tire size change vs rpm change

[Racer9]

I'm going to 17" from 22" tires and wheels, and I might also have to go to a smaller tire to clear the fenders after the lowering kit.
Currently my 22" tires are 32", I'm looking at 17" tires that are 30". Has anyone swapped tire sizes that much? how much rpm change did you see?

 

[Wild one]

I'm going to 17" from 22" tires and wheels, and I might also have to go to a smaller tire to clear the fenders after the lowering kit.
Currently my 22" tires are 32", I'm looking at 17" tires that are 30". Has anyone swapped tire sizes that much? how much rpm change did you see?

Add roughly 7% to your rpm/tach numbers.
32" is roughly 7% taller then 30",so your rpm for any given mph is going to increase right around 7%
ie: if your cruise rpm used to be 2,000 rpm,it'll now be roughly 2140 rpm.

 

[Racer9]

Thank you a whole bunch! That isn't enough of a change to worry about.
Since this truck is more of a street/strip ride, not my dd, and that will help a bit on the track, I can live with it.
Thanks again.

 

[Jeepwalker]

Plug in your tire size specs below and it'll tell you what you can expect. You can have your truck's tire size updated in it's computer (with a 'good' scan tool or the dealer), so the odometer will read correctly. You can do it yourself with AlfaOBD software (do a search on this forum).

Tire Size Comparison

Tire size comparison the easy way. Use our tire size comparison calculator to compare diameter, width, sidewall, circumference and revolutions per mile.

tiresize.com

 

[RamDiver]

I'm going to 17" from 22" tires and wheels, and I might also have to go to a smaller tire to clear the fenders after the lowering kit.
Currently my 22" tires are 32", I'm looking at 17" tires that are 30". Has anyone swapped tire sizes that much? how much rpm change did you see?

Try out this online calculator to learn the specific impact of different sized tires.

AFAIK, your DOT will have a specific speedo error tolerance which is considered legal.

Here in Ontario, Canada, within 4% is acceptable. That's not saying you couldn't go beyond that, just don't forget when you're rushing to go somewhere or, correct the error with AlfaOBD or Jscan.

Tire Size Calculator

The best tire size calculator. Use our tire size calculator to find tire size specs and suggested tire sizes. Then check prices for all available tire sizes.

tiresize.com

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[RamDiver]

Plug in your tire size specs below and it'll tell you what you can expect. You can have your truck's tire size updated in it's computer (with a 'good' scan tool or the dealer), so the odometer will read correctly. You can do it yourself with AlfaOBD software (do a search on this forum).

Tire Size Comparison

Tire size comparison the easy way. Use our tire size comparison calculator to compare diameter, width, sidewall, circumference and revolutions per mile.

tiresize.com

We were obviously typing at the same moment in time.

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[Travelin Ram]

Here in Ontario, Canada, within 4% is acceptable.

I’m curious, do you mean tire sizes are constrained to +/- 4% from OEM, or speedometer accuracy must be within 4%? Or that the Mounties give you 4% of grace on speed enforcement?

Not that any of those things ever mattered to me.

 

[DILLIGAF]

Sure glad Berta dosent worry about insignificant things like that. No wonder I retired here.

 

[RamDiver]

I’m curious, do you mean tire sizes are constrained to +/- 4% from OEM, or speedometer accuracy must be within 4%? Or that the Mounties give you 4% of grace on speed enforcement?

Not that any of those things ever mattered to me.

I suppose I wasn't very clear on the 4%. Fixed.

A 4% speedo error is considered acceptable by big brother. I believe that's an Ontario rule.

When I posted last night, I was near comatose after a long day followed by a huge pig out on Indian food.

Sure glad Berta dosent worry about insignificant things like that. No wonder I retired here.

I might be looking to move back there if the next federal election is again dominated by fools.

.

 

[DILLIGAF]

Your wallet will also thank you

 

[Racer9]

Plug in your tire size specs below and it'll tell you what you can expect. You can have your truck's tire size updated in it's computer (with a 'good' scan tool or the dealer), so the odometer will read correctly. You can do it yourself with AlfaOBD software (do a search on this forum).

Tire Size Comparison

Tire size comparison the easy way. Use our tire size comparison calculator to compare diameter, width, sidewall, circumference and revolutions per mile.

tiresize.com

Thank you. I have the Diablo so I should be able to correct the speedo.
One other issue I thought of;
Will different size front vs rear tires cause any errors in the computer?
I'm debating running wider rear vs narrower fronts and trying to match diameter is difficult.
Yes truck is 2WD. I'm basically wondering if speed sensors are just side to side or if they are also front to back, possibly affecting ABS, or anti-spin.

That calculator says that the tires are a 6.5% (Wild one was corrrect!) difference, so at a speedo reading of 60mph, my actual speed will be 56. I can correct that, and I was more concerned about RPM, but it shouldn't be much of an issue.
Thanks again.

 

[RamDiver]

Thank you. I have the Diablo so I should be able to correct the speedo.
One other issue I thought of;
Will different size front vs rear tires cause any errors in the computer?
I'm debating running wider rear vs narrower fronts and trying to match diameter is difficult.
Yes truck is 2WD. I'm basically wondering if speed sensors are just side to side or if they are also front to back, possibly affecting ABS, or anti-spin.

We rotate the tires on street vehicles to even out the wear over time.

How would you elect to rotate tires if the front and rear were different?

.

 

[Racer9]

I wouldn't worry about it. The truck will likely only see a couple thousand miles a year, and a bit of track time, so the rears will probably need replaced first.

Oh, and thanks Jeepwalker for that tire calculator, I've saved it for future research.

 

[Wild one]

Thank you. I have the Diablo so I should be able to correct the speedo.
One other issue I thought of;
Will different size front vs rear tires cause any errors in the computer?
I'm debating running wider rear vs narrower fronts and trying to match diameter is difficult.
Yes truck is 2WD. I'm basically wondering if speed sensors are just side to side or if they are also front to back, possibly affecting ABS, or anti-spin.

That calculator says that the tires are a 6.5% (Wild one was corrrect!) difference, so at a speedo reading of 60mph, my actual speed will be 56. I can correct that, and I was more concerned about RPM, but it shouldn't be much of an issue.
Thanks again.

You can get away with about a 3" differance in height between the front and rear,before the wheel speed sensors start to go crazy on you,and put the truck into limp mode.I run 295/45/20's and 305/50/20's as my street set-up,and can get away with running my short little 26" drag radials that i use on the 1/8th mile tracks,with the 295 fronts,but on my 2 buddies trucks that have 33" front tires,those short little drag radials will put both trucks into limp mode (both are 2015 R/T's with 22's on the front)

 

[Racer9]

That's what I was concerned about. I'm having a rough time finding 30" x 17'" drag tires. Most are closer to 25".

 

[Wild one]

That's what I was concerned about. I'm having a rough time finding 30" x 17'" drag tires. Most are closer to 25".

I run the 305/45/17 Street R's which are about a 28" tall DR on the 1/4 mile tracks,but you can get them in 315/50/17 which is roughly 29.4" tall,or the 315/55/17's which is just over 30.6" tall.The bigger versions are noticably heavier,so take that into consideration.The heavier the tire,the harder it is to accelerate or deaccelerate it.The old school rule of thumb is every pound of rotating weight is equilevent to 4lbs of static weight.I use the little 26" tall 275/40/17's on the 1/8th mile tracks to help offset the 3.55's i have in the rear diff

ET Street® R

ET Street® R

www.mickeythompsontires.com

 

[Racer9]

Thank you once again.
I'm debating what to do with tires. I can stick with the 22's, but I have a set of 17" wheels. What I would like to do is find a narrow front and at least 10" rear to run all the time. Of course my OEM wheels are probably 8" maybe 8.5" so I'm sort of limited on tire sizes. I could get a pair of wider wheels and race tires for the back and just put them on when I run, but that's another grand plus that I'd rather not spend, unless I can get close to that by selling the 22's. I probably won't do anything with tires until spring, but I'd like to have a plan before that.
Over the winter I am lowering the truck, installing ARH long tube headers, and adding the Fastman throttle body. Already have cat back exhaust and Varam CAI.
I'm going to pull the rear diff cover to see what it looks like and may be adding a Detroit locker.

 

[Wild one]

Thank you once again.
I'm debating what to do with tires. I can stick with the 22's, but I have a set of 17" wheels. What I would like to do is find a narrow front and at least 10" rear to run all the time. Of course my OEM wheels are probably 8" maybe 8.5" so I'm sort of limited on tire sizes. I could get a pair of wider wheels and race tires for the back and just put them on when I run, but that's another grand plus that I'd rather not spend, unless I can get close to that by selling the 22's. I probably won't do anything with tires until spring, but I'd like to have a plan before that.
Over the winter I am lowering the truck, installing ARH long tube headers, and adding the Fastman throttle body. Already have cat back exhaust and Varam CAI.
I'm going to pull the rear diff cover to see what it looks like and may be adding a Detroit locker.

What you'll spend on one of Dans throttle bodies,save that money,and put it towards a 6.4 truck intake manifold instead.Dans/Fastman throttle bodies are good,but you're along ways off needing one yet.The 485 horse SRT 6.4's use the exact same throttle body as the trucks do,and even on them the throttle body is one of the last things you do

 

[Racer9]

Very interesting. I'll look into it. I haven't ordered the TB yet.
I located a pair of Cooper Discoverer H/T's for a great price, they are 11" wide, 29.9 tall.

 

[Surge One]

You can get/buy a programmer that will calibrate your up or down size on tires/rims. That's how I kept mine to where the rpm and speedo needed to be. Otherwise the changes do impact your transmission shifting. I went up on rims and tires on my truck. Prior to reprogramming the tranny shifted oddly. I programmed the truck to set it at the tire/rim specs and it shifted properly. Just my experience and information.