Okay, technical question for the experienced bullnose owner here.

My '84 is a daily driver and a street truck. It hauls some weight, and sees dirt roads, but I don't wheel or deal with snow/ice. I need to be able to maintain 65mph comfortably and economically (Texas, big long roads).

So right now, I'm pretty happy with my gearing. The speedo reads a little slow, but I'm turning 1900 at 55, and 2100 at 65. Haven't gone faster because of the old rubber. Averaging 18-20 with mixed driving 75 hwy 25 city.

Right now I have 31" rear tires on my truck, with an unmodified 300 six, NP435 granny 4 speed, and I believe a 2.75 rear end.

The tires need replacing eventually (rears are old, fronts are small and worn) so I figured what the heck, and bought some vintage Ford 5-bolt steelies (with OE chrome dog dish caps!) at a swap meet for that 50's look, figuring I would paint them and put the new rubber on them.

However, my old steelie wheelset is 15 x 6 diameter, and too small to run the 31 x 10.50 R15s that I currently have on the 15 x 7 wagon wheels.

I can run 235/75 R15s or 30 x 9.50 R15s on these old wheels.

This would gear it down 7.3% and 3.3% respectively.

I'd like to know the "sweet spot" of these motors for power and economy. I know they can rev really low, but what is a good freeway cruising RPM for engine wear and fuel mileage? Like I said I am happy right now at 1900-2100rpm.

Basically, if I gear down / bump the cruising speed at 65-70 by a couple hundred RPM and it doesn't have a big effect on fuel mileage or engine wear, that's fine. But if I'm currently in the sweet spot and would have to cruise at 55 to maintain my current fuel economy and wear and tear, I will need to choose another option.

Have any of y'all faced the same conundrum and how did you fix it?

Thanks!

 

Sounds like it already does better than most other peoples. If it ain't broke, don't fix it.

 

Well, the other problem is I can't afford $600+ for a set of new streetable 31 x 10.50 R15s.

235/75s are way cheaper, and 30 x 9.50s are a couple hundred less. And no one sells their used 31s around here until they are worn to the carcass, whereas 30 x 9.50s and 235/75s are out there.

 

31" tires and 2.75 gears would give you 1639 rpm at 55 mph and 1937 at 65. To get the speeds you listed you probably have 3.08s (1836 at 55 and 2170 at 65).

Dropping to 235/75-15 (28.9" tall) would change it to 1969 and 2328 rpm.

30" tires would give you 1897 and 2242 rpm.

If I were you I'd go with 30" tires and not change the gears. I doubt you'll notice the difference, and changing gears is either expensive or a lot of picky work. Either way, not really worth it for a small change.

 

I think you might be at the point where lifting your rpms much (if you actually are at 2100 now), is going to hurt your mileage.......but happy reading

https://www.ford-trucks.com/forums/1...-9l-300-a.html

 

The "sweet spot" for my 4.9L with regard for gas mileage is 1600-1800 RPMs... right at the max torque. With manual O.D., 3.08 rear gears and stock 215/75 X 15 tires, it turns approximately 1500 RPMs @ 55 mph or 1950 RPMs @ 70 mph.

 

My truck has the 300 with 3.08s and the NP435, and it's all around good on mileage and power, and with the granny gear, it's perfect for towing cars. However, the old Bronco has the OD trans and 31s, and it noticeably struggles off the line

 

Thanks.

I'll probably just try to find some good 30s and put them on, but out of curiousity, how bad is a rear end swap? I would think with a compatible/OE rear end it would be simple.

That's a great gearing combo, basically it's like driving a diesel with that spec I'd think.

Unfortunately I have a non-OD manual trans, so 1:1 is my cruising gear. If overdrive units didn't end up being so expensive that would be my next thought.

 

Looks like you can get a 5 speed overdrive trans for about $400-$600 in Texas. I assumed you had the 3.08 rearend and it's 2wd. Apparently you can't change something with the speedo gear on the Mazda 5 speed, so you have to specify what rearend gear you have when you go buy one. Looks like they slipped one in the list with a 2.73 ratio though.

1988
Transmission Assembly
Ford Truck F250 M.T., 5 speed, Mazda and Toyo Kogyo, 4x2, 2.73 ratio,MANUAL TRANS-150K 150,000 A C9920 $500 Arcadia Auto Parts USA-TX(Dallas) Request_Quote 214-339-5622 / 214-331-2234 Request_Insurance_Quote
1990
Transmission Assembly
Ford Truck F150 MT,5SPD,4X2,3.08 RATIO 3117A $550 K & K Motors, Inc. USA-TX(Graham) Request_Quote 1-800-766-2009 Request_Insurance_Quote
1989
Transmission Assembly
Ford Truck F150 CUSTOM,4.9,5 Sp,FWD K7M582 $Call Charlies Auto & Truck Salvage USA-TX(Tomball) Request_Quote 281-351-1314 Request_Insurance_Quote
1990
Transmission Assembly
Ford Truck 150 013142 $Call Hoelewyn Auto Salvage, Inc. USA-TX(Angleton) E-mail 1-800-259-7276
1989
Transmission Assembly
Ford Truck 150 XLT,302,5 Sp,RWD,Tilt,Floor 16825 $600.14 Manuel's Used Auto Parts USA-TX(Pasadena) Request_Quote 1-800-435-5748 Request_Insurance_Quote
1992
Transmission Assembly
Ford Truck F150 5.0 AUT 4*2, GRAY-GRAY P2879 $400 Brother's Used Auto Parts USA-TX(Mission) Request_Quote 956-580-3349 Request_Insurance_Quote
1991
Transmission Assembly
Ford Truck F150 EXT 2WD,4.9L GOOD ART 002298 $Call For Less Used Auto Parts USA-TX(Houston) E-mail 713-734-1621
Year
Part
Model Description Miles Part
Grade Stock# US
Price Dealer Info

 

Update ya'll.

Today I unexpectedly picked up a set of 30 x 9.50 tires with about 50-60% of tread left for free on Craigslist. They're BF Goodrich Baja Champions. Currently they're mounted on a set of Dodge six lug rims but I can pop them off easily enough.

Am I correct in assuming these would fit on the set of 15 x 6 steelies I have? It's the same tire size that I have on my Ranger, but I don't know if the factory Ranger wagon wheels are 15x6 or 15x7.

If not, I'm thinking of keeping the current 31s on the rear, putting the two best 30's up front, and keeping two 30" spares in the bed (why not? ).

 

I use Tire Rack for checking specs on tires
https://www.tirerack.com/tires/tires...omCompare1=yes
it lists 6.5-8.5 rim but I would give it a try if I needed tires and they were the only rims I had.


BTW I had 31x10.5x15 tires on a 10" wheel and they did work but the tires did not wear good. When time for new tires, same make I had on the 10", I went with 8" wheels and had great wear.


I don't think you would had an issue being only a half inch skinner.
Dave ----

 

On a skinny rim, the tire will tend to wear in the middle. But you can compensate for this some with tire pressure. So just keep an eye on the tire wear, if they are wearing too much in the middle lower the tire pressure 5lbs.

 

Skinny (or wide) rims MIGHT also affect handling. Personally I'd try to stay in the recommended zone, but realistically I doubt most of us would ever notice the difference.