I have a stock 87 carbed wagon, and want to put bigger tires on it (I'm in the process of building a 2" lift.) Does anyone know about how much fuel mileage changes with bigger tires? Right now I'm getting about 21 mpg running regular unleaded.

Thanks,

Tom

Pretty hard to say, as there are so many factors involved. Honestly, on my lifted wagons, I didn't see much of a difference if I was careful how I drove them (both FI '88s on ~28" tires....means you don't use 5th gear any more).

 

 

The important thing you have to do is figure out your odometer conversion to make up for the tire size. Best way is to use a GPS while on the highway to figure out distance traveled compared to an odometer reading (the longer, the better). Not quite as accurate, but still a good benchmark, is to figure out your speedometer discrepancy (with a GPS, or even a radar speed sign), or even just calculate the tire size difference....

I'm running a 2" lift and 27" tires on my SPFI Loyale. I still use fifth gear, quite a lot. But more for cruising. If you plan to accelerate at all or go up any hill, gotta go into fourth or third, depending on your speed. But for me, I haven't noticed much gas mileage change. Maybe a little less, but only a couple MPGs.

 

For me, my speedometer isn't too far off. When the speedo reads 35 MPH, I'm really going 38 MPH. When the speedo reads 60 MPH, I'm really going 66 MPH. It's about a 1.0:1.1 ratio. which isn't that far off. I don't do conversions when I fill up for gas because it's only a few miles off, but I haven't noticed much more fuel consumption.

 

If you do more city driving, stop-and-go, hills, etc., you're gonna get worse mileage. If you do more freeway driving, you're actually gonna get better gas mileage. At speed, the wheels are going a longer distance per every revolution, but during city driving, it requires more power to turn the wheels. I personally do a 50/50 mix of driving, so that's why I don't notice much different in the gas mileage.

 

I used to get 25-30 MPG when I was stock, but now I probably get 20-25 MPG.

Wheel weight makes more of a difference than tire size (though bigger tires weigh more as well of course).  Being so close to the leverage point makes the weight matter a lot more than you would expect.

I'm running a 2" lift and 27" tires on my SPFI Loyale. I still use fifth gear, quite a lot. But more for cruising. If you plan to accelerate at all or go up any hill, gotta go into fourth or third, depending on your speed. But for me, I haven't noticed much gas mileage change. Maybe a little less, but only a couple MPGs.

 

For me, my speedometer isn't too far off. When the speedo reads 35 MPH, I'm really going 38 MPH. When the speedo reads 60 MPH, I'm really going 66 MPH. It's about a 1.0:1.1 ratio. which isn't that far off. I don't do conversions when I fill up for gas because it's only a few miles off, but I haven't noticed much more fuel consumption.

 

If you do more city driving, stop-and-go, hills, etc., you're gonna get worse mileage. If you do more freeway driving, you're actually gonna get better gas mileage. At speed, the wheels are going a longer distance per every revolution, but during city driving, it requires more power to turn the wheels. I personally do a 50/50 mix of driving, so that's why I don't notice much different in the gas mileage.

 

I used to get 25-30 MPG when I was stock, but now I probably get 20-25 MPG.

 

Just using the 35/38mph number, that's more than an 8% discrepancy. And Speedometers are calibrated to read high, so I bet if you compare your odometer to a GPS, you'll find it's even more than that. Tire size calculations (just 23 v 27") calculate to about 17%. That is significant!

Tman, you will have speedometer error. Easiest way to look at proposed new tire sizes and speedo error is go online to 'discounttire.com' .

Across the top of the page are various tabs. Click on "Info Center". The left side of the page has "Tire Information". Go about half way down on the left side and find and click on "Tire calculator". You will be able to see speedo error.

Determining your gas mileage will be trial and error.