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Does changing tire size affect performance?

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wlrdlr49

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I have a 98 GMC 3500 will putting taller tires on it have any effect on the performance of the engine? I went from 225/75/16 to 265/75/16? Thanx in advance for your help.
 
Yes it does. It is just like changing your differential gears. I haven't had enough coffee to do the math yet, but that is like going from 4.10s to 3.73s. You are probably going to lose a little on acceleration but pick up mpg.
 
Thanks. I am having some problems with its general performance. It misses occasionally + when I go up + down hills. Does it make a diff if it is not level? I still have the 225,s on the front.
 
The tires should have no effect other than the operating rpms, hp and torque at a particular vehicle speed. Anthing like a miss I think you should fill out the diagnostic check list from the stickies. There are much much wiser wrenches than I here that can help you diagnose your truck from the check list.
 
I have 4:10's and I changed to 265 tires in 19.5 wheels. The gearing now is in the neighborhood of a 3:48. My rpm's now is 1800 at 60 and that does help with the fuel milage. I 'did' lose a lot of low end grunt but get a few rpms up and it come back to life real quick.
 
wlrdlr49, I have an issue with the transmission going in and out of overdrive also, but only at 45 mph. The main highway here is 45 but at a steady 47mph there is no overdrive problem.
 
I had not checked at what speed mine starts doing it but it continues at highway speed.
 
Not enough of a tire size difference to cause problems. I went from 225/75 to 235/85, about two inches taller and barely noticeable. I then went up to 255/85 another 2 inches it's about a 33x9, and I can feel a difference power-wise, oddly I have lost some top end, seems like I shouldn't have. But the tire size change won't cause the problems you're having, you might have lift pump problems. I just changed mine for the second time, it was doing the same thing you describe on the hills. The overdrive question I tried to address in your other thread.
This is kind of the shotgun approach to diagnostics, you really should do the checklist.
 
Thnx Dan I am trying to do the checklist but I,m not smart enough to copy & paste. I need to paste a big L on my forehead !
 
Yeah, I have the big L. Try to highlight the whole thing, right click on it and copy it, then you can put it in a new post and paste it.
 
I hate to differ w/ some of you guy's but tire size will affect fuel economy, speed (inaccurate), and when the trany shifts. Depending on how much of a difference from stock size you go will depend on how much off your speedo will be as well as your fuel economy. My burb calls for 235's stock. I have 265's. According to my gps and mile markers, I am approximately 2 mph faster (I think, it has been over a year since I checked it; I know it is 2 mph i cant remember if it is +/-). My MPG's are lower when I figure them to the odometer however I get more mpgs when I figure miles on my gps. In addition if your tires have an aggressive off road tread your mpgs will decrease.
 
Rodd, I'm not arguing that point at all, my speedo is off a bit, I don't bother calculating fuel mileage anymore because it's too much math, but it actually seems better, not worse. I do 90% of my driving right in a sweet spot of around 40-50, in OD and not leaning into the throttle hardly at all. I think I probably use a bunch more fuel getting the truck moving, but at my cruising speed I'm turning lower rpms without lugging the engine, so overall I think that gives me the better fuel economy. I do have very aggressive tires, but that's a trade-off I'm always willing to make.

I do realize the final drive ratio being so different alters the shift points and speed sensors, and the speed at which lockup occurs. But I feel like the truck doesn't know the difference, if it thinks we're going 40 and it engages, but we're actually going 45, it's all good in my book. I'm not going to tell.
Honestly, I never look at the speedometer. I drive so much I have a pretty good feel for how fast I'm going, so I never get stopped for speeding.
 
635 said:
wlrdlr49, I have an issue with the transmission going in and out of overdrive also, but only at 45 mph. The main highway here is 45 but at a steady 47mph there is no overdrive problem.
Click to expand...

The tcc lockup point is 45. You are feeling it unlock if you drop below 45 and lock when you get back to 45 (Assuming you truck is like mine, I don't know what gears you have).
 
wlrdlr49 said:
I have a 98 GMC 3500 will putting taller tires on it have any effect on the performance of the engine? I went from 225/75/16 to 265/75/16? Thanx in advance for your help.
Click to expand...

That isn't much of a difference so you should barely notice it.

Do you have a 4x4? if so, you mentioned you have the 225's on the front, this is a big NO NO, :nono: This well cause all kinds of drivetrains issues if used in 4wd and will also cause your ABS light to come on.
 
Detriot Dan i am looking to do the same as you did on my 3500. Are you able to run the 255/85's without spacers?? What is the brand, style of the tires? I am looking at Maxxis. According to thier tire chart I should be OK. I really don't want spacers 8 lugs per wheel are enough I don't want 16. Thanks Joe
 
On my 9500lbs work truck switching from 225/75 to 235/85 dropped my rpms on highway OD about 300rpm. Also allows me to drive more in 3rd without windint up to much. I remember an initial power loss feeling off the line, but either my brain compensated or the computer compensated, because now it feels as torquey as ever, almost to the point that if it was any more so it would be too much... Was about a 2" size upgrade.
 
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