Upsizing tires- Fit? Speedo? TPMS? Performance?

[ErikTheRed]

I just spent the past hour searching for info on tire upsizing, found some helpful stuff but I'm looking for specific answers to my specific questions....

1- Fit. I have a 2024 Bighorn 3500 SRW, stock tires are Trashfarce AT 275-70-18. I want to upsize to 295-70-18. I will use the Bighorn wheels and keep everything stock, I do not plan to change the springs or add a level kit. Will there be any fit or turning/rubbing issues?

2- Going from the 275-70-18 to a 295-70-18, according to online tire size calculators, at 70 mph, will result in about a 2 mph diff in speedo displayed vs actual (speedo shows 70, actually going 72). I can live with that, if the calculator is accurate. What have you fellas experienced? I've read that JScan can adjust the speedo, but does JScan work on a 2024?

3- What about the TPMS? Based on what I've read, if I'm understanding correctly, the upsized 295 tire's inflation pressures are within the parameters of the TPMS so there shouldn't be any issues. Amiright?

4- Any performance changes? I assume the overall ride will probably be a little better, but what about towing? Changing the tire size will change the final drive ratio, but is it enough to even matter? I'd think not. How was your fuel economy affected? Probably not a noteworthy difference there either. Any noticeable shift point concerns with the Aisin?

5- Without looking, I assume my spare is the same stock 275-70-18 size. I should probably order a 5th tire to replace the spare too, yeah?

Thanks for any info, people! I'll wait for a few replies so I can feel better before putting the credit card to work.

 

[Deleted member 47449]

2- Going from the 275-70-18 to a 295-70-18, according to online tire size calculators, at 70 mph, will result in about a 2 mph diff in speedo displayed vs actual (speedo shows 70, actually going 72).

I have the same 2024 Ram 3500 truck with the Aisin. I would check using your current tires when you're doing 70 MPH on a flat highway to a GPS speed APP displaying what you're actually doing.

I wanted to upgrade to 295 70R18 3PMSF winter rated tires immediately after purchase when we towed the same day from Colorado in the Fall to Wisconsin. Stuck with the stock size due to no 3PMSF tires were available without waiting 3 to 5 days. The new 275 70R18 Toyo C/T's when I'm driving 65 MPH on our local I-70 highway mountain curve warning sign it displays 64 MPH.

The 35 12.50R17 Toyo C/Ts on our 2016 Ram 2500 Tradesman Power Wagon I reprogrammed based on a GPS speed APP. When I'm doing 65 the I-70 sign displays 65.

 

[ErikTheRed]

The new 275 70R18 Toyo C/T's when I'm driving 65 MPH on our local I-70 highway mountain curve warning sign it displays 64 MPH.

So is your speedo off by 1 mph, or the mountain curve warning sign? LOL Either way, your tires are stock size so it doesn't help me determine what the larger 295's would do. With your 35's on the 2016, you mentioned you reprogrammed the speedo but you didn't mention how much correction was needed to get it to read accurately.

I'm not gonna bother trying to correct for a 1 or 2 mph misreading from the speedo (or even if I can correct it). I guess what I'd like to know is, from someone who has upsized from the 275 to a 295, what real effect did it have on speedo reading? Online calculators are nice but not exactly real world experience.

 

[Deleted member 47449]

My suggestion was to first check your speedometer at 70 MPH against what you're actually going based on GPS. If you're 1 or 2 MPH currently under on the speedometer, getting the larger size will put you going closer to 70 with no changes.

Hopefully, you hear from someone with real world experience.

 

[Deleted member 47449]

This is what I have observed that speedometers are usually factory installed with the OEM sized tires to display a speed higher than what you're actually going. Google AI actually had a good response.

 

[Sponge60]

I put 35 12.50 20's on my stock wheels. TPMS works correctly and it caused a 1mph difference clocked by a GPS app. I believe the speed difference is a percentage, so the faster you go, the higher the difference. I saw no difference at 35 mph, and the 1 mph at 55. Haven't tried it on the interstate, but I don't usually do more than 5 over anyway.

 

[Wanderlust073]

There’s about a thousand speedometer calibration calculators/charts available on google. They’re accurate. It’s math.

Going to 34” tires from the stock 33’s doesn't really force you to buy a full size spare and won’t likely make a noticeable difference in any other regard.

 

[Brutal_HO]

https://www.wheel-size.com/calc/?wheel1=275-70-18X7.5ET45&wheel2=295-70-18X7.5ET45&fcl=50mm&wcl=30mm&scl=50mm&sr=0mm

Depending on brand an actual revs/mile. they are ~3% bigger (on spec).

Probably 10lbs heavier per tire.

Your HO won't notice either.

 

[ErikTheRed]

Bull Creek, thanks for posting that info about the stock speedos being a little under actual speed. I didnt know that. But I guess if thats the case, upsizing the tires should actually bring the speedo reading closer to actual speed... so its kinda like fixing it.

 

[Deleted member 47449]

Bull Creek, thanks for posting that info about the stock speedos being a little under actual speed. I didnt know that. But I guess if thats the case, upsizing the tires should actually bring the speedo reading closer to actual speed... so its kinda like fixing it.

If you look at the OEM 20 inch wheel Transforce tires on the Ram 3500 SRW the height dimensions are slightly taller by about 0.2" at 33.6" versus 33.4" tires for the 18 inch wheel as I recall. Not much, but the speedometer isn't recalibrated. Different manufacturers have different height dimensions for the same sized tires too.

 

[Rockcrawlindude]

My opinion

Your tires are close enough in size that your Speedo will be about as accurate as mine is and I “corrected” mine but didn’t bother to fine tune it after it was a few mph off.

Your tires are close enough in size that I wouldn’t bother replacing the spare. I have a factory spare tire and am running tires taller than you.

Tpms doesn’t care about size. Will function the same unless you lower the threshold

 

[Deleted member 47449]

ErikTheRed

Most tire sellers list the diameter so that you can calculate the speedometer difference. Here's the 2024 Ram 3500 SRW 18" and 20" Transforce OEM sizes.

 

[AH64ID]

I always calibrate the odo, not the speedo. The speedo is very close when the odo is accurate and I’d rather not rack up miles faster than reality. There is also a difference between the speedo and the odo on every rig I’ve checked in the last 20 years, with the speedo reading slightly faster.

Use a standalone GPS and drive at least 50 miles to see the difference run your ODO and the GPS.

AlfaOBD and JScan make speedo adjustments quick and easy.

If you look at the OEM 20 inch wheel Transforce tires on the Ram 3500 SRW the height dimensions are slightly taller by about 0.2" at 33.6" versus 33.4" tires for the 18 inch wheel as I recall. Not much, but the speedometer isn't recalibrated. Different manufacturers have different height dimensions for the same sized tires too.

Funny you bring this up. My truck has factory 20’s but I run 18’s during hunting season. The first time I installed the 18’s I calibrated the Speedo. When I went back to the OEM setting for the 20’s it was off and I ended up back at the setting for the 18’s. Both are within 1% of the GPS when comparing ODO miles on long drives.

 

[Deleted member 47449]

Both are within 1% of the GPS when comparing ODO miles on long drives.

We have a very straight long section of highway near our home with an 8% gradient. Everywhere else nearby has mountain road corners. I was attempting to recalibrate the 35 inch tires on our 2016 Power Wagon against a GPS APP on the 8% grade and it was noticeably far off than what my speedometer was stating. Basic geometry is that I was traveling a further distance by gaining 8 feet in elevation for every 100 feet traveled than if I was driving on a flat road. Figured that the GPS speed APP was reporting accurately using the flat distance between 2 points, but it did not account for the additional distance of going up. At least that's what I had experienced. Going down after recalibration it was crazy off.

 

[AH64ID]

Elevation will have an effect on most GPS’s (my watch has a setting to calibrate distance for this when running), but it should have been exactly the same going down as it was up.

 

[Deleted member 47449]

Elevation will have an effect on most GPS’s (my watch has a setting to calibrate distance for this when running), but it should have been exactly the same going down as it was up.

Yes, it was the exactly same going down until I got onto flatter terrain with curves and figured that the recalibration wasn't right.

 

[AH64ID]

Yes, it was the exactly same going down until I got onto flatter terrain with curves and figured that the recalibration wasn't right.

All that being said on a 8% grade traveling 100' horizontally the driven distance is 100' 4", or approx. 0.32% error and should be indiscernible on your speedo. Something else seems amiss.

 

[Deleted member 47449]

All that being said on a 8% grade traveling 100' horizontally the driven distance is 100' 4", or approx. 0.32% error and should be indiscernible on your speedo. Something else seems amiss.

You are right.

I must have been doing something incorrect at the time with the recalibration. On a mile it's just 80 feet further distance. That makes me trust the GPS speed Apps.

Our home's road grade per a bicycle website...

 

[ErikTheRed]

Does AlfaOBD or JScan work on a 2024? I know very little (and by very little, I mean pretty much nothing) about either of those apps or how to use them. But it was my understanding there is no getting into the computers on 2023 and later trucks? Which app is better, or more importantly, easier to use for an old schlubb who is braindead with technology gizmos? It looks like JScan is free for Android, AlfaOBD is $50. Where do you fellas get the little plug-in module to make them work?

*I see Alfa shows up to 2023 but does not list 2024. JScan only shows to 2022.

 

[Deleted member 47449]

Suggestion versus the Alfa.

AEV ProCal SNAP for 2019-2024 Ram HD

Take control of your 2019+ HD Ram 2500/3500/4500/5500 with the AEV ProCal SNAP module. This module customizes vehicles and optimizes fuel efficiency.

www.aev-conversions.com

We have in our family a 2014 Ram 2500 4x4 Cummins pickup with 285 75R18 tires. The speedometer was recalibrated with an AEV Procal. Also, I used another AEV Procal on our 2016 Ram 2500 4x4 35 12.50R17 tires for its speedometer recalibration.