Tire Size and Speed???

silentcrow · #1 01/02/08, 05:41 PM

I got a used '98 Chevy Silverado this past spring. It's actually going about 4 MPH less than the speedometer shows. Could this be due to a smaller tire size than it was meant to have? I don't know what manufacturer recommendation is since the truck didn't come with a manual. Where do I find the recommended tire size? Thanks!

uncle Will in In. · #2 01/02/08, 05:49 PM

Smaller diameter tires will make it travel slower than is supposed to be traveling.
I have a 96 half ton silverado and it uses 325 X 15 tires. It doesn't have 4 wheel drive.

js2743 · #3 01/02/08, 05:58 PM

should be in the drivers side door jam area on a tag that is posted there it should tell tire size and the gears and every thing.

Raptor · #4 01/02/08, 07:47 PM

Call your local Chevy dealer and ask if they can recalibrate the computer system to the size tire you have on your truck.

Phantomfyre · #5 01/02/08, 08:36 PM

I've got a '97 K1500 Silverado that I'm pretty sure still had the stock tires on when I bought it. I replaced the tires with the same size right after I bought it. The truck also had a 3" body (not suspension) lift kit on it when I bought it, but I can't imagine a body lift would affect this. I've noticed the speedo runs about 10% low. So if my gauge shows 30 MPH, I'm actually only doing about 27; if I see 60, I'm really doing 54. At least according to all those "YOUR SPEED" dealies that highway dept. puts out on holidays. So I just do the math to get my speed right, and when I do that, I match the gun. YMMV, literally.

Up North · #6 01/03/08, 12:49 AM

Quote:

Originally Posted by Phantomfyre

I've got a '97 K1500 Silverado that I'm pretty sure still had the stock tires on when I bought it. I replaced the tires with the same size right after I bought it. The truck also had a 3" body (not suspension) lift kit on it when I bought it, but I can't imagine a body lift would affect this. I've noticed the speedo runs about 10% low. So if my gauge shows 30 MPH, I'm actually only doing about 27; if I see 60, I'm really doing 54. At least according to all those "YOUR SPEED" dealies that highway dept. puts out on holidays. So I just do the math to get my speed right, and when I do that, I match the gun. YMMV, literally.

I suspect that a previous owner changed the rear axle gear ratio but neglected to change the speedometer to correlate to the altered rear end.

Up North · #7 01/03/08, 12:53 AM

Quote:

Originally Posted by silentcrow

I got a used '98 Chevy Silverado this past spring. It's actually going about 4 MPH less than the speedometer shows. Could this be due to a smaller tire size than it was meant to have? I don't know what manufacturer recommendation is since the truck didn't come with a manual. Where do I find the recommended tire size? Thanks!

Which model Silverado do you own?...1500, 2500,or 3500? C or K series? If you provide this info, perhaps some kind soul can look it up in their manual for you

comfortablynumb · #8 01/03/08, 12:55 AM

my speedo shows 5 MPH faster than I am really going.... so I can go 50 in a 45 and feel like a rebel.

Dont try to stop me, I'm out of control.
grrrrr

Big Dave · #9 01/03/08, 04:51 AM

My youngest just payed a ticket for going ten miles over the limit. He has over sized his tires. If you go to your tire dealer they can tell you what is the proper size for any vehicle. They can also tell you where to buy the equipment to recalibrate your vehicle to what ever size tire you want to put on your vehicle. It ain't cheap

silentcrow · #10 01/03/08, 05:47 AM

It's a 1500, but I don't know the series. There are no lift kits or other alterations. It is 4x4 extended cab (3 door). Trying to find actual tire dealers around here is a joke. My choice is my mechanic (I'm starting to wonder about him), other mechanics well known for "lick and stick", or Wally World. I refuse to have anything to do with the dealerships...They are more dishonest than the used dealers most of the time.

Last trip I made was this past Sunday. I had my cruise set at 75 MPH, yet was being "blown away" by other drivers, and cops didn't even look at me twice. Posted speed limit was 65 (I-80). During the summer, those speed warning signs at construction areas showed I was 4 MPH off. Mechanic said it was a "speed sensor" located in something I needed replaced anyway. I doubted it, but since I needed the part, it didn't matter. New tires were put on, but were the same size as the ones that were on it when I bought it. It always felt like it set kinda low, but I was used to driving a '97 F-150 so I brushed it off. I'll try to see if there is anything remaining of the stickers on the door jamb. I do know this is the last Chevy I'll ever get, unless I can find a really old one.

foxtrapper · #11 01/03/08, 05:55 AM

Showing a few mph higher than you're actually going at interstate speeds is normal. Intentionally so. You can tune it with different size tires, there's plenty of calculators on the web for this. I certainly wouldn't go out of my way to correct it.

silentcrow · #12 01/03/08, 06:32 AM

Quote:

Originally Posted by foxtrapper

Showing a few mph higher than you're actually going at interstate speeds is normal. Intentionally so. You can tune it with different size tires, there's plenty of calculators on the web for this. I certainly wouldn't go out of my way to correct it.

It also does it at low speeds.

**Cabin Fever** · #13 01/03/08, 07:22 AM

My 1999 K1500 4x4 came from the factory with 265/75-16 tires. This is the tire size for pickup with the Z-71 option package. The K1500's without the Z-71 package came with 245/75-16 tires.

Silentcrow, what size tire is on your K1500 right now?

stonykill · #14 01/03/08, 08:22 AM

tire size will effect the speedo. Being you have a 3 inch body lift I would imagine the tire size is larger than stock. What size tire is on it?

More importantly tire size effect final drive ratio, therefore gas mileage/performance. Taller tires than stock have the same effect as putting a numerically lower set of gears in the differential. Shorter tires have the oposite effect. Therefore with taller tires, you will see a slight increase in gas mileage. This will be negated however if they are taller and much wider. The added rolling restistance of the wide tire contact to the ground, adds to the drag on the truck.

BTW if you are off 4 mph at highway speeds, you are off different amounts at slower and higher speeds. It's just the way a speedo works.

As an example of taller tires in a real life situation.... My 93 grand cherokee, full time 4wd, 318 V8, came stock with p235/75 r 15's. Roughly 28 3/4 tall x 9 inches wide.My highway mileage was just over 18 mpg. I needed ground clearance as my Jeep spends a lot of its time working offroad on my property. I added 30/9.5-15 inch tires. My highway mileage went up to an even 19 mpg. By replaceing the restrictive stock exhaust system (after it rotted off of course) with a flowmaster exhaust system and a new set of plugs and wires, I get a solid 20 mpg highway. Not bad for full time 4wd, v8, and 165,000 miles. Plus I work this Jeep, and need the power.

So I guess what I'm getting at, is if oyu know how far off your speedo is, who cares?

silentcrow · #15 01/03/08, 08:31 AM

Quote:

Originally Posted by Cabin Fever

My 1999 K1500 4x4 came from the factory with 265/75-16 tires. This is the tire size for pickup with the Z-71 option package. The K1500's without the Z-71 package came with 245/75-16 tires.

Silentcrow, what size tire is on your K1500 right now?

Well, I looked. That is what is on it...LT245/75R 16 (not a Z-71). What else could be causing the problem?

crafty2002 · #16 01/03/08, 08:35 AM

Tell me what the speed is showing when you know for sure you are going 4 MPH faster and what the radius of the tire is, and I can figure out what size tire you need to get the speedometer showing the correct speed. If you have a tack in it and tell me the RPM's the engine is turning at the speed you give me it would make it easier to figure plus I can tell you what the rear end ratio is.

Nearly all the car or trucks I have had when I was working out of town, I would change the rear end to a lower ratio or put really tall tires on the rear end so if (or rather when) I got a speeding ticket I could have the speedometer checked and the judge would throw it out of court.
I liked it when I bought a Ford with a 3.25 ratio rear end for the road. I could swap in a 2.75 and run exactly 65 MPH and it would show 55 MPH with the correct tires.
The really tall tires worked best for this because I could act dumb when the officer walked up and nearly every time they would be the one to explain to me that the tires on the back looked to big for whatever I was driving and let me go with a warning. I would always mark the mile marker on the map in RED.
Don't let the same cop pull you twice.

I'll have to look up the formula but I think it is

tire radius x RPM
______________
168 x rear end ratio

You can put a chalk mark on the bottom of a read tire and one on the drive shaft and have someone else pull forward "SLOWLY" and and count the times the driveshaft turns before the mark on the tire gets back to the exact position on the bottom.
It should be 3 point or 4 point something number of turns for your truck. That will tell you the rear end ratio regardless of your tire size.
I just checked and I was right. I do still remember useless things to know, LOL.
Lets assume you have a 14" tire radius and are going down the road and your engine is turning 3,000 RPM's.
Now lets assume you have a 4.11 rear gear.
14 x 3,000= 42,000. Now 4.11 x 168 = 690.48. 42,000 devided by 690.48 is 60.827 MPH

Hope this helps some.

stonykill · #17 01/03/08, 08:56 AM

Quote:

Originally Posted by silentcrow

Well, I looked. That is what is on it...LT245/75R 16 (not a Z-71). What else could be causing the problem?

as someone else previously stated, the previous owner probably changed the gear ratio.

I personally don't see a problem. If you know how far off the speedo is, adjust your driving accordingly. Maybe its the homesteader in me, maybe I'm just cheap, but if it ain't broke, I don't fix it. I know my speedo is 2 to 4 mph off, depending on the speed I'm driving, and drive accordingly. Besides a ticket for only 4 mph over the speedlimit most likely won't stick. Now if your going 14 mile per hour over, thats a different story.

**Cabin Fever** · #18 01/03/08, 09:26 AM

It this were my pick-up, I'd not let it bother me. Who knows, maybe the speed warning display could have been off by a few mph. Look at it htis way, you are less likely to get a speeding ticket and your odometer is showing less miles than actual which may result in a higher resale price.

foxtrapper · #19 01/03/08, 12:09 PM

Quote:

Originally Posted by silentcrow

It also does it at low speeds.

Yes, at a ratio. If it's 4 miles high at 60, it'll be 2 miles high at 30, and 1 mile high at 15.

It's not a big deal, and is oem typical.

245/75-16 vs 265/75-16 is 2.43 mph at 65 mph. Less than what you're seeing. And that's only if the tires are precisely that size. Which they are not. From manufacturer variation to tread wear.

idahodave · #20 01/03/08, 12:38 PM

The truck once had big tires on it (why else the body lift?) and the original owner had the speedo re-calibrated for the bigger tires. When he sold, he kept the big tires, put the oem ones back on, and didn't recal the speedo.

96 chev has software calibration for speedo, which can be done at dealer or shop with recal equipment.