Light Truck Tires - Width and Diameter

[wjohn]

I recently purchased a '90 Dodge Dakota 4WD since I needed a truck. My '90 Civic hatch is great but just doesn't quite cut it when I need to haul much of anything. I didn't really want 4WD, but other than that it was everything I wanted - V6, manual transmission, 8' bed, and 2,000 pound payload package. After 6 months of serious searching I figured I wasn't going to do much better. At least it has 3.55 gears instead of 3.90s.

Anyhow, it needs tires. The current wheels have a silly offset, one is smashed, and I lucked into a set of factory aluminum wheels at a junkyard last weekend so I'm on my way. The stock tire size for this particular truck is 235/75R15. I think I am going to go with Michelin LTX M/S2 tires in the stock size but I thought I'd ask a couple of pretty open-ended questions here first in case anyone has some good insight

Really, I'd like to be running some narrower tires than 235s. My old Dakota had 205/75R15s and was a 2WD auto (yuck on the auto part). I knocked out 22-23 MPG at 65 MPH with 500-800 pounds in the bed with that truck, which was decent considering the trans. For my current truck, I would like to stay close to the same overall diameter so that I don't have to change out the speedometer gear. It's not difficult to change, just pricey for what it is.

The other issue caused by the smaller diameter from the 205 sizes is that they result in slightly higher engine speeds. Granted, it's not a lot, but a 205/75R15 would run about 2430 RPM at 70 MPH vs. 2280 RPM with the 235s. I wouldn't even be concerned about that for city driving (narrower would be obviously better if I never ran out of gears) but I plan on mainly doing highway travel in this truck. 70 MPH is the fastest I'd ever conceivably go.

The other big issue is that Michelin doesn't seem to make anything in the 205/75R15 size that would work. I've been happy with their tires so far and will continue to give them business as long as that remains true... or if someone else out there has a light truck tire that gets significantly better fuel economy.

Like I said, I'm pretty sure I'll just stick with the 235s but wasn't sure if anyone had any info regarding a similar tire sizing situation. I did see the link to the Car and Driver article (I'd link to it here, but I'm not allowed to as this is my first post) and noticed that the drop in fuel economy from 195/65R15s to 225/45R17s was 23.3 to 22.8 MPG. That's ballpark for my truck's MPG figures, although there may be a bit more increase in Cd since more of my tires would be exposed with the taller ground clearance. Granted, I'm not changing wheel size in this situation (stock 15x6s with either tire) but the width difference is similar.

Anyway, I'm definitely rambling at this point. Anybody have any suggestions or recommendations? Anybody able to stop me before I put 235s on and get stuck with them for 70K miles?

[oil pan 4]

Trucks tend to be over powered and putting smaller tires on them usually hurts milage.

I don't think any one has put smaller diameter drive wheels on anything and seen an improvement with out regearing the differential or transmission.

Then when you put smaller narrower tires on you lose load capacity. If you go with inch narrower and smaller diameter tires you can forget about that 2,000lb load rating.
You can get away with smaller tires on a car, be cause you usually don't carry that much weight.

Put load range C 31x10.5 tires on. Or load range C 30x9.5 tires.
I went from P235/75R15 tires to 31x10.5R15 tires on my suburban back in 2008, milage did not suffer, may have even gone up a little. It rode much better.
I plan to put 30x9.5 H/T tires up front, and leave the 31'' A/Ts on the back since I have a 2wd. I don't have to worry about over loading the tires any more.

[wjohn]

It looks like I can find 215/75R15s with adequate load capacity for my rear GAWR - definitely not Michelins, though. I almost wish I had 16" rims so I could run 215/85R16s, which are narrower but very close to the same diameter as the 235/75R15s and still have good load capacity.

I guess the 30x9.5s would only be about 6 mm wider but an inch taller than the 235s. I'd still have to change the speedometer gear. I'll run the numbers and check prices this afternoon - thanks for the idea.

[oil pan 4]

Just make sure you don't price 30 inch load range C tires against Pmetric car tires. Do price comparison againt LT tires.

Usually you only want to go an inch or 2 taller at most.
Had I gone with 30 inch tires I may have seen more of a fuel economy boost.
But it was hard to argue with free tires when the P235/75R15 were failing left and right.
Then I kind of got hooked on the 31 inch tire.

[CapriRacer]

First, you need to understand that tire size plays a small role in the rolling resistance of a tire. Within a given size, the rolling resistance can vary widely - up to 60%. And changing tire size has a much smaller affect than choosing the right tire.

You will want to stay away from LT tires. They are built with materials that a bit more rugged, but that also means worse RR.

Since your truck originally came with 235/75R15's you don't really have much in the way of options for size anyway. I'd stick with the 235/75R15's and select a tire that best meets your needs.

[Hersbird]

I got my last 235/75/15s Goodyear Wranglers AT at Wal-Mart and they were under $100 each mounted and balanced. That was 10 years ago though...
Now I buy everything through discounttiredirect.com

[oil pan 4]

Originally Posted by CapriRacer

First, you need to understand that tire size plays a small role in the rolling resistance of a tire. Within a given size, the rolling resistance can vary widely - up to 60%. And changing tire size has a much smaller affect than choosing the right tire.

You will want to stay away from LT tires. They are built with materials that a bit more rugged, but that also means worse RR.

Since your truck originally came with 235/75R15's you don't really have much in the way of options for size anyway. I'd stick with the 235/75R15's and select a tire that best meets your needs.

How do you think load range C flotation size compares to LT tires for RR in tires that are very close to the same size?

[wjohn]

My rear GAWR is 3650 so I need tires with a load capacity of at least 1,825 pounds. A tad more would be nice. Most of the truck/SUV 235s have load ratings of at least 2000 (that's a load index of 104) so I'm generally safe with them. I have done some more research and decided to just stick with the stock tire size for now.

Here's a twist - the Michelin LTX M/S2 tires in my 235/75R15 size are not classified as LRR. Apparently some sizes of the LTX are LRR and some aren't. That's kind of frustrating. It would have been a fairly obvious choice for me to go with the Michelins if they were LRR.

I see that General Grabber HTS tires have approximately the same weight as the Michelins but additionally are LRR. They obviously aren't rated for as many miles as the Michelins are although I may give them a look. The factory tires were AS Goodyear Wranglers. Those don't seem to be LRR, either. Actually, the more I look the more it seems like the Generals are the only LRR tires in this size and class.

I'm not in a big rush since I need to replace the timing chain and redo most of the brake system before I get tires, but I'd like to figure out what I want in case a rebate or sale pops up. Hopefully I'll have the other work finished on the truck in the next couple of weekends. I can drop the wheels off at a shop anytime, though.

Thanks for all of the input so far! I appreciate it... if anyone here has any experience with the General Grabbers, I'd be curious to hear about it.

[CapriRacer]

Originally Posted by oil pan 4

How do you think load range C flotation size compares to LT tires for RR in tires that are very close to the same size?

I think flotation sizes ARE LT tires.

If you look carefully, you will find that some folks include the letters "LT" AFTER the size - which is the proper way to designate the size. Those letters are frequently omitted - as is the letter P in front of some P metric sizes.

But as I said before, the range of RR within a given size is HUGE!! So a comparison of a flotation sized tire to a regular LT tire, while somewhat problematic, is small, small potatoes.

[elhigh]

Originally Posted by wjohn

I recently purchased a '90 Dodge Dakota 4WD since I needed a truck. My '90 Civic hatch is great but just doesn't quite cut it when I need to haul much of anything. I didn't really want 4WD, but other than that it was everything I wanted - V6, manual transmission, 8' bed, and 2,000 pound payload package. After 6 months of serious searching I figured I wasn't going to do much better. At least it has 3.55 gears instead of 3.90s.

Here's a situation where I don't think the "buy a trailer instead" suggestion is going to work. The Dakota is just starting to feel the load by the time a trailer-equipped Civic is at the outer edge of its capacity.

I heat with wood. I have a truck, mostly because I had the truck long before I ever owned a car. In the fullness of time I have acquired a Forester which could tow a ton, and under that paradigm I could get a LOT done with just a car and a trailer.

But if I just had the Civic hybrid? According to all the manuals, you can't tow with a HCH at all.

For the longest time I've been tooling around with passenger tires on my truck. The one time I fitted LT tires, my mileage took a big dip and stayed there for the next 30,000 miles. I was very glad when those lossy donuts wore out. You might consider passenger tires instead, if you can find them in the right size. If you never go offroad and don't often load up to the limit of the chassis, you might do very well.