgage for one of these

A41 4-Speed Automatic PerfectFit™ Kit for B-Body

Put it in drive and watch the gas stations go by.
Numerous magazines have reviewed the kit: Popular Hot Rodding, Hot Rod, Mopar Action and several blogs say that this transmission will bring your car "into the 21st century!"



The A41 4 Speed Automatic Transmission
View the Installation Manual and View the Floor Shifter Installation Manual

• Specifications
• EZ-TCU Controller
• Kit Components

A41 Ratio: 3.06, 1.63, 1.00, 0.70

Available in three stages:

Stage 1 - 450 Torque
Stage 2 - 550 Torque
Stage 3 - 650 Torque



Features:

• Low RPM Cruising
• Great Shift Quality
• Dyno Tested
• Stand Alone Electronic Controller
• Lockup Torque Converter with Optional Stall Speeds
• No Tunnel Cutting

The A41 is an excellent way to replace your tired 904, 727, 518, Powerglide, TH350 (Turbo 350), TH400 (Turbo 400), 200R4 (200-4R) or 700R4 with modern technology that offers better acceleration and a 30% reduction in cruising rpms.

 

[383man]

Use a 28 inch tall tire, 275-60-15, you’ll be happy.

That's what I would do. Heck I run a 30" tall tire on my 63 with 4.30's and turn about 3200 rpm at 60 to 65 mph on the highway. Ron

 

[CoronetDarter]

For that kinda dough I'd consider converting to manual and getting a Tremec 5 spd. I love the one in the Dart.

 

[themechanic]

Wishful thinking. I prefer an automatic.

 

[AJ/FormS]

From 25.5 tire to 28" tire is 10% increase same as from 3.55s to 3.23s .
But I don't think anybody would ever put a 25.5 on a Coronet.
The point is that from min to max on the tires is about the same as 1 gear swap. From 27s tho, you'd have to go to a 29.7, to affect the same gearchange. So IMO not really doable.

With an automatic, you're likely better off with a loc-up convertor , 5% slip at 3000 rpm is 150 rpm or half a gear swap.