# 1990 4Runner is too slow. Replace diff gears? **URL:** \ **Category:** Factual Questions\ **Created:** [May 30, 2006, 8:46pm UTC](https://boards.straightdope.com/t/1990-4runner-is-too-slow-replace-diff-gears/358791 "2006-05-30T20:46:58Z")\ **Posts on this page:** 6\ **Page:** 2
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [May 31, 2006, 2:46pm UTC](https://boards.straightdope.com/t/1990-4runner-is-too-slow-replace-diff-gears/358791/21 "2006-05-31T14:46:39Z")
[QUOTE=astro] I’m not a vehicle maven, but I am astounded you guys are talking about gear changeups on a 16 year old incipient beater truck intended for commuting. Aren’t mian gear replacement mods like that at least a thousand dollars plus? [/QUOTE] We’re only talking about it because the OP raised the question. No one here is advocating it, and the high cost has been mentioned as one reason not to jump into it. ---
**Author:** ![Belrix](https://avatars.discourse-cdn.com/v4/letter/b/ed655f/32.png) [@Belrix](https://boards.straightdope.com/u/Belrix)\ **Post date:** [May 31, 2006, 4:20pm UTC](https://boards.straightdope.com/t/1990-4runner-is-too-slow-replace-diff-gears/358791/22 "2006-05-31T16:20:07Z")
Hi - OP returning here. Yeah - I thought the diff gear was a plug-and-play replacement. I’m pretty much scrapping that idea. The tire size, **Rick** , is 31x10.5xR15. Seller said they were a stock size. The manual is missing for this truck so I’m taking his word on it. Their BF Goodrich T/A’s, too. I don’t know the tire pressure right now, my guage is missing, but they look pretty high right now - not excessively squatty at all. I’ll get a guage after work today. The door panel says 25 lbs - sounds low - any verification? I talked with a friend last night and he had an interesting idea. The “TV” cable, ie: tranmission downshift cable, might be poorly adjusted. When it downshifts, it seems really early - there’s a lot of pedal travel left. It may be out of adjustment. It may be that I can hit these speeds easily in overdrive if it didn’t downshift due to the transmission valve release. By the way, addressing an earlier note, when it downshifts, it accelerates nicely - I got it to 80. When I back off the throttle to allow to drop back to overdrive (fourth?), though, it slows back to 65. Leaning on the gas a little more makes it downshift. So - If I want to move the valve-open point out some, do I add slack toward the throttle lever or away from it? ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [May 31, 2006, 4:51pm UTC](https://boards.straightdope.com/t/1990-4runner-is-too-slow-replace-diff-gears/358791/23 "2006-05-31T16:51:58Z")
[QUOTE=Belrix] The tire size, **Rick** , is 31x10.5xR15. Seller said they were a stock size. The manual is missing for this truck so I’m taking his word on it. Their BF Goodrich T/A’s, too. I don’t know the tire pressure right now, my guage is missing, but they look pretty high right now - not excessively squatty at all. I’ll get a guage after work today. The door panel says 25 lbs - sounds low - any verification? So - If I want to move the valve-open point out some, do I add slack toward the throttle lever or away from it? [/QUOTE] On the tire pressure label the tire size should also be listed (it is on my car anyway) According to [this site](http://www.top4runners.com/ja/tech.html) the correct size tire is a P265/70R16 tire. Which unless I am mistaken has a smaller diameter than a 31" tire. I saw a site last night (my google fu is weak this AM) that said the standard tire was a 27" tire. This agrees with my somewhat hazy memory of those trucks. Putting a 31" tire where a 27" is designed would limit the acceleration, top speed, and make the speedo read slow. Looking a little further the [tire rack](http://www.tirerack.com/tires/SelectTireSize.jsp?autoMake=Toyota&autoModel=4Runner+4wd&autoYear=1990&autoModClar=) says your car could have come with either 27 or 31" tires. I am fairly sure that Toyota would have fitted different gears for the different size tires. If your car came with 27" and got swaped for 31 it would limit the top speed. As far as adjusting the kick down cable goes, the looser you make the cable, the less it will want to down shift. The tighter the cable the more it will want to down shift. Adjust in small increments. ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [May 31, 2006, 6:51pm UTC](https://boards.straightdope.com/t/1990-4runner-is-too-slow-replace-diff-gears/358791/24 "2006-05-31T18:51:52Z")
[QUOTE=Gary T] This suggests that you aren’t familiar with what’s involved in a ring and pinion gear replacement. Both the ring gear and the pinion gear must be replaced as a set, and it’s a pretty big job, with a pretty big price tag. Setting those gears up is a specialty, and needs to be done by someone who **KNOWS** what he’s doing. [/QUOTE] I concur completely. Setting up hypoid gears correctly is advanced wrenching. I’ve done two sets as a shade tree mechanic, and while I got the first one done in a day, the second one required several days to get a pattern I was happy with. I also spent about a day and a half making special tools (case spreader and yoke holder) that would have been very expensive assuming I could find a source. ( I couldn’t) The Toyota trucks do use a detachible carrier, so it is possible that you could buy a carrier pre-loaded with the new gear ratio and just swap it in…I know that is commonly done with 9" ford diff’s. The curious can google for “Randy’s Ring & Pinion” to see what is involved. ---
**Author:** ![Belrix](https://avatars.discourse-cdn.com/v4/letter/b/ed655f/32.png) [@Belrix](https://boards.straightdope.com/u/Belrix)\ **Post date:** [June 2, 2006, 3:46pm UTC](https://boards.straightdope.com/t/1990-4runner-is-too-slow-replace-diff-gears/358791/25 "2006-06-02T15:46:38Z")
Just an update. I tweaked at the TV cable and gave the truck another 5 mph or so. That’s good. The door tag says the tires should be a 225/75r15, which according to the great interweb, should be around 28.5 inches in dia. Theres 31-inchers on it now. Assuming linear behavior, then the odometer ought to be readiing about 8% low. My 65MPH speedo mark should be just about 70 mph real speed. I stopped by my local convenience store and aired up the tires. It became clear, after dumping a lot of air into the tire, that the pump’s pressure guage was wrong. I thought I over-inflated but the guage I bought that afternoon still shows the back tires about 3 lbs low. (Door sticker says 35 lbs rear, 25 lbs front on that p225 tire). Does it sound strange to say that I think I can detect that it seems to be rolling easier? I’ll have to find a spot of flat ground and give it another run up to hiway speeds again. For now, I think things are better if not yet complete fixed. The new problem is that, somewhere along the way, I’ve developed an exhaust leak. Given the sound’s location, I suspect right around where the multiple exhaust pipes join to make a single. I’ll slide under the car this weekend. If I’ve burst the seam there, any idea of what a welder would charge to fix? ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [June 2, 2006, 4:59pm UTC](https://boards.straightdope.com/t/1990-4runner-is-too-slow-replace-diff-gears/358791/26 "2006-06-02T16:59:47Z")
If it is just a split seam, not much. If the pipes have rusted out, more, lots more. [Previous page](https://boards.straightdope.com/t/1990-4runner-is-too-slow-replace-diff-gears/358791.md?page=1)