Hell ya its nice! I'm back to getting very roughly 13-14 mpg city (without the speedo calibrated tho) but I've been driving most of the week on $25!!! I'm stoked to see what I get highway ON THE WAY TO MOAB!

 

Don't mean to hijack but I have a similar question. I'm not planning on running 33s but I have the 8.25 diff with 3.55 gears on 235/75. Would going to 3.73 or even 4.10 gears make any improvements on mpg or is the the tire size not really affecting much as the oem tires were 215/75 ???

 

If your final gearing is close to stock now, as in turning similar rpms to stock at same speed, there's no need to calibrate the speedo.

 

I know & as far a RPMs go I'm way better off than i was...
Before; 2800-2900 RPMs at 65
Now; it holds about 2000-2100 RPMs

Around 35-45 mph my gage is withing 2-3 mph, its when I'm doing 65-70 mph that my gauge reads about 8 mph faster than I'm going. Either way im not to far off!

 

Ya it would help, & you'll go threw tires faster than I do! lol

 

Wait, you went with higher gears (lower numerically)? Or did you mean 1800-1900? You must not have gone with enough gear though if you're still 8 mph off, I'm only 10 at that speed and I haven't regard yet (my stock is 235/75 and I'm on 33s). Did you go 4:10 on 33s?

If your gauge reads 8 mph high at 65 you're still off by 12.3%, that means if it's really off by that much and you calculate 14 mpg you're only getting 12 mpg or so

 

I know that when I was running 31"s on 3.55 gears I was 5 miles under (speedo 65mph = actual speed 70mph)

Now running 35"s on 4.56 gears I'm about 8 miles over (speedo 65mph = actual speed 57mph)

4.56 is the recommended ratio for 35" tires.

 

Also take into account that my jeep weighs about 600lbs more than stock, its an '89, with 217k miles!!!

 

That's off by quite a bit, how have you been finding your actual speed?

Calcs show you're pretty much right on though, 4.56 would be exact for 36.7" tires and 35s should show 61.6 or so at 65 speedo. That's using true 35 though, and we all know that most 35s are more like 34 or less.

 

Found my actual speed via CB, BSing with a truck driver! Lol my tires are pretty warn too, so maybe a true 33"-33.5" tire... I don't really care what the speedo says I just roll with the flow anyways

 

What would make it go through tires faster?? Heavier gearing spinning the tires leang some rubber down????

 

Yep I can break mine loose on asphalt & its locked in the rear! Lol