JLSpeed

Ok veterans, sorry if this is a repost, but I must ask the experienced: Is there something wrong with my truck??

I recently put a 2.5in suspension lift and 305/70/16 BFG AT's on my 97 Tacoma..... beforehand I was pretty much averaging 22-23mpg with 31x10.50 BFG AT's. Now I'm around 15mpg......at best....and this is with a 2.7L. Is this what is to be expected with 33's? Is there something wrong with my truck? Would re-gearing to 4.56's or 4.88's help? Any input would be greatly appreciated. Thanks in advance!

SCYOTA

I would bet that regearing could bring your truck back to 18-20 MPG's. Also remember that your odometer is no longer accurate because of the increased tire size.

JLSpeed

SCYOTA, you do actually bring up another point I'd been wondering about - would regearing bring my odometer back to accuracy?

sloppy85toy

you will never get the mileage completely back but regearing will help so your motor wont have to work so hard.

JLSpeed

Would you guys agree that 12-15mpg is about right with a 2.7L with 305/70's though? I'm basically looking for reassurance that my truck isn't f**ed up. I just can't help but think that the 4cyl should do better than that, even with 33's.

Dare44

my 22re got 13-16 with 33's and 4.88 gearing, but i always have my foot in it

bwhyit

When calculating your fuel mileage are you recaluculating your odometer miles based in your tire size increase? If not your milage may not be as bad as you think.

JLSpeed

That's a good point actually - I have not been taking tire size into account. I believe 33's throw off the speedo by about 10-12miles for every hundred - that sound about right?

jdsdj98

I'd say more. I'd been skeptical of my speedo (and thus odo) for a while now, and on my last tank of gas kept track of odo mileage vs. Google maps. My 31's yield a 13% under read, meaning that for every 87 miles on my odometer, I had traveled 100 miles. What had been consistently mid-17's for mpg mixed driving on the odometer, I'm actually getting over 20 mpg (additionally, you can reverse calculate how much mileage/speed to add to your truck's indications using these figures. I now add 15% to my speedo/odo for an accurate read). By that measure, I'd say the circumference of a 33 yields a signficantly larger odometer error.

It's a pain in the butt, but for your next tank, keep tabs of every trip to work, the store, your buddy's house, etc. on your truck's odometer, and then trace it on Google maps (which allows click & drag mapping). Calculate the per trip differential as a percentage, then do the same for the totals at the time of your next fill-up, just to make sure your tracking has been right. Like I said, I just did this last week and it gave me a better idea of how far off things are.

94fuelie

I would think he should get better mileage with bigger wheels and tires because the engine turning slower at the same highway speed than it did with 15's. But the wheels and tires may be heavier, too.

jdsdj98

Remember that a new vehicle rolls off the assembly line with an accurately geared speedomoter/odometer with regard to a specific OEM tire size.

A LARGER than spec tire rolls a further distance with a single revolution (Π * diameter = circumference, or distance traveled). So, over the course of a mile, and subsequent miles, a LARGER than spec tire would have made fewer revolutions than a smaller tire to cover that same distance. Think now in terms of time, where for an hour of driving he traveled 60 actual miles. Since for one hour that wheel was making fewer revolutions than a normal sized wheel would, and since the speedometer/odometer are driven by driveshaft revolutions in the transfer case, the speedometer and odometer will reflect a slower speed and shorter distance traveled than is actually the case.

Conversely, a smaller tire/wheel yields the opposite result, showing a mileage and speed overage.

tacoma728

The 2.7 can turn 35s decently, but depending on what type of driving you do for the majority, albeit, highway or city, a bigger tire would give you better mileage. That is if you stayed mainly on the highway. I would keep my stock gear ratio and apply 35s. You're fuel mileage will go up because it will actually make the final drive ratio higher, hence less RPMs.

JLSpeed

I hear the point about lower engine speed on the highway, however, I rarely use fifth gear now - I have no power whatsoever in said gear at the lower RPM; therefore, I'm usually in fourth on the highway.... I'm also usually on the gas just to maintain speed. I find that it's easiest to maintain constant speed without being on the gas around 40-50 mph - that's what sprung my question about gears; my thinking being: if I'm having no trouble maintaining speed with very little throttle pressure at 40-50mph, if I were to switch to 4.56's or 4.88's, i'd hopefully be able to maintain 60-70mph at the same engine speed as I currently experience at 40-50.....without giving it half - to 2/3's throttle. This, in theory (in my crazy mind) might bring back my gas mileage. Does this make any sense?

4Rocker

Funny noone has said anything about rolling resistance or moment of inertia. A taller and wider tire has more rotating mass that is further from the center, requiring more power to turn it. True, a bigger tire will reduce your engine rpms at highway speed, but it takes more power to move them so you will be using more gas at lower rpms than you would at a higher, lower load rpm.

02TAC2

Like 4 rocker said, teh supposed benefits or the alrger diameter are offset by the increased weight and loss of leverage teh smaller tires applied. It takes more force to get teh larger tires rolling, and unless regeared requires more force to keep it rolling. Think of it this way hold out a stick with a 10 pound weight on it. Not add a couple inches to the stick and see if it is easier or harder to hold out. You axle is you and the tire is the stick it applying force to.

jjrgr21

if you put your lift and tires on recently, then you are experiancing winter gas +lift and tires like me. i was getting 20-22 mpg hwy w/a 3.4, and now i'm at about 15-16 hwy//11-13 mpg city. this is with 255/85/16 BFG's (33's), i'm gonna regear to 4.56 to help w/ towing, but i'll be going back to a smaller a/t tire.

JLSpeed

This is basically what I was getting at; Thanks, 4Rocker for putting my remedial understanding of science into actual terminology - so, again, am I correct in assuming at 4.56's or 4.88's would probably bring back my MPG?

4Rocker

If your truck came stock with 28" tires and a manual transmission, the 4.56 should correct your gearing for 31" tires. As to weather or not it will restore fuel economy, it should restore some, but I dont have any experience to know how much. Hopefully it will get pretty close as I think this summer I might try to track down some 4.56 diffs for my truck too.

JLSpeed

uh-oh now it looks like i've had some conflicting information - I was always under that impression that I have 4.11's stock - yes, my truck is the "LX" so stock it had steelies and, yes, I believe `28's, and a 5speed. With the 31's my speedo was off by about 4-5mph at 60mph.