teamjbl

2000 4runner limited with rear locker (4.30 gears), 3.4, automatic.

Help me figure this out: with the stock 265/70R16 tires (when I had them on) my speedo was about 2.5 MPH fast when traveling 60mph (so if the speedo reads that I am going 60, I am really going about 57.5). I installed 265/75R16 tires, which are about 1" taller than stock and the speedo reads that when I am traveling 60 MPH, I really am going 60 MPH. I got one of those Eclipse navigation head units that use GPS and the speed sensor wire going to the dash cluster to verify the speed was dead on, along with those trailers that the highway patrol parks along side construction zones that tells you your speed.

All is good, everything is normal. Now I discover Sonoran Steel, these forums, and pictures with other peoples 4runners with 285/75R16 tires and how cool they look and how well they perform. So on go these larger tires (BFG MT, which are just a hair under 33" tall).

Now I really notice the power loss in first and overdrive, crawling up steep hills, and I had to floor it to do 90 MPH (with a supercharger). The brakes also don't work as well in hard stopping. I expected this, nothing big, nothing unexpected.

On went the Tundra brakes, stopping power is corrected, stops now like stock... cool! Now lets tackle the power problem:

Using a few different tire/gear calculators available online http://members.optusnet.com.au/~zerosin/gearcalc.html , I input 265/70R16, and get 30.6", input 285/75R16 and get 32.8". I plug these numbers into the gear ratio calculator and get a recommended new gear ratio of 4.6. I order and have installed Yukon 4.56 gears, which is pretty damn close. The 4wd shop recommends 4.88 because of my bumpers and winch, but I decline. I pick up the truck, and the install went fine. All is good. Break the gears in for 50 miles babying everything not going over 35MPH.

While I do get a little more power (as far as I can recall), my speedo still reads two mph fast at 60 instead of two MPH slow! I verified this with the gps and again with the construction radar speed trailers. My Nissan 4-banger pickup has more top end power than my supercharged 6 cylinder 4runner! I floor it over 60 and the thing struggles to go any faster. The only thing I can think of was that the wrong gears were installed or something. The 4wd shop pulled the differentials out, and delivered them to a gear specialty shop with the new gears, and then the newly rebuilt differentials were picked up and reinstalled. I didn't physically look at the boxes or count ring and pinion teeth to verify anything...

I am to the point where I am thinking of going back to 265/75R16 tires to get performance back. I have been combing the forums getting 265/75R16 to 285/75R16 photo comparisons, and the difference between the two hightwise isn't that much to really notice a difference. The width, however...

I don't want to spend the money again on a gear swap, when it is so much cheaper to do a tire swap. What the hell happened? Am I totally nuts?

Please chime in and help! Thanks in advance!

teamjbl

I should have probably added a few of these:
maybe two of these:
five of these:
and probably ten of these:

I can hear you thinking: change tires, and give me that 6 month old set of 285/75R16 BFG MT that you are using now!

then you can add one of these:

ninjarunner

It sounds like you have other issues. I have a 99 supercharged on 315/75/16. i have the stock 4.10 gears and I havent had a power problem. I can do 90 no prob, it is a little sluggish from the 85's but not bad. i did not even notice a difference when i went from stock to 285's. my low end is fine too, i can peel out my mudders on concrete with my gearing as well. i would get the 7th injector kit or the UDR kit put in and then tune it.

teamjbl

You can 'peel out' with those big tires on there? The only time I can do that is on slick blacktop after it rains. I just ordered the URD 7th injector add-on kit from URD along with their tuning tools. If I can figure out how to install the control module, fuel pump, and then tune everything... Thanks for the input.

teamjbl

Reading over gadget's original site and I believe i'm leaning out. He reported the same thing originally how it was hard to go over 90mph. Time for the AIC and pump