bigger tires, tranny shift points?
 

I've had my truck for a few years now, and I always thought my speedo was off some. This past weekend I was doing 70 mph, and my wife who was following me said I was doing more like 80. I emailed my vin to a local dealer and they said the original tire size was P235/75R15XL. The size that has been on there since I got it is 31x10.5/70 R15. I know I can get a tuner to fix the speedo/odometer issue, but they also said the tranny shift points would be off. From what I've read on the tuners for a 99 3.9 auto tranny you can't change tranny shift points. Do I have many options to get things right? Are tranny shift points different for different tire sizes? Also Superchips is the only one I've found that specifically says it's for a 99 Dakota 3.9, are there any others that will work, if so pro's and con's of each?

Thanks guys and gals

RE: bigger tires, tranny shift points?
 

you could go back to the original tires, 235's are only a little more then an inch shorter then your 31's and it sounds like most of your problems would go away

RE: bigger tires, tranny shift points?
 

Yellowjacket,

Just to share my experience with you.........

I've owned my '98 Dak 4x4 since new. It came equipped with 31x10.5R15 owl tires new as a factory option and I have always kept that size tire on the truck. You may want to "re-verify" that your truck did not have the 31x10.5 tire option. I believe that this size tire was standard if the truck came with the factory wheel flares. My factory sticker lists the P235/75R15 as the standard tire, then lists the 31x10.5R15 as a $400 option.

I had never been happy with the way that the truck (automatic) shifted. It seemed very awkward and would at times seem to "search" for the proper gear. I would always lock out the overdrive in "around town" driving which seemed to help a bit. I never considered until I read your post that possibly it was due to the tire option (although this was a factory option on my truck).

What I can say is that recently I purchased the superchips tuner for my truck through Strippermotorsports for about $310 delivered. I personally believe this to be the best money that I have spent on my truck. In addition to a slight noticeable power increase, the shift points have smoothed out remarkably. It is almost like driving a new truck. I no longer feel the need to lock out the overdrive and I am very pleased with the way that the truck accelerates and shifts.

These are just my own personal observations, I have no affiliation with Superchips but I am so far very pleased with their product.

I hope that this information is helpful to you.

Regards,
Bob

RE: bigger tires, tranny shift points?
 

hey bob did you use the superchips computer before or after you added the exhaust?

RE: bigger tires, tranny shift points?
 

Kurby,

I used the Superchips tuner AFTER I put the Cat back exhaust and hi-flow cat on.

The reason I replaced the cat and muffler was that the originals needed to be replaced because of age (rusted, worn). The Magnaflow components have a real nice tone and sound and they MAY have added a little noticeable power but I have nothing to support this other than feel.

I used the Superchips tuner to improve driveability and I was very pleased with the result on my 5.2 although I have no experience in using this on the V6.


Bob

RE: bigger tires, tranny shift points?
 

thanks, i have a truck exactly like yours (except its a 99) and i was considering the superchips but was upset that they didnt offer any dyno evidence. i also have the 31" tires and have noticed the transmission sucking at picking the right gear so your shifting improvement helps out. i havent noticed any problems with the speedo though

RE: bigger tires, tranny shift points?
 

Thanks for the input guys. I knew the 31's were a factory option because back in November I needed new tires and went to Goodyear's website. They listed a standard size of P215 something, and then the two optional sizes of the P235's and 31's. The truck had 31's when I bought it, so I stuck with those. Since it had aftermarket rims when I got it, I started to wonder if someone didn't find out you could go up to the 31's and put them on there without changing anything else. Kurby, if the 31's are actually 31" tall, according to Goodyear's website the 235's are 28.9" tall, so that's over 2" taller. Anyway as far as shifting my truck acts that way too. It seems to upshift too soon, and seems like I have to floor it to get it to downshift sometimes and just to me never seems like it's in the right gear at times. Since I just dropped around $500 on tires that are still almost new, I hate to go buy more tires. Thanks for the input again though. Any others are still welcome.

RE: bigger tires, tranny shift points?
 

If goodyears website says that then im sure its accurate, i used like math and stuff to figure it out and it doesnt account for variances in tread depth. but 2 inches on a tire is still only lifting your truck 1"

RE: bigger tires, tranny shift points?
 

It's only a height increase of ~1", but the math is more complex than that. Check the rolling distance (circumference) for each.

28.9"= 90.8" per revolution = 697.8 revolutions per mile
31" = 97.4" per revolution = 650.5 revolutions per mile

The difference is almost 50 revolutions of the tire per mile. It doesn't seem significant, but over 20,000 miles that's 1,000,000 fewer revolutions at the tire/wheel bearing, and somewhere around 370,000 fewer revolutions of the motor (assuming 3.92 and .69 OD), if my math is correct. That's the good news about the larger tires. The bad news is that as diameter increases, the torque required to spin them increases exponentially which puts more stress on your axles, gears, tranny/t-case, engine, u-joints, etc. Not a big increase in this case, as going from a 31 to a 40 would be, but still it's an added stress, and if an engine is close to underpowered with smaller tires, the final drive ratio change with larger tires coupled with the increase in power required for similar performance will cause the engine to feel that much more inadequate.

RE: bigger tires, tranny shift points?
 

Thanks guys. Just ordered the Max micro tuner so I'll see how that does.