Overall Diameter
+0.45 in
+1.31%
34.36 in34.81 in
315/70R17 vs 305/70R18 Tire Size Comparison
315/70R17 vs 305/70R18: overall diameter changes by +0.45" (+1.31%), section width changes by −10 mm, sidewall height changes by −0.28", and static ground clearance shifts by about +0.22 in. Speedometer impact is about +1.31% at 60 mph indicated. This pair requires a different wheel diameter (17" → 18"). These figures are dimensional calculations only — they do not confirm vehicle fitment.
Tire Size Comparison Calculator
Key Specification Differences
Section Width
−0.39 in
-3.17%
12.40 in12.01 in
Sidewall Height
−0.28 in
-3.17%
8.68 in8.41 in
Circumference
+1.41 in
+1.31%
107.95 in109.36 in
Speedometer & Revs-per-Mile Impact
Speedometer Difference
+1.31%
+1.31%
60 mph60.8 mph true
Revolutions per Mile
−8
-1.29%
586.9579.4
Side-by-side proportional comparison of Original tire 315/70R17 and New tire 305/70R18 on a shared ground baseline. Overall diameter 34.36" versus 34.81" (difference +0.45"). Section width 12.40" versus 12.01". Wheel diameter 17" versus 18".
Diameter 34.36" → 34.81" (+0.45")Width 12.40" → 12.01"Wheel 17" → 18"
Comparison uses nominal dimensions calculated from the selected tire sizes. Actual manufacturer measurements may differ.
- Steering responseShorter sidewall may reduce flex, although tire construction, pressure and vehicle setup also affect response.
- Ride comfortShorter sidewall generally transmits more impact harshness.
- Sidewall complianceReduced sidewall height typically means less vertical compliance over sharp edges; construction still matters.
- Effective gearingThe larger rolling diameter produces slightly taller effective gearing.
- Road-use characterWidth and profile changes can alter road feel and noise; they do not by themselves prove better grip, snow performance or towing ability.
Driving & Vehicle Impact
Speedometer at selected speed
60 mph indicated ≈ 60.8 mph actual
Based on the selected vehicle-speed input.
Ground-clearance effect
Approximately +0.22"
Half of the overall diameter increase.
Sidewall behaviour
Shorter sidewall
May reduce flex and increase impact harshness.
Effective gearing
Approximately +1.31% taller
Slightly fewer wheel revolutions per road mile.
Wheel requirement
18-inch wheel required
The original 17-inch wheel cannot be reused.
Installation complexity
Moderate verification required
Wheel, load, clearance and offset checks remain.
Checks Triggered by This Comparison
Replacement wheel required
- 305/70R18 requires a 18" wheel
- The 17" wheel used by 315/70R17 cannot be reused
Larger envelope check
- Overall diameter increases +0.45"
- Verify fender and suspension clearance at full lock and compression
The ±3% diameter threshold is a comparison screen, not confirmed vehicle fitment.
Alternative Comparison Paths
Alternative sizes are dimensional comparisons only. Verify wheel width, load rating, offset and vehicle clearance.
What changes when switching from 315/70R17 to 305/70R18?
Plus-sizing and wheel use
This is a plus-size change from a 17" to a 18" wheel.
The 17" wheel cannot be reused for the 18" tire.
+1.31% diameter change is inside the site’s ±3% comparison threshold.
Dimensional direction
305/70R18 is taller than 315/70R17 (+0.45") and narrower (−10 mm).
Sidewall height decreases by 0.28"; static ground clearance increases by 0.22".
Road speed, revs and gearing
Theoretical road speeds: 30 indicated = 30.4 mph actual; 60 indicated = 60.8 mph actual; 75 indicated = 76.0 mph actual.
Revolutions per mile change −7.6 (-1.29%), producing taller effective gearing.
Tire models sold in both sizes
3 indexed brand/model names exist in both sizes: Goodyear Discoverer® S/T MAXX™, Goodyear Discoverer® STT Pro™, Goodyear Wrangler Territory® MT. Service descriptions and specifications can differ by size.
Popular Tires Available in 305/70R18
Touring
Discoverer S/T Maxx
305/70R18 · 126Q · Load Range E
All-Terrain
Discoverer Stt Pro
305/70R18 · 126Q · Load Range E
All-Terrain
Wrangler Territory MT
305/70R18 · 126R · Load Range E
Models shown are matched to exact-size records in TireReference's tire database. Availability and specifications may change; confirm with the manufacturer or retailer. Data and Calculation Standards
Popular Comparison Pairs
Common size upgrades from the Tire Reference database.
Frequently Asked Questions
Does this comparison confirm vehicle fitment?
No. It performs a dimensional comparison of 315/70R17 and 305/70R18, not a complete vehicle fitment analysis.
Confirmed fitment still needs model year, trim, factory tire specification, wheel width, offset, bolt pattern, hub bore, brake clearance, suspension and fender clearance, plus load and speed-rating requirements.
What this tool can quantify from size (and published catalog fields when present): diameter difference (+1.31%), width difference (−10 mm), and wheel diameter requirement (17" → 18").
No approved rim-width range is listed for either size in this dataset, so rim fit must be confirmed from the specific tire maker’s documentation.
Load index and speed rating are not populated for both sizes in this dataset.
A diameter change within ±3% is only a common screening guideline — not a fitment guarantee for any specific vehicle.
How accurate are the comparison results?
Nominal tire dimensions for 315/70R17 and 305/70R18 are calculated from the tire-size code using standard geometric relationships, so the arithmetic for those nominal values is exact.
Real mounted tires can still differ because of tread design, approved measuring rim, inflation, load, remaining tread depth and casing construction.
For this pair, overall diameter moves +1.31% (34.36" → 34.81") and section width moves −10 mm (-3.17%).
This comparison is running on nominal size-code calculations for both 315/70R17 and 305/70R18. Prefer manufacturer-published overall diameter and revs/mile when you have selected a specific tire model.
Speedometer and gearing figures are theoretical: they assume the indicated speed is based on the original rolling circumference with no cluster recalibration.
How are tire dimensions calculated?
For metric sizes, sidewall height equals section width × (aspect ratio ÷ 100), wheel diameter is converted with 1 in = 25.4 mm, and overall diameter equals wheel diameter plus two sidewalls.
Circumference is π × overall diameter, and revolutions per mile equal inches per mile divided by that circumference.
Applied here: 315/70R17 uses 315 mm width and aspect ratio 70 on a 17" wheel → 8.68" sidewall and 34.36" overall diameter; 305/70R18 works out to 8.41" sidewall and 34.81" overall diameter.
The width code is nominal section width, not tread width, and the aspect ratio is a percentage of that section width.
Will the speedometer be affected?
Yes — the larger rolling circumference on 305/70R18 travels farther per wheel revolution, so at a given indicated speed the vehicle typically moves slightly faster than the cluster shows.
Circumference changes +1.41" (+1.31%), which is the same percentage order as the diameter change for this geometric model.
At an indicated 60 mph, the theoretical road speed is approximately 60.8 mph (+1.31% versus 315/70R17).
Actual cluster behaviour can still differ after tire wear, load, temperature or any OEM speedometer calibration.
Do I need new wheels?
Yes for bead-seat diameter: 315/70R17 is an R17 tire and 305/70R18 is an R18 tire (+1" wheel change). An R17 tire will not correctly seat on an R18 wheel.
Plan a 18" wheel with appropriate width for 305 mm section width, then confirm offset, bolt pattern, hub bore, brake clearance and load rating on the target vehicle.
No approved rim-width range is listed for either size in this dataset, so rim fit must be confirmed from the specific tire maker’s documentation.
Can different tire sizes use the same wheel?
Only when wheel diameter matches, the rim width sits in the approved range for both tires, load and speed requirements are met, and the bead-seat type is compatible.
315/70R17 (R17) and 305/70R18 (R18) cannot use the same wheel diameter; an R17 tire and an R18 tire do not share a bead seat.
A wider tire does not automatically require a wider wheel, but mounting outside the published rim range can distort the tread profile and change handling and load behaviour.
No approved rim-width range is listed for either size in this dataset, so rim fit must be confirmed from the specific tire maker’s documentation.
How much clearance should I check?
Clearance must be checked through the suspension and steering travel, not only when the vehicle is parked on level ground.
Inspect full steering lock, suspension compression, inner sidewall to strut or control arm, outer shoulder to fender or liner, brake and wheel-barrel clearance, and tire growth/deflection under load — plus any wheel offset or width change.
For this pair, half the overall diameter change approximates the static radius or ground-clearance shift: about +0.22 in from the +1.31% diameter move (34.36" → 34.81").
Width change (−10 mm) spreads around the wheel centreline only when wheel width and offset stay unchanged; a different offset moves the whole package inward or outward.
Can I mix tire sizes front and rear?
Only when the vehicle manufacturer approves a staggered or mixed setup — do not create one casually from a comparison tool.
Mixing 315/70R17 and 305/70R18 across axles would put roughly +1.31% (+0.45") of rolling-circumference difference between front and rear if one size sat on one axle and the other on the opposite axle.
ABS, stability control and especially AWD/4WD systems can be sensitive to axle circumference mismatch; transfer-case and differential hardware may see extra stress, and tire rotation patterns become limited.
Follow the placard or OEM staggered specification. There is no universal “safe” percentage that applies to every drivetrain.
Why do actual tire dimensions differ between brands?
Two products sharing the 315/70R17 or 305/70R18 size code can still publish different overall diameters and revolutions per mile.
Measuring rim width, tread pattern, sidewall and casing construction, rated load and pressure, manufacturing tolerances, and new-versus-worn tread all shift the measured envelope.
Use the size-code geometry on this page for planning, then prefer the manufacturer’s published diameter and revs/mi for the exact model you intend to buy.
What is the difference between nominal and published tire dimensions?
Nominal values come from the tire-size formula applied to the size code — for example 34.36" overall diameter on 315/70R17 and 34.81" on 305/70R18.
Published values come from physical measurement or manufacturer specifications for a specific brand and model, and can differ from that formula result.
Nominal figures are useful for generic size-to-size comparison; published figures are preferred for exact model-to-model analysis.
This comparison is running on nominal size-code calculations for both 315/70R17 and 305/70R18. Prefer manufacturer-published overall diameter and revs/mile when you have selected a specific tire model.
How does tire size affect effective gearing?
A larger rolling diameter on 305/70R18 produces taller effective gearing: fewer wheel revolutions per road mile and usually a small drop in engine RPM at a given true road speed.
revs/mi moves −8 (-1.29%). At 60 mph indicated, theoretical engine speed shifts with that circumference change — exact RPM also depends on final-drive ratio and transmission gear.
Acceleration response can feel slightly softer with taller gearing or sharper with shorter gearing; treat those effects as directional rather than certain for every powertrain.
Why does sidewall height matter?
Sidewall height sets how much vertical rubber is available to deflect under load.
For this pair sidewall changes −0.28" (8.68" → 8.41", -3.17%). Lower sidewalls generally reduce flex; taller sidewalls generally add compliance.
Construction, inflation pressure, load and tread design still matter — sidewall height alone does not determine handling or comfort.
- Calculated by TireReference (Tire Reference)
- Report a calculation issue
- Calculator logic updated: July 2026