i have an '86 FSB 5.0 xlt that had a set of 31's on her when i got her 11 years ago...then i went up to 32's...and about two years ago - we went up to 33's...naturally - with 200+k on the motor - she wasnt havin' an easy time of it...in fact - it seemed downright painful for her.

so a few months ago - i had a set of 4.56 gears added and now shes like a new truck again!!

yesterday i took her out wheelin' for the first time since i had the gears installed - to a place about 75 miles away...we filled up in town and headed out.

i noticed as we were getting close to home that the trip odometer was reading 150 miles - but WHATS THIS - I ONLY USED A QUARTER TANK OF GAS???!!!!

that is UNHEARD OF from a truck that ive NEVER seen better than 10mpg in...

am i crazy - or this a benfit of gearing down on an older truck with big tires?!!?

if the gauge isnt screwy - and im not blind - then WOOHOO!!!

ill find out tomorrow when i gas her up - and if she only takes 8-9 gallons - they OMIGOSH!!!! wow.

i freekin' love that truck - always have - always will.

drj

 

Did you update your speedometer gear after changing tire size and gear ratio? If not, it might be fooling you.

 

Being that it's an 86, does it have the vehicle speed sensor (or mechanical speedo drive gear) on the output of the transfer case? If so, the odometer will definitely increment faster than actual miles on the trip. But if you've taken this trip before and know that it IS 75 miles one way, then hey, eat it up. Then again, your speedometer will say 90 when you're doing 65-70.

Cody

 

Not all gas gauges are indicative of what is in the tank. When mine reads half, it is actually 2/3 empty; a difference of nearly 4 gallons from where it whould be.

 

i checked the GPS and the mileage is pretty darn close...now i KNOW it cant be that all of a sudden im getting 20 mpg - but i DO believe its defintely better than the usual 9-10mpg (old tires/gearing)...

i cant wait till the motors replaced...shes going in to the garage to be transformed in to a prerunner this winter!!



drj

 

Although I'm not entirely certain of this, since it comes from observation of diffrent years of vehicles, but what I have seen is you can in some cases actually improve you mpg by increasing your gear ratio. From a pure mathmatical standpoint bigger wheels and a lower gear ratio gives you the best mpg, but from what I have seen this only works to a point. I have a 92 F150 with a I6, 5spd, 33" tires, and 3.09 rear end; avg mpg is 15-17 driving speeds between 45-70 going to work and back. My friend with a 86 F150, I6, 4spd, 33" tires, and a 4:10 rear end get about the same mpg going 45-55 driving to work. If you figure anything over 55 or 60 I'm in od then our mileages really are pretty close comparison. But with the 4:10 gears his truck dosn't have much problem towing a car trailer where as mine is a dog and my mileage drops a lot more than his. I think what is happening is that even though the engine speeds may be a little higher at the higher speeds it is actually using less, fuel per cycle to create the force needed to move the truck which brings the mpg back in line.

 

I forgot to finish my point: With your bigger tires your mileage would naturaly increase, although the shift in diff size should have offset that and likely reduced it depending on what your old gears were your engine may be running at a higher rpm than before but using less gas beause of increased torque output. Pay attention to how your gas pedle feels and think back if it feels like you arn't pushing it as far down now.

 

ITS OFFICIAL!!

i just took her to the station and filled her back up - and she took 9.8 gallons...

so that brings her up to 15 MPG!!!!

thats a 50% increase over the old setup!!!

not too shabby for a 20yr old with 230,000+ miles on the o.g. motor!!!

now i cant wait to get my 'one-fitty' geared up - she only gets about 10mpg right now - same problem - OVERWHEELED & UNDERGEARED.

drj

 

you will get better around town mileage with 4.56s. your highway mileage (55+ mph) will suffer because your rpms go up.. If you had 3.08 gears, you'll notice an all around improvement since you had to be ******* it all the time to kept your speed up. If your bronc has automatic overdrive, then you are set (better rpms crusin').

 

actually - the 4.56 gears were a MAJOR improvement on the highway - and not so much around town...as for the RPMs - 30 mph @ 2000 rpms in drive - is no different for the truck than doing 60 mph @ 2000 rpms in OVERdrive...and - i never drive her @ 80 anyway...shes an old truck.

drj

 

Yes, There is a sweat spot on the motor/final drive ratio. It's where the engine doesn't have to work very hard to keep it's speed and it is not turning unnesasserialy fast. It;s a matter of effeciency. I am hoping to find it soon in my truck.

 

if its a 302 with 33's - then it might be 4.56s @ 65mph...im still in shock!!

the next mystery to solve is the 150...shes only getting 10mpg turning a 315/75/16... i was thinking 4.10's might do it...but now im leaning towards 4.56s for her too!!

drj