ive never messed much with gear ratios i normaly used 3.55s for towing but with this truck and no od i want better fuel and lower rpm

the truck now gets 16 around town 19 mpg at a constant 50mph
at 65 mph im runnin about 2800 rpm

so goin thru a ratio calculator with 30 inch tires with a 1.00 final ratio out of the trans and rving 2250 rpm at 65 mph id need to swap my 3.55 for a 3.08

anyone runnin a 3.08 and tow now i dont tow often anymore but when i do its between 1000 and 4000 lbs between truck and trailer

any sugestions?

 

If you're running stock tires, your numbers are off by a bit. 235/75-15s are a skosh under 29" diameter and roll at about 720 revs/mile. With 3.55 gears that gives about 2775 RPM @ 65 MPH which agrees reasonably well with your present tach reading. With 3.08s you'd see about 2400 ROM @ 65. To get down to the RPM range you're looking for you'd need 2.73s which would give about 2125 RPM @ 65.

What tranny are you running in a '90 that you don't have an OD?

 

na i got a set of dayton 30x9.50 on the stock 15 x7.5

borg warner t18 4 speed granny 1st gear

 

Those roll at about 705 per mile so at 65:
3.55 = 2710
3.08 = 2350
2.73 = 2085

With a T-18 and 3.08s or 2.73s you're actually going to be using the granny gear in daily driving rather than starting in 2nd like you probably do with the 3.55s

 

haha that get a little getting use too lol im leaning more twords the 308 for towing i know for towing i sould stay with my 355 but im working fartherway now so i wanted to squieze some more mpg out of it even tho im gettin pretty good compaired to other people i know.

if i stay at a constant 50 or 55 i get close to 20 mpg but at 65 im getting 13 to 14 worst then around town at 16 to 17

 

plus i dont like runinn my truck at almost 3000rpm for an hour especialy i6 v8s wouldnt bother me as much but at 3600 rev limiter.

i dont mind if its a little slower thats what i got my cutlass for lol

now i gotta c if i can fine one in the junk yard in good shape

 

Lots of guys run the T-18/3.08 combo and tow with it. You may need to use 3rd gear a bit more when towing but that's hardly a deal breaker. The reason late model manual trannied trucks have such a low tow rating is the crappy Mazdog tranny. You don't have that problem

 

yeah i love these t18 especialy for towing/mudding but ware its my daily driver n my jobs and hour drive eachway i dont like reving it so high so long

 

how much more power did you add???


if you're wanting to increase mileage why make the motor work harder???



you need to look at a 4.10 swap

 

with 4.10 id be pumpin a ton of unnessisary rpm 4.9s max at 3600 rpm im tryin to lessin the rpms for highway without overdrive im revin 2900+ on the highway and im not towing all the time bout once or twice a month and im doing alot of long distance highway driving

as far as engine and trans stock engine but everything has been cleaned rebuilt or replaced stock intake stock headers and cats thrush turbo exhaust trans is stock and stock style clutch pressure plate and throwout bearing

 

so you'll kill your mileage by going to a 3.08 gear as the truck doens't have enough power to propel itself empty now, let alone with a higher gear


if it's better highway mileage you want, swap in an overdrive transmission for less money

 

few and far between ware i live the second ones avalible its snatched up and id have to get the crossmember clutch hose and change the clutch and pressure plate from the 11 in to the 10 in plus if i go with the m5 ive tore the syncros towin and the zf is so hard and expensive to find here its not even funny and ill never go back to an automatic i blew 6 in 2 years and 1 was perfesionaly rebuilt and one was brand new from the factory

 

WRONG. The 300 has its torque peak at 1800-2000 RPM and delivers best mileage in or slightly below that range. The 3.08/T-18 combo works very, very nicely behind a 300. 2.73s would would probably deliver even better mileage. Remember that Ford once offered 2.47 gears as an economy option for the 300.

 

My truck was built with a 300, NP435, 3.08, & ~30" tires. Now it has a 4.9L (EFI), NP435, 3.08, & ~33". I tow, wheel, & set the cruise above 90mph (calibrated & GPS-confirmed)for hours at a time.

.

I hated 3.55s for the short time I ran them.

 

I have heard of F-150's with the 300 and 2.73's getting 25 mpg on the highway.