tire size vs MPG

[bluefordcat]

<_< 94 FSB, 351, running stock tire & rims 235-75-15 If I go up to 32X15 will my MPG increase because of less revolutions turned and engine RPM is slower. or will MPG get worse because of taller tires?

 

ALSO Anybody have a clue how to figure a final drive ratio? IF? I remember it has 373 gears under it now. you let off the gas now and she slows right down not much coasting here............................

 

one more ?? changing out the e40d for a 5 speed Manuel. will it help with power & MPG? or not enough change to ***** with the headache of swapping it out?

 

I'm not an off roader or into hill climbing or mudding , drive it every day to work , stores, and in snow (deep) during winter time. more less it is babied.........

 

[bronconite]

Your mileage will decrease for multiple reasons. The new tires will add weight, and unsprung weight at that. They will be wider which increases rolling friction. I know Broncos already have the aerodynamics of a billboard, but the higher you go the worse it gets.

I've thought about a 5-speed conversion myself. From what I've heard 1st is pretty low, so you can go with a bigger tire, like a 33" without regearing and keep your low end grunt.

 

[Seabronc]

Calculations are done with a 1:1 gear ratio which is what you get in 3rd gear if everything is in proper working order, (no slippage). A formula for figuring RPM at a given cruise speed is MPH x gear ratio x 336 / tire size. For ex. 65 x 3.73 x 336 / 33 = 2468.58 RPM 2469 x OD ratio .8 = 1975 . Not sure of the OD ratio since I don't have one and only think in the 1:1 mode

In my experience, if you stay with the narrower 10.5 width tire, you can go to a 33 with out any real noticeable negative effects such as loss of MPG or in steering control, (whit 12.5s my experience with steering is that they seem to want to grab every crack in the road and follow it :-& which means you need to be paying attention at all times). The tire size increase will require recalibrating the speedometer to get accurate MPG readings but will also reduce the RPMs, (if you don't re-gear). The increased weight can be compensated for by putting your truck on a diet, (get rid of the weight you are just carrying around and not using like the junk in the back and even removing the rear seat and only putting it in if you really need it. If you are not off roading, look under the truck and see if you have a transfer case skid guard, it weighs about 50 pounds, take it off, you don't need it, keep it in the garage along with the rear seat, you might need them some day.

Good luck,

>-

 

[dsgb9840]

i have a 94 with 6" lift and 35 bfg muds. before the lift i got 10-11 city and 12-15 hwy. after i get 7.4 MPG city...and haven't ran a tank through on the highway.. i short around town where u have to stop and get those tires going again your mpg should go down...on the highway it should go up a little cuz in 4th gear at 80 i was about at 2300 rpms with 31" tires, now i run at about 1600 RPM's = better gas mileage ..... until u hit the brakes.